Class 11 Chemistry Notes

Chapter 8: Organic Chemistry – Some Basic Principles and Techniques

Topic 1: Introduction to Organic Chemistry

1. Organic Chemistry

Definition

Organic Chemistry is the branch of chemistry that studies carbon compounds, especially those containing carbon-hydrogen (C–H) bonds.

Examples:

Methane (CH₄)
Ethanol (C₂H₅OH)
Acetic acid (CH₃COOH)

2. Why is Carbon Special?

Carbon is unique because it has:

✔ Tetravalency

Carbon has 4 valence electrons.
It forms four covalent bonds to complete its octet.

✔ Catenation

Carbon atoms can bond with one another.
This forms long chains, branched chains, and rings.

Result: Millions of organic compounds are possible.

3. Elements Commonly Present in Organic Compounds

Besides carbon, organic compounds may contain:

Hydrogen (H)
Oxygen (O)
Nitrogen (N)
Sulphur (S)
Phosphorus (P)
Halogens (F, Cl, Br, I)

4. Importance of Organic Compounds

Organic compounds are present in:

✔ DNA and RNA

✔ Proteins

✔ Carbohydrates

✔ Fats and oils

✔ Medicines

✔ Fuels

✔ Plastics

✔ Dyes

✔ Polymers

5. Historical Background

Vital Force Theory

Proposed by J. J. Berzelius.
It stated that organic compounds could only be produced by living organisms due to a mysterious “vital force.”

Why was it rejected?

In 1828, Friedrich Wöhler prepared urea from ammonium cyanate, proving that organic compounds can also be made in laboratories.

Reaction

NH₄OCN → NH₂CONH₂

Ammonium cyanate → Urea

This experiment marked the beginning of modern organic chemistry.

Quick Facts (Exam Ready)

Property	Information
Father of Organic Chemistry	Friedrich Wöhler (modern development)
Organic chemistry studies	Carbon compounds
Carbon valency	4
Special property	Catenation
Theory rejected	Vital Force Theory
First organic compound synthesized in laboratory	Urea
Year	1828

Remember These Keywords

Organic Chemistry
Carbon
Tetravalency
Catenation
Covalent Bond
Vital Force Theory
Wöhler Synthesis
Urea

One-Line Revision

Organic chemistry is the study of carbon compounds.
Carbon forms four covalent bonds.
Carbon shows catenation.
Organic compounds are essential for life.
Wöhler disproved the Vital Force Theory by synthesizing urea in 1828.

Topic 2: Tetravalency of Carbon & Shapes of Organic Molecules

Class 11 Chemistry Notes

Chapter 8: Organic Chemistry – Some Basic Principles and Techniques

Topic 2: Tetravalency of Carbon & Shapes of Organic Molecules

1. Tetravalency of Carbon

Definition

Carbon has 4 valence electrons, so it forms four covalent bonds to complete its octet.

Example:

CH₄ (Methane)

Carbon can form bonds with:

Carbon (C)
Hydrogen (H)
Oxygen (O)
Nitrogen (N)
Halogens

2. Hybridisation of Carbon

Definition

Hybridisation is the mixing of atomic orbitals to form new orbitals of equal energy called hybrid orbitals.

There are three important types of hybridisation in organic chemistry.

A. sp³ Hybridisation

Formation

1s + 3p orbitals mix
Forms 4 sp³ hybrid orbitals

Geometry

Tetrahedral

Bond Angle

109.5°

Example

Methane (CH₄)
Ethane (C₂H₆)

Characteristics

Only single bonds (σ bonds)
Highest bond length
Lowest bond strength

B. sp² Hybridisation

Formation

1s + 2p orbitals mix
Forms 3 sp² orbitals
One p orbital remains unhybridised

Geometry

Trigonal Planar

Bond Angle

120°

Example

Ethene (C₂H₄)
Benzene

Characteristics

One double bond
Contains 1 σ bond + 1 π bond

C. sp Hybridisation

Formation

1s + 1p orbitals mix
Forms 2 sp orbitals
Two p orbitals remain unhybridised

Geometry

Linear

Bond Angle

180°

Example

Ethyne (C₂H₂)
Hydrogen cyanide (HCN)

Characteristics

Triple bond
Contains 1 σ bond + 2 π bonds

3. Comparison of Hybridisation

Property	sp³	sp²	sp
Shape	Tetrahedral	Trigonal Planar	Linear
Bond Angle	109.5°	120°	180°
s-character	25%	33%	50%
Example	CH₄	C₂H₄	C₂H₂
Bonds	Single	Double	Triple

4. Effect of s-Character

As s-character increases:

Electrons stay closer to the nucleus.
Carbon becomes more electronegative.
Bond length decreases.
Bond strength increases.

Order

sp > sp² > sp³

Electronegativity:
sp > sp² > sp³

Bond Strength:
sp > sp² > sp³

Bond Length:
sp < sp² < sp³

5. Important Examples

Compound	Hybridisation	Shape
CH₄	sp³	Tetrahedral
C₂H₆	sp³	Tetrahedral
C₂H₄	sp²	Trigonal Planar
HCHO	sp²	Trigonal Planar
C₂H₂	sp	Linear
HCN	sp	Linear

Exam Points ⭐

✔ Carbon is always tetravalent.

✔ sp³ → Single bond

✔ sp² → Double bond

✔ sp → Triple bond

✔ Greater s-character = Stronger and shorter bond.

Quick Revision

Carbon forms four covalent bonds.
Hybridisation determines the shape of molecules.
sp³ → 109.5° → Tetrahedral
sp² → 120° → Trigonal Planar
sp → 180° → Linear
Bond strength: sp > sp² > sp³
Bond length: sp < sp² < sp³

Topic 3: Sigma (σ) Bond and Pi (π) Bond

1. Covalent Bonds in Organic Compounds

Carbon atoms form covalent bonds by sharing electrons.

There are two main types of covalent bonds:

Sigma (σ) bond
Pi (π) bond

2. Sigma (σ) Bond

Definition

A sigma bond is formed by the head-to-head overlap of atomic orbitals along the internuclear axis.

Features of σ Bond:

✔ Strongest covalent bond

✔ First bond formed between two atoms

✔ Electron density is maximum between two nuclei

✔ Allows free rotation around a single bond

✔ Present in all single bonds

Types of Overlap in σ Bond

1. s–s overlap

Example:

H–H in hydrogen molecule

2. s–p overlap

Example:

C–H bond in methane

3. p–p overlap

Example:

Some carbon-carbon bonds

3. Pi (π) Bond

Definition

A pi bond is formed by the sidewise overlap of parallel p-orbitals.

Features of π Bond:

✔ Formed after formation of a sigma bond

✔ Weaker than sigma bond

✔ Electron cloud is present above and below the plane of bonding atoms

✔ Restricts rotation around double bonds

✔ More reactive than sigma bonds

4. Formation of Multiple Bonds

Double Bond (C=C)

Contains:

1 Sigma (σ) bond
1 Pi (π) bond

Example:

Ethene (CH₂=CH₂)

Triple Bond (C≡C)

Contains:

1 Sigma (σ) bond
2 Pi (π) bonds

Example:

Ethyne (CH≡CH)

5. Difference Between σ and π Bonds

Sigma (σ) Bond	Pi (π) Bond
Formed by head-on overlap	Formed by sideways overlap
Stronger bond	Weaker bond
Electron density along axis	Electron density above and below axis
Allows free rotation	Restricts rotation
Present in single, double and triple bonds	Present only in multiple bonds
More stable	More reactive

6. Importance of π Bonds

π electrons are easily available for attack by reagents.
Therefore, compounds containing double and triple bonds are generally more reactive.

Example:

Ethene reacts easily because of its π bond.

Important Examples ⭐

Methane (CH₄)

4 σ bonds
No π bond

Ethene (C₂H₄)

5 σ bonds
1 π bond

Ethyne (C₂H₂)

3 σ bonds
2 π bonds

Quick Revision

σ bond = head-on overlap
π bond = sidewise overlap
Every single bond contains only σ bond.
Double bond = σ + π
Triple bond = σ + 2π
π bonds are weaker and more reactive.

Topic 4: Structural Representation of Organic Compounds

1. Structural Formula of Organic Compounds

Organic compounds can be represented in different ways to show the arrangement of atoms.

The main types are:

Lewis (Dot) Structure
Complete Structural Formula
Condensed Structural Formula
Bond-Line Formula

2. Lewis Structure (Electron Dot Structure)

Definition

A Lewis structure represents:

Atoms
Covalent bonds
Lone pairs of electrons

using dots and lines.

Rules:

Shared electron pairs are shown as lines.
Lone pairs may be shown as dots.
Each line represents one covalent bond.

Example: Methane (CH₄)

Carbon shares four electron pairs with four hydrogen atoms.

3. Complete Structural Formula

Definition

A structural formula in which all atoms and all bonds are shown clearly is called a complete structural formula.

Representation of Bonds:

Bond Type	Representation
Single bond	–
Double bond	=
Triple bond	≡

Examples:

Ethane

CH3–CH3

Ethene

CH2=CH2

Ethyne

CH≡CH

4. Condensed Structural Formula

Definition

A simplified form of structural formula where similar groups are written together is called a condensed formula.

Examples:

Ethane:

Complete:

CH3–CH3

Condensed:

CH3CH3

Hexane:

Complete:

CH3–CH2–CH2–CH2–CH2–CH3

Condensed:

CH3(CH2)4CH3

5. Bond-Line Formula (Skeletal Formula)

Definition

A simplified representation where only carbon-carbon bonds are shown as lines is called a bond-line formula.

Rules:

✔ Carbon atoms are not written.

✔ Hydrogen atoms attached to carbon are also not written.

✔ Carbon atoms are assumed at:

Line ends
Corners
Junction points

✔ Heteroatoms like O, N, Cl, Br are written.

Example:

A zig-zag line represents a carbon chain.

Each corner or end represents a carbon atom.

6. Importance of Bond-Line Formula

Advantages:

Easy to draw large organic molecules.
Saves time.
Used commonly by organic chemists.
Makes complex structures easier to understand.

7. Comparison of Structural Representations

Type	Shows	Example
Lewis Structure	Electrons and bonds	CH₄ with dots
Complete Structure	All atoms and bonds	CH₃–CH₃
Condensed Formula	Groups together	CH₃CH₃
Bond-Line Formula	Carbon skeleton only	Zig-zag lines

Important Exam Points ⭐

✔ One dash (–) represents a single bond.

✔ Two dashes (=) represent a double bond.

✔ Three dashes (≡) represent a triple bond.

✔ In bond-line structures, carbon and hydrogen atoms are usually omitted.

✔ Every carbon atom in bond-line formula has enough hydrogen atoms to complete valency 4.

Quick Revision

Organic compounds can be represented in four ways.
Lewis structures show electrons.
Complete formulas show every atom.
Condensed formulas simplify structures.
Bond-line formulas show only carbon skeletons.
Carbon atoms are understood at line ends and corners.

Topic 5: Three-Dimensional Representation of Organic Molecules

1. Need for 3D Representation

Organic molecules are three-dimensional (3D) structures.

A normal structural formula is drawn on paper (2D), but actual molecules have atoms arranged in space.

To represent the actual shape of molecules, special methods are used.

2. Wedge-Dash Representation

The wedge-dash formula is used to show the 3D arrangement of atoms on a 2D paper.

There are three types of bonds:

A. Normal Line (—)

Meaning:

Represents a bond lying in the plane of the paper.

Example:

C — H

B. Solid Wedge (▲)

Meaning:

Represents a bond coming out of the plane of the paper towards the observer.
The broad end of the wedge is closer to the observer.

C. Dashed Wedge (—)

Meaning:

Represents a bond going behind the plane of the paper away from the observer.

3. Summary of Wedge-Dash Symbols

Symbol	Meaning
—	Bond in the plane of paper
▲	Bond coming towards observer
⋯	Bond moving away from observer

4. Molecular Models

Definition:

Molecular models are physical models used to understand the three-dimensional shape and arrangement of atoms in organic molecules.

They help in visualising:

Bond angles
Molecular shape
Arrangement of atoms

5. Types of Molecular Models

There are three common molecular models:

1. Framework Model

Features:

✔ Shows only bonds connecting atoms.

✔ Atoms themselves are not shown.

✔ Focuses on the bonding pattern.

Advantage:

Helps understand the arrangement of bonds.

2. Ball-and-Stick Model

Features:

✔ Atoms are represented by balls.

✔ Bonds are represented by sticks.

✔ Shows both atoms and bonds.

Example:

Ethene (C₂H₄) is better represented using this model.

3. Space-Filling Model

Features:

✔ Shows actual size of atoms.

✔ Based on van der Waals radii.

✔ Bonds are not clearly visible.

✔ Shows the volume occupied by atoms.

6. Comparison of Molecular Models

Model	Shows	Does Not Show
Framework	Bonds only	Size of atoms
Ball-and-stick	Atoms + bonds	Actual atom size
Space-filling	Atom size and volume	Clear bonds

Important Exam Points ⭐

✔ Solid wedge → bond towards observer.

✔ Dashed wedge → bond away from observer.

✔ Normal line → bond in plane of paper.

✔ Molecular models help understand 3D shapes.

✔ Space-filling model is based on van der Waals radius.

Quick Revision

Organic molecules have 3D structures.
Wedge-dash notation represents 3D arrangement on paper.
Solid wedge shows bonds coming out.
Dashed wedge shows bonds going behind.
Three molecular models:
1. Framework model
2. Ball-and-stick model
3. Space-filling model

Topic 6: Classification of Organic Compounds

1. Classification of Organic Compounds

Organic compounds are classified mainly on the basis of:

Structure of carbon skeleton
Presence of functional groups
Arrangement of carbon atoms

The major classes are:

Acyclic or Open-chain compounds
Cyclic or Closed-chain compounds

2. Acyclic Compounds (Open Chain Compounds)

Definition:

Compounds in which carbon atoms are arranged in straight chains or branched chains are called acyclic compounds.

They do not contain rings.

Types:

A. Straight Chain Compounds

Carbon atoms are arranged in a continuous chain.

Examples:

Ethane:

CH3–CH3

Propane:

CH3–CH2–CH3

B. Branched Chain Compounds

Carbon atoms form side chains attached to the main chain.

Example:

Isobutane:

     CH3
      |
CH3–CH–CH3

3. Cyclic Compounds (Closed Chain Compounds)

Definition:

Compounds containing a ring or closed structure of carbon atoms are called cyclic compounds.

They are divided into:

Homocyclic compounds
Heterocyclic compounds

4. Homocyclic Compounds

Definition:

Cyclic compounds containing only carbon atoms in the ring are called homocyclic compounds.

They are further divided into:

A. Alicyclic Compounds

Definition:

Cyclic compounds that have properties similar to aliphatic compounds are called alicyclic compounds.

Examples:

Cyclopropane:

      CH2
     /   \
  CH2---CH2

Cyclohexane:

     (CH2)6 ring

Characteristics:

✔ Non-aromatic

✔ Carbon atoms form rings

✔ Show properties similar to open-chain compounds

B. Aromatic Compounds

Definition:

Cyclic compounds containing aromatic rings with special stability are called aromatic compounds.

Example:

Benzene (C₆H₆)

Characteristics:

✔ Contain delocalised π electrons

✔ Usually follow Huckel’s rule

✔ More stable than ordinary cyclic compounds

5. Heterocyclic Compounds

Definition:

Cyclic compounds containing atoms other than carbon in the ring are called heterocyclic compounds.

The atoms present may be:

Nitrogen (N)
Oxygen (O)
Sulphur (S)

Examples:

Pyridine

Contains nitrogen atom in the ring.

Furan

Contains oxygen atom in the ring.

Thiophene

Contains sulphur atom in the ring.

6. Classification Chart

                 Organic Compounds
                         |
          --------------------------------
          |                              |
       Acyclic                        Cyclic
    (Open chain)                 (Closed chain)
          |                              |
   ----------------        -------------------------
   |              |        |                       |
Straight      Branched  Homocyclic           Heterocyclic
 chain         chain        |
                         --------------
                         |            |
                    Alicyclic      Aromatic

7. Difference Between Major Classes

Type	Structure	Example
Acyclic	Open chain	Ethane
Alicyclic	Carbon ring, non-aromatic	Cyclohexane
Aromatic	Stable aromatic ring	Benzene
Heterocyclic	Ring with other atoms	Pyridine

Important Exam Points ⭐

✔ Open-chain compounds are also called acyclic compounds.

✔ Ring compounds are called cyclic compounds.

✔ Homocyclic compounds contain only carbon atoms in the ring.

✔ Heterocyclic compounds contain atoms like N, O, or S.

✔ Benzene is the most common aromatic compound.

Quick Revision

Organic compounds are classified based on carbon skeleton.
Acyclic compounds have open chains.
Cyclic compounds have rings.
Homocyclic compounds contain only carbon atoms in rings.
Alicyclic compounds are non-aromatic cyclic compounds.
Aromatic compounds have special stability due to delocalised electrons.
Heterocyclic compounds contain atoms other than carbon in the ring.

Topic 7: Functional Groups and Homologous Series

1. Functional Group

Definition:

A functional group is an atom or group of atoms present in an organic compound that determines its chemical properties and reactions.

The behaviour of an organic compound mainly depends on its functional group.

2. Importance of Functional Groups

Functional groups help to:

✔ Identify the class of organic compounds.

✔ Predict chemical reactions.

✔ Decide the naming of compounds.

✔ Compare properties of different compounds.

3. Common Functional Groups

Functional Group	Formula	Class of Compound	Example
Hydroxyl	–OH	Alcohol	Ethanol (C₂H₅OH)
Aldehyde	–CHO	Aldehyde	Ethanal (CH₃CHO)
Ketone	>C=O	Ketone	Propanone (CH₃COCH₃)
Carboxyl	–COOH	Carboxylic acid	Ethanoic acid (CH₃COOH)
Amino	–NH₂	Amine	Methylamine (CH₃NH₂)
Halogen	–X (F, Cl, Br, I)	Halo compound	Chloroethane (C₂H₅Cl)
Ether	–O–	Ether	Dimethyl ether (CH₃OCH₃)
Nitro	–NO₂	Nitro compound	Nitrobenzene

4. Homologous Series

Definition:

A homologous series is a group of organic compounds having:

Same functional group
Similar chemical properties
Same general formula
Successive members differ by a –CH₂– group

5. Characteristics of Homologous Series

1. Same Functional Group

All members contain the same functional group.

Example:
Alcohols:

CH₃OH (Methanol)
C₂H₅OH (Ethanol)
C₃H₇OH (Propanol)

All contain –OH group.

2. Difference of –CH₂–

Each successive member differs by:

–CH₂–

Example:

Methane:
CH₄

Ethane:
C₂H₆

Difference:
CH₂

3. Similar Chemical Properties

Members of the same series show similar reactions due to the same functional group.

4. Gradual Change in Physical Properties

Physical properties such as:

Melting point
Boiling point
Solubility

change gradually with increase in molecular mass.

6. Examples of Homologous Series

A. Alkane Series

General Formula:

CₙH₂ₙ₊₂

Examples:

Compound	Formula
Methane	CH₄
Ethane	C₂H₆
Propane	C₃H₈
Butane	C₄H₁₀

B. Alcohol Series

General Formula:

CₙH₂ₙ₊₁OH

Examples:

Compound	Formula
Methanol	CH₃OH
Ethanol	C₂H₅OH
Propanol	C₃H₇OH

C. Carboxylic Acid Series

General Formula:

CₙH₂ₙ₊₁COOH

Examples:

Compound	Formula
Methanoic acid	HCOOH
Ethanoic acid	CH₃COOH
Propanoic acid	C₂H₅COOH

7. Difference Between Functional Group and Homologous Series

Functional Group	Homologous Series
Specific atom/group responsible for reactions	Group of related compounds
Determines chemical properties	Members have similar properties
Example: –OH, –COOH	Example: Alcohol series

Important Exam Points ⭐

✔ Functional group controls chemical behaviour.

✔ Same functional group → similar chemical properties.

✔ Successive members differ by –CH₂–.

✔ Molecular mass increases by 14 u between consecutive members.

✔ Homologous series has the same general formula.

Quick Revision

Functional group is the reactive part of an organic molecule.
Common groups include –OH, –CHO, –COOH, –NH₂.
Homologous series contains compounds with the same functional group.
Each next member differs by CH₂.
Physical properties change gradually in a homologous series.

Topic 8: IUPAC Nomenclature of Organic Compounds (Basic Rules)

1. IUPAC Nomenclature

Definition:

IUPAC nomenclature is a systematic method of naming organic compounds according to rules given by the International Union of Pure and Applied Chemistry (IUPAC).

Purpose:

Gives a unique name to every organic compound.
Avoids confusion caused by common names.

2. Parts of an IUPAC Name

An organic compound name generally contains:

Prefix + Word Root + Suffix

Example:

2-methylpropane

2-methyl → Prefix
prop → Root word
ane → Suffix

3. Word Root

The word root indicates the number of carbon atoms present in the longest carbon chain.

Number of Carbon Atoms	Root Word
1	Meth
2	Eth
3	Prop
4	But
5	Pent
6	Hex
7	Hept
8	Oct
9	Non
10	Dec

4. Primary Suffix

The primary suffix indicates the type of carbon-carbon bond present.

Bond Type	Suffix	Example
Single bond	–ane	Ethane
Double bond	–ene	Ethene
Triple bond	–yne	Ethyne

5. Secondary Suffix

The secondary suffix represents the main functional group.

Functional Group	Suffix	Example
–OH (Alcohol)	–ol	Ethanol
–CHO (Aldehyde)	–al	Ethanal
>C=O (Ketone)	–one	Propanone
–COOH (Carboxylic acid)	–oic acid	Ethanoic acid
–NH₂ (Amine)	–amine	Ethanamine

6. Prefix

Prefix indicates:

Side chains (alkyl groups)
Substituents
Halogens

Common Alkyl Groups

Alkyl groups are formed by removing one hydrogen atom from an alkane.

Alkane	Alkyl Group	Name
Methane	CH₃–	Methyl
Ethane	C₂H₅–	Ethyl
Propane	C₃H₇–	Propyl
Butane	C₄H₉–	Butyl

7. Steps for IUPAC Naming

Step 1: Select the Longest Carbon Chain

Choose the longest continuous chain of carbon atoms.
This chain is called the parent chain.

Example:

CH₃–CH₂–CH₂–CH₃

Longest chain = 4 carbons

Name = Butane

Step 2: Number the Carbon Chain

Rules:

✔ Number from the end nearest to:

Double bond
Triple bond
Functional group
Substituent

The aim is to give the lowest possible number.

Step 3: Identify Substituents

Find groups attached to the main chain.

Examples:

CH₃– → Methyl
C₂H₅– → Ethyl
Cl– → Chloro
Br– → Bromo

Step 4: Write the Complete Name

Order:

Position + Prefix + Root Word + Suffix

Example:

CH₃–CH(CH₃)–CH₃

Step:

Longest chain = 3 carbons → Prop
Methyl group at carbon 2
Single bonds → ane

Name:

2-methylpropane

8. Important Rules

Rule 1:

The longest carbon chain must be selected.

Rule 2:

Numbering should give the lowest possible numbers.

Rule 3:

Different substituents are written in alphabetical order.

Example:

Ethyl comes before methyl.

Rule 4:

Prefixes are separated by hyphens.

Example:

2-methylbutane

9. Examples

Example 1:

CH₃–CH₂–CH₃

Carbon atoms = 3

Name:

Propane

Example 2:

CH₃–CH(CH₃)–CH₃

Parent chain = Propane

Substituent = Methyl at carbon 2

Name:

2-methylpropane

Example 3:

CH₃–CH₂–OH

Parent chain = Ethane

Functional group = Alcohol

Name:

Ethanol

Important Exam Points ⭐

✔ Root word depends on the number of carbon atoms.

✔ Suffix shows the main functional group.

✔ Prefix shows substituents.

✔ Longest chain rule is the first step.

✔ Functional group gets priority during numbering.

✔ Lowest number rule is followed in naming.

Quick Revision

IUPAC name = Prefix + Root + Suffix.
Root tells carbon number.
Primary suffix tells C–C bond type.
Secondary suffix tells functional group.
Prefix tells substituents.
Always select the longest chain and number it correctly.

Topic 9: IUPAC Nomenclature of Alkanes, Alkenes and Alkynes (Solved Examples)

1. Naming of Alkanes

Definition:

Alkanes are saturated hydrocarbons containing only single bonds between carbon atoms.

General Formula:

$C_nH_{2n+2}$ CnH2n+2

Suffix Used:

–ane

Steps for Naming Alkanes

Step 1: Select the longest carbon chain

The longest chain is the parent chain.

Step 2: Number the chain

Number from the end nearest to the substituent.

Step 3: Name the substituents

Add the position number before the substituent name.

Examples of Alkanes

Example 1:

Structure:

CH₃–CH₂–CH₂–CH₃

Carbon atoms = 4

Root word = But

Suffix = ane

IUPAC Name: Butane

Example 2:

Structure:

CH₃–CH(CH₃)–CH₃

Steps:

Longest chain = 3 carbon atoms → Propane
Methyl group at carbon 2

IUPAC Name: 2-Methylpropane

Example 3:

Structure:

CH₃–CH(CH₃)–CH₂–CH₃

Steps:

Longest chain = 4 carbons → Butane
Methyl group at carbon 2

IUPAC Name: 2-Methylbutane

2. Naming of Alkenes

Definition:

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond (C=C).

General Formula:

$C_nH_{2n}$ CnH2n

Suffix Used:

–ene

Rules for Naming Alkenes

✔ The longest chain containing the double bond is selected.

✔ Numbering starts from the end nearest to the double bond.

✔ Position of double bond is mentioned before the suffix.

Examples of Alkenes

Example 1:

Structure:

CH₂=CH₂

Carbon atoms = 2

Root word = Eth

Double bond = ene

IUPAC Name: Ethene

Example 2:

Structure:

CH₂=CH–CH₃

Steps:

Three carbon chain → Prop
Double bond at carbon 1

IUPAC Name: Prop-1-ene

(Also written as Propene)

Example 3:

Structure:

CH₃–CH=CH–CH₃

Steps:

Four carbon chain → But
Double bond at carbon 2

IUPAC Name: But-2-ene

3. Naming of Alkynes

Definition:

Alkynes are unsaturated hydrocarbons containing at least one carbon-carbon triple bond (C≡C).

General Formula:

$C_nH_{2n-2}$ CnH2n−2

Suffix Used:

–yne

Rules for Naming Alkynes

✔ Select the longest chain containing the triple bond.

✔ Number from the end nearest to the triple bond.

✔ Mention the position of triple bond.

Examples of Alkynes

Example 1:

Structure:

CH≡CH

Carbon atoms = 2

Root word = Eth

Triple bond = yne

IUPAC Name: Ethyne

Example 2:

Structure:

CH≡C–CH₃

Steps:

Three carbon chain → Prop
Triple bond at carbon 1

IUPAC Name: Prop-1-yne

Example 3:

Structure:

CH₃–C≡C–CH₃

Steps:

Four carbon chain → But
Triple bond at carbon 2

IUPAC Name: But-2-yne

4. Comparison of Hydrocarbons

Type	Bond Present	Formula	Suffix
Alkane	Single bond	CₙH₂ₙ₊₂	–ane
Alkene	Double bond	CₙH₂ₙ	–ene
Alkyne	Triple bond	CₙH₂ₙ₋₂	–yne

Important Exam Points ⭐

✔ Double bond gets priority over alkyl substituents.

✔ Triple bond position should always be given the lowest number.

✔ Alkane → saturated compound.

✔ Alkene and alkyne → unsaturated compounds.

✔ Position of multiple bonds is written before the suffix.

Quick Revision

Alkane → C–C → –ane
Alkene → C=C → –ene
Alkyne → C≡C → –yne
Choose the longest chain.
Give the lowest number to multiple bonds.
Write substituents with their positions.

Topic 10: IUPAC Nomenclature of Compounds Containing Functional Groups

1. Priority Order of Functional Groups

When a compound contains more than one functional group, the group with higher priority is selected as the main functional group and gets the suffix.

Priority Order (Higher → Lower)

Functional Group	Suffix
–COOH (Carboxylic acid)	–oic acid
–CHO (Aldehyde)	–al
>C=O (Ketone)	–one
–OH (Alcohol)	–ol
–NH₂ (Amine)	–amine
C=C (Alkene)	–ene
C≡C (Alkyne)	–yne
–X (Halogen)	Prefix

2. Naming of Alcohols (–OH)

Definition:

Alcohols are compounds containing the hydroxyl group (–OH).

General Formula:

$C_nH_{2n+1}OH$ CnH2n+1OH

Suffix:

–ol

Rules:

✔ Select the longest carbon chain containing –OH group.

✔ Number the chain from the end nearest to –OH.

✔ Replace –e of alkane with –ol.

Examples:

Example 1:

CH₃OH

One carbon → Meth
–OH group → ol

IUPAC Name: Methanol

Example 2:

CH₃CH₂OH

Two carbons → Eth
Alcohol group → ol

IUPAC Name: Ethanol

Example 3:

CH₃–CH(OH)–CH₃

Three carbons → Prop
OH at carbon 2

IUPAC Name: Propan-2-ol

3. Naming of Aldehydes (–CHO)

Definition:

Aldehydes contain the functional group –CHO.

Suffix:

–al

Rules:

✔ Aldehyde carbon is always carbon number 1.

✔ No need to mention its position.

Examples:

Example 1:

HCHO

IUPAC Name: Methanal

Example 2:

CH₃CHO

Two carbon chain
Aldehyde group

IUPAC Name: Ethanal

Example 3:

CH₃CH₂CHO

IUPAC Name: Propanal

4. Naming of Ketones (>C=O)

Definition:

Ketones contain a carbonyl group (>C=O) attached to two carbon atoms.

Suffix:

–one

Rules:

✔ Select longest chain containing carbonyl group.

✔ Give lowest number to carbonyl carbon.

Examples:

Example 1:

CH₃COCH₃

Three carbons
Carbonyl at carbon 2

IUPAC Name: Propan-2-one

(Common name: Acetone)

Example 2:

CH₃COCH₂CH₃

IUPAC Name: Butan-2-one

5. Naming of Carboxylic Acids (–COOH)

Definition:

Compounds containing the carboxyl group (–COOH) are called carboxylic acids.

Suffix:

–oic acid

Rules:

✔ Carboxyl carbon is always carbon number 1.

✔ Replace alkane “e” with “oic acid”.

Examples:

Example 1:

HCOOH

IUPAC Name: Methanoic acid

Example 2:

CH₃COOH

IUPAC Name: Ethanoic acid

(Common name: Acetic acid)

Example 3:

CH₃CH₂COOH

IUPAC Name: Propanoic acid

6. Naming of Amines (–NH₂)

Definition:

Amines contain an amino group (–NH₂).

Suffix:

–amine

Examples:

Example 1:

CH₃NH₂

IUPAC Name: Methanamine

(Common name: Methylamine)

Example 2:

CH₃CH₂NH₂

IUPAC Name: Ethanamine

7. Quick Functional Group Table

Functional Group	Formula	Suffix	Example
Alcohol	–OH	–ol	Ethanol
Aldehyde	–CHO	–al	Ethanal
Ketone	>C=O	–one	Propanone
Acid	–COOH	–oic acid	Ethanoic acid
Amine	–NH₂	–amine	Ethanamine

Important Exam Points ⭐

✔ Functional group with highest priority gets the suffix.

✔ –COOH has the highest priority among common groups.

✔ Aldehyde and carboxylic acid groups are always at carbon number 1.

✔ Alcohol numbering is based on the position of –OH group.

✔ Ketone position must always be mentioned.

Quick Revision

Alcohol → –OH → ol
Aldehyde → –CHO → al
Ketone → >C=O → one
Carboxylic acid → –COOH → oic acid
Amine → –NH₂ → amine

Topic 11: Isomerism in Organic Compounds

Class 11 Chemistry Notes

Chapter 8: Organic Chemistry – Some Basic Principles and Techniques

Topic 11: Isomerism in Organic Compounds

1. Isomerism

Definition:

The phenomenon in which two or more organic compounds have the same molecular formula but different structures or arrangements of atoms is called isomerism.

Such compounds are called isomers.

Example:

Molecular formula: C₄H₁₀

It has two structures:

n-Butane
Isobutane (2-methylpropane)

Both have the same molecular formula but different structures.

2. Types of Isomerism

Isomerism is mainly divided into:

Structural Isomerism
Stereoisomerism

A. Structural Isomerism

Definition:

Structural isomerism occurs when compounds have the same molecular formula but different arrangement of atoms in the carbon skeleton or functional groups.

Types:

Chain isomerism
Position isomerism
Functional group isomerism
Metamerism

1. Chain Isomerism

Definition:

Isomers having the same molecular formula but different arrangements of the carbon chain are called chain isomers.

Example:

Formula: C₄H₁₀

n-Butane:

CH₃–CH₂–CH₂–CH₃

Isobutane:

CH₃–CH(CH₃)–CH₃

Difference:

Same formula
Different carbon skeleton

2. Position Isomerism

Definition:

Isomers having the same carbon skeleton and functional group but different positions of the functional group or multiple bond are called position isomers.

Example 1: Alcohols

Formula: C₃H₈O

Propan-1-ol:

CH₃–CH₂–CH₂OH

Propan-2-ol:

CH₃–CHOH–CH₃

Difference:

Position of –OH group changes.

Example 2: Alkenes

Formula: C₄H₈

But-1-ene:

CH₂=CH–CH₂–CH₃

But-2-ene:

CH₃–CH=CH–CH₃

Difference:

Position of double bond changes.

3. Functional Group Isomerism

Definition:

Isomers having the same molecular formula but different functional groups are called functional group isomers.

Example:

Formula: C₂H₆O

Ethanol (Alcohol)

CH₃CH₂OH

Functional group:
–OH

Dimethyl ether

CH₃OCH₃

Functional group:
Ether

4. Metamerism

Definition:

Metamerism occurs due to different alkyl groups attached on either side of a polyvalent functional group.

Common in compounds containing:

Ether (–O–)
Amine (–NH–)
Ketone (>C=O)

Example:

Formula: C₄H₁₀O

Ethoxyethane:

CH₃CH₂–O–CH₂CH₃

Methoxypropane:

CH₃–O–CH₂CH₂CH₃

Difference:

Different alkyl groups around oxygen.

5. Comparison of Structural Isomerism

Type	Difference	Example
Chain isomerism	Different carbon chain	Butane & Isobutane
Position isomerism	Different position of group/bond	Propan-1-ol & Propan-2-ol
Functional isomerism	Different functional groups	Ethanol & Ether
Metamerism	Different alkyl groups around functional group	Ethers

Important Exam Points ⭐

✔ Isomers have the same molecular formula.

✔ Structural isomers differ in the connectivity of atoms.

✔ Chain isomerism occurs due to different carbon skeletons.

✔ Position isomerism occurs due to different locations of groups.

✔ Functional isomerism occurs due to different functional groups.

✔ Metamerism is common in ethers and amines.

Quick Revision

Same formula + different structure = Isomerism
Structural isomerism:
- Chain
- Position
- Functional group
- Metamerism
Isomers have different physical and chemical properties.

Topic 12: Stereoisomerism (Basic Concepts)

1. Stereoisomerism

Definition:

Stereoisomerism is the type of isomerism in which compounds have:

Same molecular formula
Same sequence of bonded atoms
Different arrangement of atoms in three-dimensional space

2. Types of Stereoisomerism

Stereoisomerism is mainly divided into:

Geometrical Isomerism
Optical Isomerism

A. Geometrical Isomerism

Definition:

Geometrical isomerism arises due to restricted rotation around a double bond or within a ring structure.

It occurs because a double bond prevents free rotation.

3. Conditions for Geometrical Isomerism

A compound shows geometrical isomerism when:

✔ It contains a double bond (C=C) or cyclic structure.

✔ Each carbon of the double bond has two different groups attached.

Example: But-2-ene

Structure:

CH₃–CH=CH–CH₃

Each carbon of double bond has:

H atom
CH₃ group

Therefore, it shows geometrical isomerism.

4. Types of Geometrical Isomers

1. Cis Isomer

Meaning:

Similar groups are present on the same side of the double bond.

Example:

cis-but-2-ene

Characteristics:

Same groups are together.
Usually has lower symmetry.

2. Trans Isomer

Meaning:

Similar groups are present on opposite sides of the double bond.

Example:

trans-but-2-ene

Characteristics:

Groups are opposite.
Usually more symmetrical.

5. Difference Between Cis and Trans Isomers

Cis Isomer	Trans Isomer
Similar groups on same side	Similar groups on opposite sides
Less symmetrical	More symmetrical
Usually lower melting point	Usually higher melting point
Dipole moment is generally higher	Dipole moment may be lower

B. Optical Isomerism

Definition:

Optical isomerism occurs due to the presence of a chiral carbon atom and results in compounds that rotate plane-polarised light.

6. Chiral Carbon Atom

Definition:

A carbon atom attached to four different atoms or groups is called a chiral carbon or asymmetric carbon.

It is represented by:

Example:

Lactic acid:

CH₃–CH(OH)–COOH

The middle carbon is attached to:

H
OH
CH₃
COOH

Since all four groups are different, it is chiral.

7. Enantiomers

Definition:

Pairs of optical isomers that are non-superimposable mirror images of each other are called enantiomers.

Properties:

✔ Same physical properties (except optical rotation)

✔ Same chemical properties in normal conditions

✔ Rotate plane-polarised light in opposite directions

8. Optical Activity

Definition:

The ability of a compound to rotate plane-polarised light is called optical activity.

Types:

1. Dextrorotatory (+)

Rotates light towards the right side.

2. Laevorotatory (–)

Rotates light towards the left side.

9. Racemic Mixture

Definition:

A mixture containing equal amounts of two enantiomers is called a racemic mixture.

Property:

It is optically inactive because rotations cancel each other.

Comparison: Structural vs Stereoisomerism

Structural Isomerism	Stereoisomerism
Different connectivity of atoms	Same connectivity but different 3D arrangement
2D difference	3D difference
Example: Butane & Isobutane	Example: Cis & Trans but-2-ene

Important Exam Points ⭐

✔ Double bonds cause restricted rotation.

✔ Cis-trans isomerism is a type of geometrical isomerism.

✔ Chiral carbon has four different groups attached.

✔ Enantiomers are mirror images.

✔ Racemic mixture is optically inactive.

✔ Optical isomerism is related to rotation of plane-polarised light.

Quick Revision

Stereoisomers have the same bonding pattern but different 3D arrangements.
Two types:
- Geometrical isomerism
- Optical isomerism
Cis → same side
Trans → opposite side
Chiral carbon → four different groups
Enantiomers → non-superimposable mirror images

Topic 13: Electronic Effects in Organic Molecules

1. Electronic Effects

Definition:

Electronic effects are the changes in the distribution of electrons in organic molecules due to the presence of different atoms or groups.

These effects influence:

✔ Stability of molecules
✔ Reactivity of compounds
✔ Acidity and basicity
✔ Reaction mechanisms

Types of Electronic Effects

The main electronic effects are:

Inductive Effect
Resonance Effect (Mesomeric Effect)
Electromeric Effect
Hyperconjugation

2. Inductive Effect (I Effect)

Definition:

The permanent displacement of sigma (σ) electrons along a carbon chain due to the difference in electronegativity between atoms is called the inductive effect.

Features:

✔ Operates through σ bonds.

✔ It decreases with increasing distance.

✔ It is a permanent effect.

Types of Inductive Effect

There are two types:

A. –I Effect (Electron Withdrawing Effect)

Definition:

When a group attracts electrons towards itself, it shows a –I effect.

Examples:

Groups showing –I effect:

–NO₂
–CN
–COOH
–Cl
–F

Result:

Decreases electron density.
Stabilises negative charge.

B. +I Effect (Electron Releasing Effect)

Definition:

When a group releases electrons towards the carbon chain, it shows a +I effect.

Examples:

Alkyl groups:

–CH₃
–C₂H₅
–C₃H₇

Order of +I effect:

$(CH_3)_3C^- > (CH_3)_2CH^- > C_2H_5^- > CH_3^-$ (CH3)3C−>(CH3)2CH−>C2H5−>CH3−

(Generally, more substituted alkyl groups show stronger +I effect.)

3. Applications of Inductive Effect

A. Acidity of Carboxylic Acids

Electron withdrawing groups increase acidity.

Example:

ClCH₂COOH is more acidic than CH₃COOH.

Reason:

Cl shows –I effect and increases the stability of the conjugate base.

B. Stability of Carbocations

Electron releasing groups increase carbocation stability.

Order: $3^\circ > 2^\circ > 1^\circ > CH_3^+$ 3∘>2∘>1∘>CH3+

4. Resonance Effect (Mesomeric Effect)

Definition:

The delocalisation of π electrons or lone pair electrons in a molecule is called the resonance effect.

It occurs when electrons are spread over more than one atom.

Conditions for Resonance

✔ Presence of conjugated double bonds.

✔ Presence of lone pair adjacent to a double bond.

Examples of Resonance

Benzene

Benzene has:

Six carbon atoms
Delocalised π electrons

This gives benzene extra stability.

Types of Resonance Effect

A. +R Effect (Electron Donating Resonance)

Groups donate electrons through resonance.

Examples:

–OH
–NH₂
–OR

Effect:

✔ Increase electron density.

B. –R Effect (Electron Withdrawing Resonance)

Groups withdraw electrons through resonance.

Examples:

–NO₂
–CHO
–COOH

Effect:

✔ Decrease electron density.

5. Hyperconjugation

Definition:

Hyperconjugation is the delocalisation of σ electrons of a C–H bond adjacent to a double bond or positive charge.

It is also called “no bond resonance.”

Importance of Hyperconjugation

It helps in:

✔ Stabilisation of carbocations.

✔ Stability of alkenes.

✔ Explaining electron-releasing nature of alkyl groups.

Stability of Carbocations

Carbocations are carbon atoms with a positive charge.

Order of stability: $3^\circ > 2^\circ > 1^\circ > CH_3^+$ 3∘>2∘>1∘>CH3+

Reason:

More alkyl groups → more hyperconjugation → greater stability.

6. Comparison of Electronic Effects

Effect	Electron Movement	Bond Involved	Nature
Inductive Effect	Through σ bond	Single bond	Permanent
Resonance Effect	Delocalisation of π electrons	π system	Permanent
Electromeric Effect	Complete electron transfer	Multiple bond	Temporary
Hyperconjugation	σ electron delocalisation	C–H bond	Stabilising effect

Important Exam Points ⭐

✔ Inductive effect works through σ bonds.

✔ –I groups withdraw electrons.

✔ +I groups donate electrons.

✔ Resonance increases stability by electron delocalisation.

✔ Hyperconjugation stabilises carbocations and alkenes.

✔ Greater electron delocalisation = greater stability.

Quick Revision

Electronic effects control the behaviour of organic compounds.
Inductive effect: movement of σ electrons.
Resonance: spreading of π electrons.
Hyperconjugation: delocalisation of C–H electrons.
These effects determine stability and reactivity.

Topic 14: Reaction Intermediates – Carbocations, Carbanions and Free Radicals

1. Reaction Intermediates

Definition:

Reaction intermediates are short-lived, unstable species formed during the conversion of reactants into products.

They are usually formed by:

Breaking of covalent bonds
Formation of new bonds

Important Reaction Intermediates:

Carbocations
Carbanions
Free radicals

2. Carbocations

Definition:

A carbocation is an organic ion in which a carbon atom carries a positive charge (+).

General Formula:

$R_3C^+$ R3C+

Structure of Carbocation

Characteristics:

✔ Carbon has only six electrons around it.

✔ It is electron-deficient.

✔ Carbon atom is generally sp² hybridised.

✔ Shape is trigonal planar.

✔ Bond angle is approximately 120°.

Types of Carbocations

Carbocations are classified according to the number of carbon groups attached to the positively charged carbon.

1. Primary Carbocation (1°)

Positive carbon attached to one carbon atom.

Example: $CH_3-CH_2^+$ CH3−CH2+

2. Secondary Carbocation (2°)

Positive carbon attached to two carbon atoms.

Example: $(CH_3)_2CH^+$ (CH3)2CH+

3. Tertiary Carbocation (3°)

Positive carbon attached to three carbon atoms.

Example: $(CH_3)_3C^+$ (CH3)3C+

Stability Order of Carbocations

$3^\circ > 2^\circ > 1^\circ > CH_3^+$ 3∘>2∘>1∘>CH3+

Reason:

Alkyl groups show +I effect.
Alkyl groups increase electron density.
Hyperconjugation stabilises positive charge.

3. Carbanions

Definition:

A carbanion is an organic ion in which a carbon atom carries a negative charge (–).

General Formula:

$R_3C^-$ R3C−

Structure of Carbanion

Characteristics:

✔ Carbon has a lone pair of electrons.

✔ Carbon is electron-rich.

✔ Usually sp³ hybridised.

✔ Shape is generally pyramidal.

Types of Carbanions

1. Primary Carbanion (1°)

Negative carbon attached to one carbon atom.

Example: $CH_3CH_2^-$ CH3CH2−

2. Secondary Carbanion (2°)

Negative carbon attached to two carbon atoms.

3. Tertiary Carbanion (3°)

Negative carbon attached to three carbon atoms.

Stability Order of Carbanions

$CH_3^- > 1^\circ > 2^\circ > 3^\circ$ CH3−>1∘>2∘>3∘

Reason:

Alkyl groups increase electron density.
Negative charge becomes less stable with more alkyl groups.

4. Free Radicals

Definition:

Free radicals are neutral species containing an unpaired electron.

General Formula:

$R_3C•$ R3C•

(The dot represents an unpaired electron.)

Formation of Free Radicals

They are formed by homolytic bond cleavage.

Example: $Cl_2 \rightarrow Cl• + Cl•$ Cl2→Cl•+Cl•

Structure of Free Radicals

Characteristics:

✔ Neutral species.

✔ Highly reactive.

✔ Carbon is generally sp² hybridised.

✔ Shape is approximately planar.

Stability Order of Free Radicals

$3^\circ > 2^\circ > 1^\circ > CH_3•$ 3∘>2∘>1∘>CH3•

Reason:

Alkyl groups stabilise radicals through:
- +I effect
- Hyperconjugation

5. Comparison of Reaction Intermediates

Property	Carbocation	Carbanion	Free Radical
Charge	Positive (+)	Negative (–)	No charge
Electron nature	Electron deficient	Electron rich	Unpaired electron
Hybridisation	sp²	sp³	sp²
Shape	Planar	Pyramidal	Nearly planar
Stability order	3° > 2° > 1°	CH₃⁻ > 1° > 2° > 3°	3° > 2° > 1°

Important Exam Points ⭐

✔ Carbocations are electron-deficient species.

✔ Carbanions contain a lone pair of electrons.

✔ Free radicals contain an unpaired electron.

✔ Carbocation stability increases with alkyl groups.

✔ Carbanion stability decreases with alkyl groups.

✔ Hyperconjugation plays an important role in stabilising carbocations and radicals.

Quick Revision

Carbocation → C⁺ → Electron deficient
Carbanion → C⁻ → Electron rich
Free radical → C• → Unpaired electron
Carbocation stability:
3° > 2° > 1°
Carbanion stability:
CH₃⁻ > 1° > 2° > 3°
Radical stability:
3° > 2° > 1°

Topic 15: Types of Organic Reactions

1. Organic Reactions

Definition:

The reactions in which organic compounds are converted into new organic compounds are called organic reactions.

Organic reactions involve:

Breaking of existing bonds
Formation of new bonds

2. Classification of Organic Reactions

The major types of organic reactions are:

Substitution Reaction
Addition Reaction
Elimination Reaction
Rearrangement Reaction

1. Substitution Reaction

Definition:

A reaction in which an atom or group of atoms in a molecule is replaced by another atom or group is called a substitution reaction.

General Form:

$R-X + Y \rightarrow R-Y + X$ R−X+Y→R−Y+X

Where:

R = carbon chain
X = leaving group
Y = incoming group

Example: Chlorination of Methane

$CH_4 + Cl_2 \xrightarrow{hv} CH_3Cl + HCl$ CH4+Cl2hvCH3Cl+HCl

Hydrogen atom is replaced by chlorine.

Types of Substitution Reactions

A. Nucleophilic Substitution Reaction

Definition:

A reaction in which a nucleophile replaces a leaving group is called nucleophilic substitution.

Example:

$CH_3Br + OH^- \rightarrow CH_3OH + Br^-$ CH3Br+OH−→CH3OH+Br−

Here:

OH⁻ = Nucleophile
Br⁻ = Leaving group

B. Electrophilic Substitution Reaction

Definition:

A reaction in which an electrophile replaces an atom or group is called electrophilic substitution.

Example:

Nitration of benzene:

Benzene + HNO₃ → Nitrobenzene

2. Addition Reaction

Definition:

A reaction in which two or more molecules combine to form a single product is called an addition reaction.

It generally occurs in unsaturated compounds.

General Form:

$A+B \rightarrow AB$ A+B→AB

Example:

Addition of hydrogen to ethene: $CH_2=CH_2 + H_2 \rightarrow CH_3-CH_3$ CH2=CH2+H2→CH3−CH3

Ethene → Ethane

Types of Addition Reactions

A. Electrophilic Addition

Electrophile attacks the electron-rich double bond.
Common in alkenes.

Example:

Addition of HBr to ethene.

B. Nucleophilic Addition

Nucleophile attacks electron-deficient carbon atoms.
Common in aldehydes and ketones.

3. Elimination Reaction

Definition:

A reaction in which atoms or groups are removed from a molecule, resulting in formation of a double or triple bond, is called an elimination reaction.

General Form:

$AB \rightarrow A+B$ AB→A+B

Example:

Dehydration of ethanol: $CH_3CH_2OH \rightarrow CH_2=CH_2 + H_2O$ CH3CH2OH→CH2=CH2+H2O

Ethanol gives ethene by removal of water.

4. Rearrangement Reaction

Definition:

A reaction in which the arrangement of atoms within a molecule changes to form a new structure is called a rearrangement reaction.

Features:

✔ Carbon skeleton changes.

✔ New isomeric structure is formed.

✔ Usually involves movement of atoms or groups.

5. Comparison of Organic Reactions

Reaction Type	Main Change	Example
Substitution	Replacement of atom/group	CH₄ + Cl₂ → CH₃Cl
Addition	Addition across double/triple bond	Ethene + H₂ → Ethane
Elimination	Removal of atoms/groups	Ethanol → Ethene
Rearrangement	Change in structure	Formation of new isomer

6. Important Terms

Nucleophile

Definition:

An electron-rich species that donates an electron pair is called a nucleophile.

Examples:

OH⁻
CN⁻
NH₃

Electrophile

Definition:

An electron-deficient species that accepts an electron pair is called an electrophile.

Examples:

H⁺
NO₂⁺
BF₃

Important Exam Points ⭐

✔ Substitution replaces one atom/group with another.

✔ Addition reactions occur mainly in unsaturated compounds.

✔ Elimination reactions usually produce double bonds.

✔ Nucleophiles attack electron-deficient centres.

✔ Electrophiles attack electron-rich centres.

Quick Revision

Substitution → Replacement
Addition → Combination
Elimination → Removal
Rearrangement → Change in carbon skeleton
Nucleophile = electron donor
Electrophile = electron acceptor

Topic 16: Purification of Organic Compounds

1. Purification of Organic Compounds

Definition:

The process of removing impurities from an organic compound to obtain a pure substance is called purification.

Organic compounds obtained from natural sources or chemical reactions usually contain impurities, so purification is necessary.

2. Methods of Purification

The important methods are:

Crystallisation
Sublimation
Distillation
Differential Extraction
Chromatography

1. Crystallisation

Definition:

Crystallisation is a method used to purify solid organic compounds based on the difference in solubility of a compound and its impurities in a suitable solvent.

Principle:

A pure compound forms crystals when its hot saturated solution is cooled.

Steps:

Step 1:

Dissolve impure solid in a suitable hot solvent.

Step 2:

Filter the hot solution to remove insoluble impurities.

Step 3:

Cool the filtrate.

Step 4:

Pure crystals separate out.

Step 5:

Dry the crystals.

Characteristics of a Good Solvent:

✔ Compound should dissolve well at high temperature.

✔ Compound should dissolve poorly at low temperature.

✔ Impurities should either dissolve completely or remain insoluble.

✔ Solvent should not react with the compound.

Examples:

Benzoic acid can be purified by crystallisation.

2. Sublimation

Definition:

Sublimation is the process in which a solid changes directly into vapour without passing through the liquid state.

Principle:

Some organic solids have the ability to sublime, while impurities do not.

Examples of Sublimable Compounds:

Camphor
Naphthalene
Anthracene

Advantages:

✔ Simple method.

✔ Gives pure solid compounds.

3. Distillation

Definition:

Distillation is a purification method based on the difference in boiling points of liquids.

Principle:

A liquid with a lower boiling point vaporises first and is collected separately.

Types of Distillation

A. Simple Distillation

Used when:

✔ Difference in boiling points is large.

✔ Liquid does not decompose on heating.

Example:

Separation of:

Benzene and toluene

B. Fractional Distillation

Used when:

✔ Boiling points of liquids are close.

✔ A fractionating column is used.

Example:

Separation of:

Different fractions of petroleum

C. Distillation Under Reduced Pressure

Definition:

Distillation carried out at pressure lower than atmospheric pressure is called vacuum distillation.

Used for:

✔ Liquids having very high boiling points.

✔ Compounds that decompose at their boiling point.

Example:

Purification of glycerol.

4. Differential Extraction

Definition:

The process of separating an organic compound from a mixture using a suitable solvent is called extraction.

Principle:

A compound dissolves differently in different solvents.

Steps:

Mixture is treated with a suitable solvent.
Desired compound dissolves.
Solvent layer is separated.
Compound is recovered.

5. Chromatography

Definition:

Chromatography is a technique used for separation and purification of compounds based on their different adsorption or distribution properties.

Principle:

Different components move at different rates through a stationary phase.

Types of Chromatography

A. Adsorption Chromatography

Based on:

Different adsorption abilities of substances.

Examples:

Column chromatography
Thin Layer Chromatography (TLC)

B. Partition Chromatography

Based on:

Different distribution of substances between two phases.

Example:

Paper chromatography

6. Thin Layer Chromatography (TLC)

Principle:

Different compounds travel different distances on a thin layer of adsorbent.

Rf Value:

The movement of a compound is expressed as: $Rf = \frac{\text{Distance travelled by compound}}{\text{Distance travelled by solvent}}$ Rf=Distance travelled by solventDistance travelled by compound

Comparison of Purification Methods

Method	Used For	Principle
Crystallisation	Solid compounds	Difference in solubility
Sublimation	Sublimable solids	Direct solid → vapour
Distillation	Liquids	Difference in boiling point
Extraction	Soluble compounds	Difference in solubility
Chromatography	Mixtures	Different adsorption/distribution

Important Exam Points ⭐

✔ Crystallisation is mainly used for purification of solids.

✔ Distillation separates liquids based on boiling points.

✔ Sublimation is used for compounds like camphor and naphthalene.

✔ Chromatography is used for separation of complex mixtures.

✔ TLC uses Rf value for identification.

Quick Revision

Purification removes impurities from organic compounds.
Crystallisation → solubility difference.
Sublimation → solid directly changes to vapour.
Distillation → boiling point difference.
Extraction → solvent separation.
Chromatography → adsorption/distribution difference.

Topic 17: Qualitative Analysis of Organic Compounds

1. Qualitative Analysis

Definition:

Qualitative analysis is the process of detecting the elements present in an organic compound.

Organic compounds mainly contain:

Carbon (C)
Hydrogen (H)
Nitrogen (N)
Sulphur (S)
Halogens (Cl, Br, I)

2. Detection of Carbon and Hydrogen

Principle:

Carbon and hydrogen are detected by heating the organic compound with copper(II) oxide (CuO).

During heating:

Carbon is oxidised to carbon dioxide (CO₂).
Hydrogen is oxidised to water (H₂O).

Tests:

Test for Carbon:

CO₂ produced is passed through lime water.

Reaction: $CO_2 + Ca(OH)_2 \rightarrow CaCO_3 + H_2O$ CO2+Ca(OH)2→CaCO3+H2O

Observation:

✔ Lime water turns milky.

Test for Hydrogen:

Water formed is tested using:

Anhydrous copper sulphate

Observation:

✔ White copper sulphate turns blue.

3. Detection of Nitrogen

Principle:

Nitrogen is detected by converting it into sodium cyanide (NaCN) by heating with sodium metal.

The sodium fusion extract is called Lassaigne’s extract.

Lassaigne’s Test

Steps:

Organic compound is heated with sodium.
Sodium converts nitrogen into sodium cyanide.

Reaction: $Na + C + N \rightarrow NaCN$ Na+C+N→NaCN

NaCN reacts with ferrous sulphate to form sodium ferrocyanide.
On oxidation, Prussian blue colour is obtained.

Observation:

✔ Blue colour indicates presence of nitrogen.

4. Detection of Sulphur

Principle:

Sulphur is converted into sodium sulphide (Na₂S) during sodium fusion.

Reaction: $2Na + S \rightarrow Na_2S$ 2Na+S→Na2S

Test 1: Sodium Nitroprusside Test

Observation:

✔ Violet colour indicates sulphur.

Test 2: Lead Acetate Test

Reaction: $Na_2S + Pb(CH_3COO)_2 \rightarrow PbS + 2CH_3COONa$ Na2S+Pb(CH3COO)2→PbS+2CH3COONa

Observation:

✔ Black precipitate of lead sulphide indicates sulphur.

5. Detection of Halogens

Halogens detected in organic compounds are:

Chlorine (Cl)
Bromine (Br)
Iodine (I)

Principle:

Halogens are converted into sodium halides during sodium fusion.

Examples: $Na + Cl \rightarrow NaCl$ Na+Cl→NaCl

Silver Nitrate Test

Steps:

Acidify Lassaigne’s extract with nitric acid.
Add silver nitrate solution.

Observations:

Halogen	Precipitate	Colour
Chlorine	AgCl	White
Bromine	AgBr	Pale yellow
Iodine	AgI	Yellow

6. Lassaigne’s Test Summary

Element	Compound Formed	Test Result
Nitrogen	NaCN	Prussian blue colour
Sulphur	Na₂S	Violet colour / Black precipitate
Halogen	NaX	Silver halide precipitate

7. Importance of Qualitative Analysis

✔ Helps identify elements present in organic compounds.

✔ Useful in determining the structure of unknown compounds.

✔ Forms the basis of organic compound identification.

Important Exam Points ⭐

✔ Carbon and hydrogen are detected by oxidation with CuO.

✔ CO₂ turns lime water milky.

✔ Water turns anhydrous CuSO₄ blue.

✔ Lassaigne’s extract is prepared using sodium fusion.

✔ Nitrogen gives Prussian blue colour.

✔ Halogens give precipitates with AgNO₃.

Quick Revision

Qualitative analysis identifies elements in organic compounds.
C and H → CO₂ and H₂O tests.
N → Prussian blue test.
S → Violet/black precipitate tests.
Halogens → Silver nitrate test.
Sodium fusion test is used for N, S and halogens.

Topic 18: Quantitative Analysis of Organic Compounds

1. Quantitative Analysis

Definition:

Quantitative analysis is the process of determining the amount or percentage of different elements present in an organic compound.

It helps to find the empirical formula and molecular formula of a compound.

2. Estimation of Carbon and Hydrogen

Principle:

A known mass of organic compound is heated with excess oxygen in the presence of copper oxide (CuO).

Carbon and hydrogen are converted into:

Carbon → Carbon dioxide (CO₂)
Hydrogen → Water (H₂O)

Reactions:

Formation of CO₂:

$C + O_2 \rightarrow CO_2$ C+O2→CO2

Formation of H₂O:

$2H_2 + O_2 \rightarrow 2H_2O$ 2H2+O2→2H2O

Calculation of Percentage of Carbon

The amount of carbon is calculated from the amount of CO₂ formed.

Formula: $\%C = \frac{12}{44} \times \frac{\text{Mass of } CO_2}{\text{Mass of compound}} \times 100$ %C=4412×Mass of compoundMass of CO2×100

Calculation of Percentage of Hydrogen

Hydrogen is calculated from the amount of water formed.

Formula: $\%H = \frac{2}{18} \times \frac{\text{Mass of } H_2O}{\text{Mass of compound}} \times 100$ %H=182×Mass of compoundMass of H2O×100

3. Estimation of Nitrogen

Nitrogen is estimated by two methods:

Dumas Method
Kjeldahl Method

A. Dumas Method

Principle:

Organic compound is heated with copper oxide.

Nitrogen is converted into nitrogen gas (N₂).

The volume of nitrogen gas is measured.

Reaction:

$2N + 3CuO \rightarrow N_2 + 3Cu$ 2N+3CuO→N2+3Cu

Calculation:

$\%N = \frac{28 \times \text{Volume of }N_2}{22400 \times \text{Mass of compound}} \times 100$ %N=22400×Mass of compound28×Volume of N2×100

B. Kjeldahl Method

Principle:

Nitrogen is converted into ammonium sulphate by heating the compound with concentrated sulphuric acid.

The ammonia produced is measured.

Not applicable for:

Nitro compounds
Azo compounds
Compounds containing nitrogen in ring structures

4. Estimation of Halogens (Carius Method)

Principle:

Organic compound is heated with fuming nitric acid in the presence of silver nitrate.

Halogens are converted into silver halides.

Reactions:

For chlorine: $Cl + Ag \rightarrow AgCl$ Cl+Ag→AgCl

For bromine: $Br + Ag \rightarrow AgBr$ Br+Ag→AgBr

Formula:

$\%X = \frac{\text{Atomic mass of halogen} \times \text{Mass of silver halide}}{\text{Molecular mass of silver halide} \times \text{Mass of compound}} \times 100$ %X=Molecular mass of silver halide×Mass of compoundAtomic mass of halogen×Mass of silver halide×100

5. Estimation of Sulphur

Principle:

Sulphur is oxidised to sulphuric acid, which is precipitated as barium sulphate.

Reaction: $H_2SO_4 + BaCl_2 \rightarrow BaSO_4 + 2HCl$ H2SO4+BaCl2→BaSO4+2HCl

Calculation:

$\%S = \frac{32}{233} \times \frac{\text{Mass of }BaSO_4}{\text{Mass of compound}} \times 100$ %S=23332×Mass of compoundMass of BaSO4×100

6. Estimation of Phosphorus

Principle:

Phosphorus is oxidised to phosphoric acid and precipitated as magnesium pyrophosphate.

Reaction: $2MgHPO_4 \rightarrow Mg_2P_2O_7 + H_2O$ 2MgHPO4→Mg2P2O7+H2O

7. Importance of Quantitative Analysis

✔ Determines percentage composition of elements.

✔ Helps in finding empirical formula.

✔ Helps identify unknown organic compounds.

✔ Used in chemical research and industries.

8. Qualitative vs Quantitative Analysis

Qualitative Analysis	Quantitative Analysis
Detects elements present	Determines amount of elements
Gives presence/absence information	Gives percentage composition
Uses colour tests and reactions	Uses measurements and calculations

Important Exam Points ⭐

✔ Quantitative analysis determines the percentage of elements.

✔ Carbon and hydrogen are estimated using combustion method.

✔ Nitrogen can be estimated by Dumas and Kjeldahl methods.

✔ Halogens are estimated by Carius method.

✔ Sulphur is estimated as barium sulphate (BaSO₄).

Quick Revision

Quantitative analysis → amount of elements.
C and H → combustion method.
N → Dumas/Kjeldahl method.
Halogens → Carius method.
Sulphur → BaSO₄ precipitation method.
Results help determine molecular formula.

Topic 19: Chapter Quick Revision Sheet + Important Questions

A. Complete Chapter Revision

1. Organic Chemistry

Study of carbon-containing compounds is called organic chemistry.
Carbon shows:
- Tetravalency → forms four covalent bonds.
- Catenation → forms long chains and rings.
Wöhler synthesized urea in 1828 and disproved the vital force theory.

2. Hybridisation of Carbon

Hybridisation	Shape	Bond Angle	Example
sp³	Tetrahedral	109.5°	CH₄
sp²	Trigonal planar	120°	C₂H₄
sp	Linear	180°	C₂H₂

Order of s-character:

$sp > sp^2 > sp^3$ sp>sp2>sp3

3. Sigma (σ) and Pi (π) Bonds

σ Bond	π Bond
Head-on overlap	Sidewise overlap
Stronger	Weaker
Allows rotation	Restricts rotation
Present in all bonds	Present in multiple bonds

Bond Types:

Single bond = 1 σ
Double bond = 1 σ + 1 π
Triple bond = 1 σ + 2 π

4. Classification of Organic Compounds

Acyclic Compounds

Open-chain compounds.
Example: Ethane

Cyclic Compounds

Ring structures.

Types:

Alicyclic → Cyclohexane
Aromatic → Benzene
Heterocyclic → Pyridine

5. Functional Groups

Functional groups decide chemical properties.

Group	Name
–OH	Alcohol
–CHO	Aldehyde
>C=O	Ketone
–COOH	Carboxylic acid
–NH₂	Amine
–X	Halo compound

6. Homologous Series

Characteristics:

✔ Same functional group

✔ Same general formula

✔ Successive members differ by –CH₂–

Example:

Methane → Ethane → Propane

7. IUPAC Naming

General pattern:

Prefix + Root word + Suffix

Example:

2-methylpropane

2-methyl = Prefix
prop = Root
ane = Suffix

Root Words

Carbon atoms	Root
1	Meth
2	Eth
3	Prop
4	But
5	Pent
6	Hex
7	Hept
8	Oct
9	Non
10	Dec

8. Isomerism

Same molecular formula but different structures = Isomerism

Structural Isomerism:

Chain isomerism
Position isomerism
Functional group isomerism
Metamerism

Stereoisomerism:

Geometrical isomerism
Optical isomerism

9. Electronic Effects

Inductive Effect

Movement of electrons through σ bonds.
Types:
- +I → Electron donating
- –I → Electron withdrawing

Resonance

Delocalisation of π electrons.
Increases stability.

Hyperconjugation

Delocalisation of C–H electrons.
Stabilises carbocations.

10. Reaction Intermediates

Carbocation (C⁺)

Positive charge
Electron deficient

Stability: $3^\circ > 2^\circ > 1^\circ$ 3∘>2∘>1∘

Carbanion (C⁻)

Negative charge
Electron rich

Stability: $CH_3^- > 1^\circ > 2^\circ > 3^\circ$ CH3−>1∘>2∘>3∘

Free Radical (C•)

Contains unpaired electron

Stability: $3^\circ > 2^\circ > 1^\circ$ 3∘>2∘>1∘

11. Types of Organic Reactions

Reaction	Meaning
Substitution	Replacement of atom/group
Addition	Addition across multiple bond
Elimination	Removal of atoms/groups
Rearrangement	Change in structure

12. Purification Methods

Method	Principle
Crystallisation	Difference in solubility
Sublimation	Solid → vapour
Distillation	Difference in boiling point
Extraction	Difference in solubility
Chromatography	Adsorption/distribution

13. Qualitative Analysis

Detects elements present in organic compounds.

Element	Test
Carbon	CO₂ turns lime water milky
Hydrogen	Water turns CuSO₄ blue
Nitrogen	Prussian blue colour
Sulphur	Violet/black colour test
Halogens	AgNO₃ test

14. Quantitative Analysis

Determines percentage of elements.

Element	Method
Carbon & Hydrogen	Combustion method
Nitrogen	Dumas/Kjeldahl method
Halogens	Carius method
Sulphur	BaSO₄ precipitation

B. Important Exam Questions

Very Short Answer Questions

1. What is catenation?

Answer: The ability of carbon atoms to form bonds with other carbon atoms is called catenation.

2. What is a functional group?

Answer: An atom or group of atoms responsible for the characteristic reactions of organic compounds.

3. What is a homologous series?

Answer: A series of organic compounds having the same functional group and successive members differing by –CH₂–.

4. What is a chiral carbon?

Answer: A carbon atom attached to four different groups.

5. What is a nucleophile?

Answer: An electron-rich species that donates an electron pair.

Long Answer Questions

1. Explain different types of hybridisation in carbon.

Points to write:

Definition of hybridisation
sp³, sp², sp
Shape and bond angles
Examples

2. Explain electronic effects.

Include:

Inductive effect
Resonance effect
Hyperconjugation
Applications

3. Explain methods of purification.

Include:

Crystallisation
Distillation
Sublimation
Chromatography

Final One-Day Revision Points ⭐

✔ Carbon is tetravalent and shows catenation.
✔ sp³ = tetrahedral, sp² = planar, sp = linear.
✔ Double bond contains one σ and one π bond.
✔ Functional groups decide chemical properties.
✔ IUPAC naming follows prefix + root + suffix.
✔ Isomers have the same molecular formula but different structures.
✔ Carbocation stability: 3° > 2° > 1°.
✔ Carbanion stability: CH₃⁻ > 1° > 2° > 3°.
✔ Purification removes impurities.
✔ Quantitative analysis gives percentage composition.