Class 11 Chemistry Notes

Chapter 9: Hydrocarbons

Part 1 — Introduction, Classification and Alkanes

1. Introduction to Hydrocarbons

Definition

Hydrocarbons are organic compounds that contain only two elements:

Carbon (C)
Hydrogen (H)

Examples:

Methane → CH₄
Ethane → C₂H₆
Ethene → C₂H₄
Benzene → C₆H₆

Hydrocarbons are important because they are:

Major sources of energy
Raw materials for industries
Used in the production of polymers, dyes, drugs and solvents

Common hydrocarbon-based fuels:

LPG (Liquefied Petroleum Gas)
CNG (Compressed Natural Gas)
Petrol
Diesel
Kerosene

Hydrocarbons are present mainly in:

Petroleum
Natural gas
Coal

2. Classification of Hydrocarbons

Hydrocarbons are classified based on the type of carbon-carbon bonds present.

A. Saturated Hydrocarbons

Meaning:

Hydrocarbons containing only single bonds (C–C) are called saturated hydrocarbons.

They are also called alkanes.

General formula: $C_nH_{2n+2}$ CnH2n+2

Examples:

Compound	Formula
Methane	CH₄
Ethane	C₂H₆
Propane	C₃H₈
Butane	C₄H₁₀

B. Unsaturated Hydrocarbons

Hydrocarbons containing multiple bonds are called unsaturated hydrocarbons.

They are of two types:

1. Alkenes

Contain at least one double bond (C=C).

General formula: $C_nH_{2n}$ CnH2n

Example:

Ethene:

CH₂ = CH₂

2. Alkynes

Contain at least one triple bond (C≡C).

General formula: $C_nH_{2n-2}$ CnH2n−2

Example:

Ethyne:

CH≡CH

C. Aromatic Hydrocarbons

These are special cyclic hydrocarbons containing aromatic rings.

Example:

Benzene:

C₆H₆

3. Alkanes

Definition

Alkanes are saturated open-chain hydrocarbons containing only carbon-carbon single bonds.

The first member of the alkane family is:

Methane (CH₄)

Other members are formed by replacing a hydrogen atom with a –CH₃ group.

Examples:

Methane → CH₄
Ethane → C₂H₆
Propane → C₃H₈
Butane → C₄H₁₀

General formula: $\boxed{C_nH_{2n+2}}$ CnH2n+2

4. Structure of Alkanes

Carbon in alkanes is sp³ hybridised.

Important features:

Carbon has tetrahedral geometry.
Bond angle = 109.5°
C–C bond length ≈ 154 pm
C–H bond length ≈ 112 pm

Methane structure:

The four hydrogen atoms occupy the corners of a tetrahedron.

5. Homologous Series of Alkanes

A homologous series is a group of compounds having:

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

Example:

Name	Formula
Methane	CH₄
Ethane	C₂H₆
Propane	C₃H₈
Butane	C₄H₁₀

Difference between consecutive members:

C₂H₆ – CH₄ = CH₂

6. Nomenclature of Alkanes (IUPAC Rules)

Step 1: Select the longest carbon chain

The longest chain is the parent hydrocarbon.

Examples:

5 carbons → Pentane
6 carbons → Hexane

Step 2: Number the carbon chain

Number from the side nearest to the substituent.

Example:

Wrong numbering:

CH₃–CH₂–CH(CH₃)–CH₃

Correct:

CH₃–CH(CH₃)–CH₂–CH₃

Name:

2-Methylbutane

Step 3: Name the substituents

Common alkyl groups:

Alkane	Alkyl group
Methane	Methyl (–CH₃)
Ethane	Ethyl (–C₂H₅)
Propane	Propyl (–C₃H₇)

General formula of alkyl group: $C_nH_{2n+1}$ CnH2n+1

7. Isomerism in Alkanes

Definition

Compounds having the same molecular formula but different structures are called isomers.

This phenomenon is called isomerism.

Chain Isomerism

When compounds differ due to different arrangements of carbon chains.

Example:

Molecular formula: $C_4H_{10}$ C4H10

Two structures:

1. n-Butane

CH₃–CH₂–CH₂–CH₃

2. Isobutane

(CH₃)₃CH

Both have the same formula but different structures.

8. Types of Carbon Atoms in Alkanes

Carbon atoms are classified according to the number of carbon atoms attached to them.

Primary Carbon (1°)

Attached to only one other carbon atom.

Example:

CH₃–

Secondary Carbon (2°)

Attached to two carbon atoms.

Example:

–CH₂–

Tertiary Carbon (3°)

Attached to three carbon atoms.

Example:

     CH3
      |
CH3— C —H
      |
    CH3

Quaternary Carbon (4°)

Attached to four carbon atoms.

Example:

      CH3
       |
CH3 — C — CH3
       |
      CH3

9. Preparation of Alkanes

Method 1: Hydrogenation of Unsaturated Hydrocarbons

Alkenes and alkynes react with hydrogen in presence of catalysts:

Catalysts:

Nickel (Ni)
Platinum (Pt)
Palladium (Pd)

Example: $CH_2=CH_2 + H_2 \rightarrow CH_3-CH_3$ CH2=CH2+H2→CH3−CH3

(Ethene → Ethane)

This process is called hydrogenation.

Method 2: From Alkyl Halides

(a) Reduction Method

Alkyl halides react with zinc and dilute HCl to form alkanes.

Example: $CH_3Cl + H_2 \rightarrow CH_4 + HCl$ CH3Cl+H2→CH4+HCl

(b) Wurtz Reaction

Two alkyl halide molecules react with sodium metal in dry ether.

General reaction: $2R-X + 2Na \rightarrow R-R + 2NaX$ 2R−X+2Na→R−R+2NaX

Used for preparing higher alkanes.

Method 3: Decarboxylation

Sodium salts of carboxylic acids react with soda lime to produce alkanes.

Important point:

Number of carbon atoms decreases by one.

Example:

Sodium acetate gives methane: $CH_3COONa \rightarrow CH_4$ CH3COONa→CH4

10. Physical Properties of Alkanes

1. Nature

Alkanes are non-polar.
They have weak van der Waals forces.

2. Physical State

At room temperature:

Carbon atoms	State
C₁–C₄	Gases
C₅–C₁₇	Liquids
C₁₈ onwards	Solids

3. Solubility

Insoluble in water
Soluble in non-polar solvents

Rule:

Like dissolves like

11. Chemical Properties of Alkanes

1. Substitution Reaction

Hydrogen atoms of alkanes are replaced by other atoms/groups.

Example:

Chlorination of methane: $CH_4+Cl_2 \xrightarrow{hv} CH_3Cl+HCl$ CH4+Cl2hvCH3Cl+HCl

Products formed stepwise:

Methane → Chloromethane → Dichloromethane → Chloroform → Carbon tetrachloride

2. Combustion

Alkanes burn in oxygen to produce:

Carbon dioxide
Water
Heat energy

Example: $CH_4+2O_2 \rightarrow CO_2+2H_2O+\text{heat}$ CH4+2O2→CO2+2H2O+heat

Because of high heat production, alkanes are used as fuels.

Quick Revision Box

Alkanes:

Saturated hydrocarbons
Only C–C single bonds
Formula: CₙH₂ₙ₊₂
Carbon is sp³ hybridised
First member: Methane
Main reaction: Substitution
Used as fuels

Part 2 — Alkenes

1. Introduction to Alkenes

Definition

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

They are also called olefins because ethene reacts with chlorine to form oily products.

General formula: $\boxed{C_nH_{2n}}$ CnH2n

Examples:

Alkene	Formula
Ethene	C₂H₄
Propene	C₃H₆
Butene	C₄H₈

The first stable member of the alkene family is:

Ethene (C₂H₄)

2. Structure of Double Bond (C=C)

A carbon-carbon double bond consists of:

1. Sigma (σ) bond

Formed by head-on overlapping of sp² hybrid orbitals
Stronger bond

2. Pi (π) bond

Formed by sideways overlapping of p-orbitals
Weaker than sigma bond

Structure:

Important points:

Carbon atoms in alkenes are sp² hybridised
Geometry is trigonal planar
Bond angle ≈ 120°
Rotation around C=C bond is restricted because of the π bond

3. Nomenclature of Alkenes (IUPAC Rules)

Rule 1: Select the longest carbon chain containing the double bond

Example:

CH₃–CH=CH₂

Number of carbons = 3

Name:

Propene

Rule 2: Number the chain from the side nearest to the double bond

Example:

CH₃–CH₂–CH=CH₂

Correct numbering:

CH₂=CH–CH₂–CH₃

Double bond gets lowest number.

Name:

But-1-ene

Rule 3: Replace suffix “-ane” with “-ene”

Examples:

Alkane	Alkene
Ethane	Ethene
Propane	Propene
Butane	Butene

Examples of Alkene Names

Structure	IUPAC Name
CH₂=CH₂	Ethene
CH₃–CH=CH₂	Propene
CH₃–CH=CH–CH₃	But-2-ene
CH₂=CH–CH=CH₂	Buta-1,3-diene

4. Isomerism in Alkenes

Alkenes show two main types of isomerism:

Structural isomerism
Geometrical isomerism

A. Structural Isomerism

Same molecular formula but different arrangement of atoms.

Types:

1. Chain Isomerism

Different carbon skeleton.

Example:

Formula: C₄H₈

But-1-ene

CH₂=CH–CH₂–CH₃

2-Methylprop-1-ene

      CH3
       |
CH2 = C — CH3

They have the same molecular formula but different carbon chains.

2. Position Isomerism

Different position of double bond.

Example:

But-1-ene

CH₂=CH–CH₂–CH₃

But-2-ene

CH₃–CH=CH–CH₃

The position of C=C bond changes.

B. Geometrical Isomerism (Cis-Trans Isomerism)

Cause

Restricted rotation around the C=C bond produces different arrangements in space.

For geometrical isomerism:

Each carbon of double bond must have two different groups attached.

General condition:

Types:

1. Cis Isomer

Similar groups are on the same side.

Example:

cis-but-2-ene

 CH3     CH3
   \     /
    C = C
   /     \
  H       H

2. Trans Isomer

Similar groups are on opposite sides.

Example:

trans-but-2-ene

 CH3      H
   \      /
    C == C
   /      \
  H       CH3

Difference between Cis and Trans Forms

Cis	Trans
Groups on same side	Groups on opposite sides
More polar	Usually less polar
Higher dipole moment	Lower dipole moment

Example:

cis-but-2-ene has dipole moment ≈ 0.33 D
trans-but-2-ene has almost zero dipole moment

5. Preparation of Alkenes

Method 1: From Alkynes

Alkynes can be partially reduced to alkenes.

Using Lindlar Catalyst

Catalyst:

Palladium on charcoal
Deactivated with poisons like sulphur compounds

Produces cis-alkenes.

Example: $CH \equiv CH + H_2 \rightarrow CH_2=CH_2$ CH≡CH+H2→CH2=CH2

Using Sodium in Liquid Ammonia

Produces trans-alkenes.

Method 2: Dehydrohalogenation of Alkyl Halides

Alkyl halides react with alcoholic KOH on heating.

Reaction: $R-X + KOH \rightarrow Alkene + KX + H_2O$ R−X+KOH→Alkene+KX+H2O

This reaction involves removal of:

Hydrogen atom
Halogen atom

Therefore called dehydrohalogenation.

Example:

Bromoethane → Ethene

Method 3: From Vicinal Dihalides

Vicinal dihalides contain two halogen atoms on adjacent carbon atoms.

They undergo removal of halogens to form alkenes.

General reaction: $RCHX-CHXR \rightarrow RCH=CHR$ RCHX−CHXR→RCH=CHR

6. Physical Properties of Alkenes

1. Physical State

Lower alkenes are gases.
Middle members are liquids.
Higher members are solids.

2. Solubility

Insoluble in water
Soluble in organic solvents

Reason:

Alkenes are mostly non-polar.

7. Chemical Properties of Alkenes

The π bond makes alkenes more reactive than alkanes.

Main reactions:

Addition reactions
Oxidation
Polymerisation

A. Addition Reactions

Alkenes add atoms across the double bond.

General reaction: $C=C + XY \rightarrow X-C-C-Y$ C=C+XY→X−C−C−Y

1. Hydrogenation

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

Catalyst:

Ni/Pt/Pd

Product:

Alkane

2. Halogen Addition

Alkenes react with halogens.

Example: $CH_2=CH_2+Br_2 \rightarrow CH_2Br-CH_2Br$ CH2=CH2+Br2→CH2Br−CH2Br

Product:

Vicinal dihalide

3. Addition of Hydrogen Halides (HX)

Examples:

Example: $CH_2=CH_2+HBr \rightarrow CH_3CH_2Br$ CH2=CH2+HBr→CH3CH2Br

Markovnikov’s Rule

When an unsymmetrical alkene reacts with an unsymmetrical reagent:

Hydrogen attaches to the carbon already having more hydrogen atoms.

Example:

Propene + HBr

Major product:

2-Bromopropane

8. Oxidation of Alkenes

Alkenes can be oxidised using oxidising agents.

Common oxidising agents:

KMnO₄
O₃

Ozonolysis

Ozone breaks the double bond and forms carbonyl compounds.

Used for:

Locating position of double bond

9. Polymerisation of Alkenes

Small alkene molecules combine to form large molecules called polymers.

Example:

Ethene: $n(CH_2=CH_2) \rightarrow (-CH_2-CH_2-)_n$ n(CH2=CH2)→(−CH2−CH2−)n

Product:

Polythene (polyethylene)

Uses:

Plastic materials
Packaging
Containers

Quick Revision Box: Alkenes

Property	Alkenes
Type	Unsaturated hydrocarbons
Bond	C=C
Formula	CₙH₂ₙ
Hybridisation	sp²
Bond angle	120°
Main reaction	Addition
First stable member	Ethene
Show	Structural + geometrical isomerism

Important Exam Points

⭐ Double bond contains one σ and one π bond.
⭐ π bond causes higher reactivity of alkenes.
⭐ Rotation around C=C bond is restricted.
⭐ Cis-trans isomerism occurs due to restricted rotation.
⭐ Alkenes mainly undergo electrophilic addition reactions.

Part 3 — Alkynes

1. Introduction to Alkynes

Definition

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

General formula: $\boxed{C_nH_{2n-2}}$ CnH2n−2

Examples:

Alkyne	Formula
Ethyne	C₂H₂
Propyne	C₃H₄
Butyne	C₄H₆

The simplest and first member of the alkyne family is:

Ethyne (C₂H₂)

Common name:

Acetylene

2. Structure of Triple Bond

A carbon-carbon triple bond consists of:

1. One Sigma (σ) Bond

Formed by head-on overlapping of sp hybrid orbitals
Strong bond

2. Two Pi (π) Bonds

Formed by sideways overlap of two pairs of p-orbitals

Structure:

H — C ≡ C — H

(Ethyne)

Hybridisation in Alkynes

Carbon atoms involved in triple bonds are:

sp hybridised

Geometry:

Linear arrangement
Bond angle = 180°

Example:

H — C ≡ C — H
       |
     180°

3. Important Features of Triple Bond

Feature	Value
Hybridisation	sp
Geometry	Linear
Bond angle	180°
Bonds present	1 σ + 2 π
General formula	CₙH₂ₙ₋₂

4. Nomenclature of Alkynes (IUPAC Rules)

Rule 1:

Select the longest carbon chain containing the triple bond.

Rule 2:

Number the chain from the end nearest to the triple bond.

Rule 3:

Replace suffix “-ane” with “-yne”.

Examples:

Structure	IUPAC Name
HC≡CH	Ethyne
CH₃–C≡CH	Propyne
CH₃–CH₂–C≡CH	But-1-yne
CH₃–C≡C–CH₃	But-2-yne

5. Isomerism in Alkynes

Alkynes mainly show:

1. Chain Isomerism

Different arrangement of carbon chains.

Example:

Formula: $C_5H_8$ C5H8

Possible structures:

Pent-1-yne

CH≡C–CH₂–CH₂–CH₃

3-Methylbut-1-yne

      CH3
       |
HC≡C—CH—CH3

2. Position Isomerism

Different positions of triple bond.

Example:

But-1-yne

CH≡C–CH₂–CH₃

But-2-yne

CH₃–C≡C–CH₃

6. Preparation of Alkynes

Method 1: From Calcium Carbide

Ethyne is prepared industrially by reacting calcium carbide with water.

Reaction: $CaC_2 + 2H_2O \rightarrow C_2H_2 + Ca(OH)_2$ CaC2+2H2O→C2H2+Ca(OH)2

Products:

Ethyne
Calcium hydroxide

Method 2: From Vicinal Dihalides

Vicinal dihalides contain two halogen atoms on neighbouring carbon atoms.

They undergo double dehydrohalogenation.

Example: $CH_2Br-CH_2Br \rightarrow HC\equiv CH$ CH2Br−CH2Br→HC≡CH

Steps:

Removal of first HBr → vinyl halide
Removal of second HBr → alkyne

Method 3: From Geminal Dihalides

Geminal dihalides have both halogens attached to the same carbon atom.

They can also form alkynes by elimination reactions.

7. Physical Properties of Alkynes

1. Physical State

Lower alkynes are gases.
Higher alkynes are liquids or solids.

2. Solubility

Insoluble in water
Soluble in organic solvents

Reason:

Alkynes are mostly non-polar compounds.

3. Boiling Point

Boiling point increases with:

Increase in molecular mass
Increase in carbon chain length

8. Chemical Properties of Alkynes

Due to the presence of two π bonds, alkynes undergo addition reactions.

Main reactions:

Hydrogenation
Halogen addition
Addition of hydrogen halides
Hydration
Oxidation

A. Addition of Hydrogen (Hydrogenation)

Alkynes add hydrogen to form alkenes and then alkanes.

Example:

Complete hydrogenation:

$CH\equiv CH + 2H_2 \rightarrow CH_3-CH_3$ CH≡CH+2H2→CH3−CH3

(Ethyne → Ethane)

Catalysts:

B. Partial Hydrogenation

Partial reduction gives alkenes.

Using Lindlar Catalyst

Produces:

cis-alkenes

Example: $CH\equiv CH+H_2 \rightarrow CH_2=CH_2$ CH≡CH+H2→CH2=CH2

Using Sodium in Liquid Ammonia

Produces:

trans-alkenes

C. Addition of Halogens

Alkynes react with halogens like:

Cl₂
Br₂

First addition:

Alkyne → Dihaloalkene

Second addition:

Dihaloalkene → Tetrahaloalkane

Example: $CH\equiv CH+Br_2 \rightarrow CHBr=CHBr$ CH≡CH+Br2→CHBr=CHBr

D. Addition of Hydrogen Halides (HX)

HX includes:

Example: $CH\equiv CH+HCl \rightarrow CH_2=CHCl$ CH≡CH+HCl→CH2=CHCl

Further addition gives: $CH_3-CHCl_2$ CH3−CHCl2

E. Addition of Water (Hydration)

Alkynes react with water in presence of catalysts.

Catalyst:

HgSO₄
H₂SO₄

Example:

Ethyne gives ethanal: $CH\equiv CH+H_2O \rightarrow CH_3CHO$ CH≡CH+H2O→CH3CHO

F. Oxidation of Alkynes

Strong oxidising agents break the triple bond.

Oxidising agents:

KMnO₄
O₃

Products:

Carboxylic acids
Carbon dioxide (depending on structure)

9. Acidic Nature of Alkynes

Terminal alkynes contain hydrogen attached to sp carbon.

Example: $HC\equiv CH$ HC≡CH

This hydrogen is slightly acidic.

Reason:

The carbon is sp hybridised and has high s-character.

Order of acidity: $sp > sp^2 > sp^3$ sp>sp2>sp3

Therefore:

Alkynes are more acidic than alkenes and alkanes.

10. Test for Terminal Alkynes

Terminal alkynes react with:

1. Ammoniacal Silver Nitrate

Forms silver acetylide precipitate.

2. Ammoniacal Cuprous Chloride

Forms copper acetylide precipitate.

Example: $HC\equiv CH + AgNO_3 \rightarrow AgC\equiv CAg$ HC≡CH+AgNO3→AgC≡CAg

11. Comparison: Alkane vs Alkene vs Alkyne

Property	Alkane	Alkene	Alkyne
Saturation	Saturated	Unsaturated	Unsaturated
Bond	Single	Double	Triple
Formula	CₙH₂ₙ₊₂	CₙH₂ₙ	CₙH₂ₙ₋₂
Hybridisation	sp³	sp²	sp
Bond angle	109.5°	120°	180°
Main reaction	Substitution	Addition	Addition

Quick Revision Box: Alkynes

⭐ Alkynes contain C≡C bond.
⭐ General formula: CₙH₂ₙ₋₂
⭐ First member: Ethyne (acetylene).
⭐ Carbon is sp hybridised.
⭐ Triple bond contains one σ and two π bonds.
⭐ Terminal alkynes show acidic character.
⭐ Alkynes mainly undergo addition reactions.

Part 4 — Aromatic Hydrocarbons (Benzene)

1. Introduction to Aromatic Hydrocarbons

Definition

Aromatic hydrocarbons are cyclic hydrocarbons that contain one or more benzene-like rings with special stability called aromaticity.

The most important aromatic hydrocarbon is:

Benzene (C₆H₆)

Structure:

Six carbon atoms arranged in a ring
Each carbon is sp² hybridised
Contains a delocalised π-electron system

2. Structure of Benzene

Benzene was discovered by Michael Faraday in 1825.

Molecular formula: $\boxed{C_6H_6}$ C6H6

It contains:

Six carbon atoms
Six hydrogen atoms
A cyclic planar structure

Representation:

The circle inside the ring represents delocalised π electrons.

3. Kekulé Structure of Benzene

August Kekulé proposed a structure containing:

Six carbon atoms in a ring
Alternating single and double bonds

However, this structure could not explain all properties of benzene.

Problems with Kekulé structure:

All C–C bonds in benzene have equal length.
Benzene is more stable than expected.
It does not easily undergo addition reactions like alkenes.

4. Modern Concept of Benzene Structure

According to the modern view:

Benzene is a planar hexagonal molecule.
All carbon atoms are sp² hybridised.
Each carbon forms:
- Two C–C σ bonds
- One C–H σ bond
Unhybridised p-orbitals overlap to form a continuous π-electron cloud.

Important features:

Property	Benzene
Formula	C₆H₆
Shape	Planar hexagon
Hybridisation	sp²
Bond angle	120°
π electrons	6

5. Aromaticity

Meaning

The special stability shown by benzene and similar compounds is called aromaticity.

A compound is aromatic when it satisfies:

Huckel’s Rule

A cyclic compound is aromatic if it contains: $(4n+2)\pi \text{ electrons}$ (4n+2)π electrons

where:

n = 0, 1, 2, 3…

For benzene:

Number of π electrons = 6 $4(1)+2=6$ 4(1)+2=6

Therefore benzene is aromatic.

6. Preparation of Benzene

1. From Ethyne (Acetylene)

Three molecules of ethyne combine to form benzene.

Reaction: $3HC\equiv CH \rightarrow C_6H_6$ 3HC≡CH→C6H6

This reaction is called:

Cyclotrimerisation of ethyne

2. From Sodium Benzoate

Sodium benzoate on heating with soda lime gives benzene.

Reaction: $C_6H_5COONa \rightarrow C_6H_6$ C6H5COONa→C6H6

3. From Phenol

Phenol reacts with zinc dust to produce benzene.

Reaction: $C_6H_5OH + Zn \rightarrow C_6H_6 + ZnO$ C6H5OH+Zn→C6H6+ZnO

7. Physical Properties of Benzene

1. Physical State

Benzene is a colourless liquid.
It has a characteristic smell.

2. Solubility

Insoluble in water
Soluble in organic solvents

3. Boiling Point

Boiling point:

Approximately 80°C

8. Chemical Properties of Benzene

Benzene is highly stable because of aromaticity.

It mainly undergoes:

Electrophilic Substitution Reactions

In these reactions, hydrogen atoms of benzene are replaced by electrophiles.

General reaction: $C_6H_6 + E^+ \rightarrow C_6H_5E + H^+$ C6H6+E+→C6H5E+H+

9. Mechanism of Electrophilic Substitution

The reaction occurs in three steps:

Step 1: Generation of Electrophile

An electrophile (electron-loving species) is produced.

Examples:

NO₂⁺
Br⁺
Cl⁺

Step 2: Formation of Carbocation Intermediate

The electrophile attacks the benzene ring.

A temporary loss of aromaticity occurs.

Step 3: Removal of Hydrogen Ion

The ring regains aromaticity.

Final substituted benzene is formed.

10. Important Electrophilic Substitution Reactions

A. Nitration of Benzene

Benzene reacts with concentrated nitric acid in presence of concentrated sulphuric acid.

Reaction: $C_6H_6+HNO_3 \rightarrow C_6H_5NO_2+H_2O$ C6H6+HNO3→C6H5NO2+H2O

Product:

Nitrobenzene

Electrophile: $NO_2^+$ NO2+

B. Halogenation of Benzene

Benzene reacts with chlorine or bromine in presence of Lewis acid catalyst.

Catalysts:

FeCl₃
FeBr₃

Example: $C_6H_6+Cl_2 \rightarrow C_6H_5Cl+HCl$ C6H6+Cl2→C6H5Cl+HCl

Product:

Chlorobenzene

C. Sulphonation of Benzene

Benzene reacts with fuming sulphuric acid.

Reaction: $C_6H_6+H_2SO_4 \rightarrow C_6H_5SO_3H+H_2O$ C6H6+H2SO4→C6H5SO3H+H2O

Product:

Benzenesulphonic acid

D. Friedel-Crafts Alkylation

Benzene reacts with alkyl halides in presence of aluminium chloride.

Example: $C_6H_6+CH_3Cl \rightarrow C_6H_5CH_3$ C6H6+CH3Cl→C6H5CH3

Product:

Toluene

Catalyst:

AlCl₃

E. Friedel-Crafts Acylation

Benzene reacts with acyl chlorides.

Example: $C_6H_6+CH_3COCl \rightarrow C_6H_5COCH_3$ C6H6+CH3COCl→C6H5COCH3

Product:

Acetophenone

11. Addition Reactions of Benzene

Although benzene prefers substitution reactions, under severe conditions it can undergo addition.

Hydrogenation

Benzene reacts with hydrogen in presence of nickel catalyst.

Reaction: $C_6H_6+3H_2 \rightarrow C_6H_{12}$ C6H6+3H2→C6H12

Product:

Cyclohexane

12. Oxidation of Benzene

Benzene is resistant to oxidation due to aromatic stability.

However, side chains attached to benzene can be oxidised.

Example:

Toluene: $C_6H_5CH_3 \rightarrow C_6H_5COOH$ C6H5CH3→C6H5COOH

Product:

Benzoic acid

13. Directive Influence of Substituents

When a group is already attached to benzene, it affects where the next substitution occurs.

Two types:

1. Ortho-Para Directing Groups

They direct incoming groups to:

Ortho position (2-position)
Para position (4-position)

Examples:

–CH₃
–OH
–NH₂

2. Meta Directing Groups

They direct incoming groups to:

Meta position (3-position)

Examples:

–NO₂
–COOH
–SO₃H

14. Uses of Benzene

Benzene is used in the manufacture of:

Plastics
Synthetic fibres
Dyes
Medicines
Detergents

15. Toxicity of Benzene

Benzene exposure can be harmful.

Important points:

Benzene vapours are toxic.
Long-term exposure can affect health.
It is considered a carcinogenic substance.

Quick Revision Box: Aromatic Hydrocarbons

⭐ Benzene formula = C₆H₆
⭐ Carbon atoms are sp² hybridised.
⭐ Benzene contains 6 π electrons.
⭐ Follows Huckel’s rule (4n+2).
⭐ Main reaction = Electrophilic substitution.
⭐ Common reactions:

Nitration
Halogenation
Sulphonation
Friedel-Crafts reactions

Part 5 — Complete Chapter Revision

1. Important General Formulae of Hydrocarbons

Type	Nature	General Formula	Example
Alkane	Saturated	CₙH₂ₙ₊₂	Methane (CH₄)
Alkene	Unsaturated	CₙH₂ₙ	Ethene (C₂H₄)
Alkyne	Unsaturated	CₙH₂ₙ₋₂	Ethyne (C₂H₂)
Aromatic hydrocarbon	Benzene type	C₆H₆	Benzene

2. Important Hybridisation Table

Compound	Carbon Hybridisation	Geometry	Bond Angle
Alkane	sp³	Tetrahedral	109.5°
Alkene	sp²	Trigonal planar	120°
Alkyne	sp	Linear	180°
Benzene	sp²	Planar hexagonal	120°

3. Bond Comparison

Bond	Bonds Present
Single bond (C–C)	1 σ bond
Double bond (C=C)	1 σ + 1 π bond
Triple bond (C≡C)	1 σ + 2 π bonds

4. Important Rules

A. Markovnikov’s Rule

Statement:

When an unsymmetrical reagent adds to an unsymmetrical alkene, the hydrogen atom attaches to the carbon already having more hydrogen atoms.

Example:

Propene + HBr: $CH_3-CH=CH_2 + HBr$ CH3−CH=CH2+HBr

Major product: $CH_3-CHBr-CH_3$ CH3−CHBr−CH3

Product:

2-Bromopropane

Exception: Anti-Markovnikov Addition

In presence of peroxide, HBr adds opposite to Markovnikov’s rule.

Example:

Propene + HBr + peroxide

Product:

1-Bromopropane

B. Huckel’s Rule

A compound is aromatic when it has: $(4n+2)\pi \text{ electrons}$ (4n+2)π electrons

Conditions:

Cyclic structure
Planar molecule
Continuous overlap of p-orbitals
Number of π electrons = 4n+2

Example:

Benzene: $6\pi\ electrons$ 6π electrons

For n = 1: $4(1)+2=6$ 4(1)+2=6

Therefore benzene is aromatic.

5. Important Reactions of Alkanes

1. Combustion

Alkanes burn in oxygen.

Example: $CH_4+2O_2 \rightarrow CO_2+2H_2O+\text{heat}$ CH4+2O2→CO2+2H2O+heat

Use:

Fuels
Energy production

2. Halogenation

Alkanes react with chlorine or bromine in sunlight.

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

Reaction type:

Free radical substitution

Free Radical Substitution Mechanism

Step 1: Initiation

Formation of radicals by breaking Cl₂ molecule: $Cl_2 \xrightarrow{hv} 2Cl^\bullet$ Cl2hv2Cl∙

Step 2: Propagation

Radicals react with methane molecules.

Step 3: Termination

Radicals combine to form stable molecules.

6. Important Reactions of Alkenes

1. Hydrogenation

Alkene + Hydrogen → Alkane

Catalyst:

Ni/Pt/Pd

Example: $CH_2=CH_2+H_2 \rightarrow CH_3CH_3$ CH2=CH2+H2→CH3CH3

2. Halogen Addition

Example: $CH_2=CH_2+Br_2 \rightarrow CH_2Br-CH_2Br$ CH2=CH2+Br2→CH2Br−CH2Br

Test:

Bromine water gets decolourised.

3. Hydration

Addition of water forms alcohol.

Example: $CH_2=CH_2+H_2O \rightarrow CH_3CH_2OH$ CH2=CH2+H2O→CH3CH2OH

Product:

Ethanol

4. Ozonolysis

Ozone breaks the double bond.

Used to determine:

Position of double bond

5. Polymerisation

Small molecules combine to form polymers.

Example:

Ethene: $n(CH_2=CH_2) \rightarrow (-CH_2-CH_2-)_n$ n(CH2=CH2)→(−CH2−CH2−)n

Product:

Polyethylene

7. Important Reactions of Alkynes

1. Hydrogenation

Alkyne → Alkene → Alkane

Example: $CH\equiv CH+2H_2 \rightarrow CH_3CH_3$ CH≡CH+2H2→CH3CH3

2. Addition of Halogens

Example: $CH\equiv CH+Br_2 \rightarrow CHBr=CHBr$ CH≡CH+Br2→CHBr=CHBr

3. Hydration

Ethyne gives ethanal. $CH\equiv CH+H_2O \rightarrow CH_3CHO$ CH≡CH+H2O→CH3CHO

Catalyst:

HgSO₄/H₂SO₄

8. Important Reactions of Benzene

1. Nitration

Reagents:

Concentrated HNO₃
Concentrated H₂SO₄

Reaction: $C_6H_6+HNO_3 \rightarrow C_6H_5NO_2+H_2O$ C6H6+HNO3→C6H5NO2+H2O

Product:

Nitrobenzene

2. Halogenation

Reagents:

Cl₂/FeCl₃
Br₂/FeBr₃

Example: $C_6H_6+Cl_2 \rightarrow C_6H_5Cl+HCl$ C6H6+Cl2→C6H5Cl+HCl

Product:

Chlorobenzene

3. Sulphonation

Reagent:

Fuming H₂SO₄

Product:

Benzenesulphonic acid

4. Friedel-Crafts Alkylation

Reagents:

Alkyl halide
AlCl₃

Example: $C_6H_6+CH_3Cl \rightarrow C_6H_5CH_3$ C6H6+CH3Cl→C6H5CH3

Product:

Toluene

9. Important Named Reactions

1. Wurtz Reaction

Preparation of higher alkanes.

General reaction: $2R-X+2Na \rightarrow R-R+2NaX$ 2R−X+2Na→R−R+2NaX

Reagent:

Sodium + dry ether

2. Kolbe’s Electrolysis

Preparation of alkanes by electrolysis of sodium salts of carboxylic acids.

Example:

Sodium acetate gives ethane.

3. Friedel-Crafts Reaction

Used for introducing alkyl or acyl groups into benzene.

Catalyst:

AlCl₃

Types:

Alkylation
Acylation

4. Ozonolysis

Used for locating double bond position in alkenes.

10. Important Tests

Bromine Water Test

Purpose:

Detection of unsaturation.

Observation:

Alkene/alkyne decolourise bromine water.
Alkane does not react easily.

Baeyer’s Test

Reagent:

Cold dilute KMnO₄

Observation:

Purple colour disappears due to oxidation of unsaturated compounds.

Test for Terminal Alkynes

Reagents:

Ammoniacal AgNO₃
Ammoniacal CuCl

Formation of precipitate confirms terminal alkyne.

11. Important Differences

Alkane vs Alkene

Alkane	Alkene
Saturated	Unsaturated
Only single bonds	Contains double bond
Less reactive	More reactive
Substitution reactions	Addition reactions
CₙH₂ₙ₊₂	CₙH₂ₙ

Alkene vs Alkyne

Alkene	Alkyne
C=C bond	C≡C bond
sp² hybridisation	sp hybridisation
120° angle	180° angle
One π bond	Two π bonds

12. One-Day Revision Sheet

Remember:

✅ Alkane formula → CₙH₂ₙ₊₂
✅ Alkene formula → CₙH₂ₙ
✅ Alkyne formula → CₙH₂ₙ₋₂
✅ Methane → first alkane
✅ Ethene → first alkene
✅ Ethyne → first alkyne
✅ Benzene → aromatic hydrocarbon
✅ Alkane reaction → substitution
✅ Alkene reaction → addition
✅ Benzene reaction → electrophilic substitution
✅ C=C contains σ + π bond
✅ C≡C contains σ + 2π bonds
✅ Benzene follows Huckel’s rule