Chapter 6: Haloalkanes and Haloarenes

Part 1 – Introduction, Classification & Nomenclature (Class 12 Notes)

1. Organohalogen Compounds

Organic compounds in which one or more hydrogen atoms are replaced by F, Cl, Br, or I are called organohalogen compounds.

Two Main Types

Haloalkanes	Haloarenes
Halogen attached to an sp³ carbon of an alkyl group	Halogen attached directly to an sp² carbon of a benzene ring

Examples

CH₃Cl → Haloalkane
C₂H₅Br → Haloalkane
C₆H₅Cl (Chlorobenzene) → Haloarene

2. Importance

Organohalogen compounds are widely used as:

Industrial solvents
Refrigerants
Medicines
Anaesthetics
Pesticides
Starting materials for organic synthesis

Some naturally occurring compounds also contain halogens.

3. Classification of Halo Compounds

A. Based on Number of Halogen Atoms

Type	Meaning	Example
Monohalo	One halogen atom	CH₃Cl
Dihalo	Two halogens	CH₂Cl₂
Trihalo	Three halogens	CHCl₃
Polyhalo	More than three halogens	CCl₄

B. Compounds with sp³ C–X Bond

(i) Alkyl Halides (Haloalkanes)

General Formula:

R–X

where

R = Alkyl group
X = F, Cl, Br, I

Example:

CH₃CH₂Cl

Classification of Alkyl Halides

Depends on the carbon attached to halogen.

Primary (1°)

Halogen attached to carbon connected with only one carbon.

Example:

CH₃CH₂Cl

Secondary (2°)

Halogen attached to carbon connected with two carbons.

Example:

CH₃CHClCH₃

Tertiary (3°)

Halogen attached to carbon connected with three carbons.

Example:

(CH₃)₃CCl

Easy Memory Trick

Count the number of carbon atoms attached to the carbon bearing the halogen.

(ii) Allylic Halides

Halogen attached to an sp³ carbon adjacent to a double bond.

General Structure

CH₂=CH–CH₂X

Example

CH₂=CHCH₂Br

(Allyl bromide)

(iii) Benzylic Halides

Halogen attached to an sp³ carbon attached to a benzene ring.

General Structure

C₆H₅CH₂X

Example

C₆H₅CH₂Cl

(Benzyl chloride)

C. Compounds with sp² C–X Bond

(i) Vinylic Halides

Halogen attached directly to a carbon of a double bond.

Example

CH₂=CHCl

(Vinyl chloride)

(ii) Aryl Halides

Halogen attached directly to the benzene ring.

Example

C₆H₅Cl

(Chlorobenzene)

Quick Comparison

Compound	Halogen Attached To
Alkyl halide	sp³ carbon
Allylic halide	sp³ carbon next to C=C
Benzylic halide	sp³ carbon next to benzene
Vinylic halide	sp² carbon of C=C
Aryl halide	sp² carbon of benzene

Exam Tip: Students often confuse benzylic and aryl halides. In benzylic halides, the halogen is not directly attached to the benzene ring; in aryl halides, it is attached directly.

4. Geminal and Vicinal Dihalides

Geminal (Gem) Dihalides

Both halogen atoms are present on the same carbon atom.

Example

CH₃–CHCl₂

Vicinal (Vic) Dihalides

Halogen atoms are present on adjacent carbon atoms.

Example

CH₂Cl–CH₂Cl

Memory Trick

Gem → Same carbon
Vic → Neighbouring carbons

5. Nomenclature

Common Name

Rule:

Alkyl group + Halide

Example

CH₃Cl

→ Methyl chloride

IUPAC Name

Rule:

Treat halogen as a substituent.

Example

CH₃CH₂Br

→ Bromoethane

Important Examples

Structure	Common Name	IUPAC Name
CH₃Cl	Methyl chloride	Chloromethane
CH₃CH₂Cl	Ethyl chloride	Chloroethane
CH₂Cl₂	Methylene chloride	Dichloromethane
CHCl₃	Chloroform	Trichloromethane
CCl₄	Carbon tetrachloride	Tetrachloromethane
CH₂=CHCl	Vinyl chloride	Chloroethene
CH₂=CHCH₂Br	Allyl bromide	3-Bromopropene
C₆H₅CH₂Cl	Benzyl chloride	Chlorophenylmethane

6. Rules for IUPAC Naming

Select the longest carbon chain.
Number the chain to give the halogen the lowest possible number.
Write the halogen as a prefix:
- Fluoro
- Chloro
- Bromo
- Iodo
Arrange different substituents alphabetically.
Use di-, tri-, tetra- when identical halogens are present.

Example

CH₃CH₂CH(Cl)CH₃

Longest chain = Butane

Halogen at carbon 2

Name = 2-Chlorobutane

Example

(CH₃)₃CBr

Longest chain = Propane

Substituents:

Bromo at C-2
Methyl at C-2

Name = 2-Bromo-2-methylpropane

NCERT Important Facts

Haloalkanes contain sp³ C–X bonds.
Haloarenes contain sp² C–X bonds.
Primary, secondary and tertiary haloalkanes depend on the carbon attached to halogen, not the total number of carbons.
Geminal = same carbon.
Vicinal = adjacent carbons.
Allylic and benzylic halides are generally more reactive than ordinary alkyl halides because of the stability of the intermediates formed in reactions.

1-Minute Revision

Haloalkane: Halogen on sp³ carbon.
Haloarene: Halogen on benzene carbon (sp²).
Primary: Halogen carbon attached to 1 carbon.
Secondary: Attached to 2 carbons.
Tertiary: Attached to 3 carbons.
Allylic: Next to C=C.
Benzylic: Next to benzene.
Vinylic: On C=C.
Aryl: On benzene ring.
Geminal: Same carbon.
Vicinal: Adjacent carbons.

Part 2 – Nature of C–X Bond & Preparation of Haloalkanes

1. Nature of the Carbon–Halogen (C–X) Bond

Why is the C–X bond polar?

Halogens are more electronegative than carbon.

Therefore,

Carbon acquires a partial positive charge (δ⁺)
Halogen acquires a partial negative charge (δ⁻)

Cδ+  ——  Xδ−

Because of this polarity:

Carbon becomes electron deficient (electrophilic).
It is easily attacked by nucleophiles.
Haloalkanes readily undergo substitution reactions.

2. Effect of Halogen Size

Moving down Group 17:

F → Cl → Br → I

Atomic size increases.

Therefore,

C–X bond length increases.
Bond strength decreases.
Bond becomes easier to break.

Order of Bond Length

C–F < C–Cl < C–Br < C–I

Reason:
Larger halogen atoms form longer bonds.

Order of Bond Strength (Bond Enthalpy)

C–F > C–Cl > C–Br > C–I

Reason:
Shorter bonds are stronger.

Easy Memory Trick

Longer bond = Weaker bond = More reactive

Reactivity Towards Bond Breaking

RI > RBr > RCl >> RF

Reason:

Iodine leaves most easily (best leaving group).
Fluorine forms the strongest bond and is the least reactive.

3. Methods of Preparation of Haloalkanes

There are four important methods in NCERT.

Alcohols
     ↓
Haloalkanes

Hydrocarbons
     ↓
Haloalkanes

Alkenes
     ↓
Haloalkanes

Halogen Exchange
     ↓
Haloalkanes

Method 1: From Alcohols (Most Important)

Alcohols are the best starting materials because they are easily available.

General reaction:

ROH  →  RX

The –OH group is replaced by a halogen atom.

Reagents Used

(i) Hydrogen Halides

(ii) Phosphorus Halides

PCl₃
PCl₅
PBr₃

(iii) Thionyl Chloride (SOCl₂)

Most important reagent for boards.

Reaction

ROH + SOCl₂

↓

RCl + SO₂↑ + HCl↑

Why is SOCl₂ preferred?

Because both by-products,

SO₂
HCl

are gases and escape from the reaction mixture.

Hence,

Product remains almost pure.
Purification becomes easy.

Exam Question: Why is SOCl₂ preferred over HCl?

Answer: Gaseous by-products (SO₂ and HCl) escape, giving pure alkyl chloride.

Lucas Reagent

For preparing alkyl chlorides using HCl.

Lucas reagent =

Conc. HCl + Anhydrous ZnCl₂

ZnCl₂ acts as a catalyst.

Reactivity of Alcohols

3° Alcohol > 2° Alcohol > 1° Alcohol

Reason:

Tertiary alcohols form more stable carbocations and react faster.

Method 2: From Hydrocarbons

(A) From Alkanes

Reaction:

Free radical halogenation.

Example

CH₄ + Cl₂

UV light

↓

CH₃Cl + HCl

Characteristics

Free radical mechanism
Gives a mixture of products
Low yield of desired compound

Not preferred for laboratory preparation because many isomers and polyhalogenated products are formed.

(B) From Alkenes

Two important reactions.

(i) Addition of Hydrogen Halides

General reaction

Alkene + HX

↓

Haloalkane

Example

CH₃CH=CH₂ + HBr

↓

CH₃CHBrCH₃

Major product follows Markovnikov’s rule.

Markovnikov Rule:
Hydrogen attaches to the carbon already having more hydrogen atoms, while the halogen goes to the more substituted carbon.

(ii) Addition of Halogens

Example

CH₂=CH₂ + Br₂

↓

CH₂Br–CH₂Br

Product:

Vicinal dibromide.

Important Application

Bromine solution loses its reddish-brown colour in the presence of a double bond.

Therefore,

Bromine test is used to detect unsaturation (C=C).

Method 3: Halogen Exchange Reactions

Very important for board exams.

(A) Finkelstein Reaction

Used for preparing alkyl iodides.

Reaction

RCl or RBr + NaI
      Dry acetone
↓

RI + NaCl/NaBr

Key Points

Reagent: NaI
Solvent: Dry acetone
NaCl or NaBr precipitates out.
The precipitation drives the reaction forward (Le Chatelier’s principle).

(B) Swarts Reaction

Used for preparing alkyl fluorides.

Reaction

RCl/RBr

+ AgF / SbF₃ / Hg₂F₂ / CoF₂

↓

RF

Remember

Swarts → Fluorides

Named Reactions Summary

Reaction	Product Formed	Reagent
Alcohol → Alkyl chloride	RCl	SOCl₂ / HCl
Finkelstein	RI	NaI + dry acetone
Swarts	RF	AgF, SbF₃, Hg₂F₂, CoF₂
Free radical halogenation	Haloalkane	Cl₂/Br₂ + UV light
Alkene + HX	Haloalkane	HCl, HBr, HI
Alkene + Br₂	Vicinal dibromide	Br₂/CCl₄

NCERT Important Facts

C–X bond is polar because halogens are more electronegative than carbon.
Bond length increases from C–F to C–I.
Bond strength decreases from C–F to C–I.
Reactivity: RI > RBr > RCl >> RF.
SOCl₂ is the preferred reagent for preparing alkyl chlorides.
Lucas reagent = Conc. HCl + ZnCl₂.
3° alcohol > 2° alcohol > 1° alcohol in reactivity.
Finkelstein reaction prepares alkyl iodides.
Swarts reaction prepares alkyl fluorides.
Bromine decolourisation is a test for unsaturation.

30-Second Revision

C–X bond is polar (Cδ⁺–Xδ⁻).
Bond length: C–F < C–Cl < C–Br < C–I.
Bond strength: C–F > C–Cl > C–Br > C–I.
Reactivity: RI > RBr > RCl >> RF.
Best reagent for RCl: SOCl₂.
Finkelstein → RI.
Swarts → RF.
Alkene + Br₂ gives vicinal dibromide.
Bromine test detects C=C double bonds.

Part 3 – Preparation of Haloarenes & Physical Properties

1. Preparation of Haloarenes

Haloarenes cannot be prepared by ordinary substitution reactions used for haloalkanes because the aryl C–X bond is stronger. NCERT describes the following important methods.

Method 1: Direct Halogenation of Benzene (Most Important)

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

Chlorination

Benzene + Cl₂
      FeCl₃ / AlCl₃
↓

Chlorobenzene + HCl

Bromination

Benzene + Br₂
      FeBr₃
↓

Bromobenzene + HBr

Role of Catalyst

FeCl₃, AlCl₃ and FeBr₃ are Lewis acids.
They generate the electrophile (Cl⁺ or Br⁺), which attacks the benzene ring.

Exam Point: Chlorination or bromination of benzene does not occur easily without a catalyst.

Method 2: From Diazonium Salts

This method is mainly used to prepare chloro-, bromo- and iodobenzene.

Aromatic diazonium salts are obtained from aromatic amines and then converted into haloarenes.

(A) Sandmeyer Reaction ⭐

Copper(I) salts replace the diazonium group by Cl or Br.

Reactions

ArN₂⁺Cl⁻ + CuCl

↓

ArCl + N₂

ArN₂⁺Cl⁻ + CuBr

↓

ArBr + N₂

Key Points

Catalyst: CuCl or CuBr
Nitrogen gas is evolved.
Used for preparing chlorobenzene and bromobenzene.

(B) Gattermann Reaction

Uses copper powder and hydrogen halides.

ArN₂⁺Cl⁻

Cu + HCl

↓

ArCl

Cu + HBr

↓

ArBr

Difference

Sandmeyer	Gattermann
CuCl/CuBr used	Cu powder + HCl/HBr
More common	Less common

(C) Preparation of Iodobenzene

ArN₂⁺Cl⁻ + KI

↓

ArI + N₂

No copper catalyst is required.

(D) Preparation of Fluorobenzene

Balz–Schiemann Reaction ⭐

Diazonium tetrafluoroborate

Heat

↓

Fluorobenzene + BF₃ + N₂

This is the preferred method for preparing aryl fluorides.

Named Reactions Summary

Reaction	Product	Reagent
Sandmeyer	ArCl / ArBr	CuCl / CuBr
Gattermann	ArCl / ArBr	Cu + HCl/HBr
KI reaction	ArI	KI
Balz–Schiemann	ArF	Diazonium tetrafluoroborate + Heat

2. Physical Properties

Physical properties depend mainly on:

Molecular mass
Shape of molecule
Intermolecular forces

(A) Physical State

Lower Members

Usually gases or volatile liquids.

Examples

CH₃Cl → Gas
C₂H₅Cl → Gas

Higher Members

Generally liquids or solids because molecular mass increases.

(B) Colour

Most haloalkanes and haloarenes are

Colourless
Sweet smelling

Exposure to light for a long time may cause slight yellowing due to decomposition.

(C) Solubility

In Water

Haloalkanes and haloarenes are almost insoluble.

Why?

Water molecules are strongly held together by hydrogen bonding.

Halo compounds cannot form hydrogen bonds with water.

The energy released during hydration is not sufficient to break the hydrogen bonds already present in water.

Hence,

Very low solubility in water.

In Organic Solvents

They dissolve readily in

Benzene
Ether
Chloroform
Carbon tetrachloride

Reason:

Like dissolves like.

(D) Boiling Point

Boiling point depends on

Molecular mass
Surface area
Strength of intermolecular forces

Effect of Halogen

As halogen size increases,

RF < RCl < RBr < RI

Boiling point increases because:

Molecular mass increases.
Van der Waals forces become stronger.

Effect of Carbon Chain

Longer carbon chain

↓

Higher molecular mass

↓

Higher boiling point.

Effect of Branching

More branching

↓

Smaller surface area

↓

Weaker intermolecular attraction

↓

Lower boiling point.

Example:

n-Butyl chloride

>

tert-Butyl chloride

(E) Density

Density generally increases with heavier halogens.

Fluoro < Chloro < Bromo < Iodo

Most alkyl chlorides are lighter than water.

Many bromides and iodides are denser than water.

(F) Melting Point

Melting point depends on:

Molecular symmetry
Crystal packing

More symmetrical molecules pack better in crystals and therefore usually have higher melting points.

NCERT Important Facts

Chlorination of benzene requires FeCl₃/AlCl₃ catalyst.
Sandmeyer reaction uses CuCl or CuBr.
Gattermann reaction uses Cu powder + HCl/HBr.
Balz–Schiemann reaction is used to prepare aryl fluorides.
Halo compounds are insoluble in water because they cannot form hydrogen bonds.
Boiling point increases with molecular mass.
Branching decreases boiling point.
Heavier halogens generally increase density.

One-Page Revision

Preparation of Haloarenes

Method	Product
Benzene + Cl₂/FeCl₃	Chlorobenzene
Benzene + Br₂/FeBr₃	Bromobenzene
Sandmeyer	ArCl / ArBr
Gattermann	ArCl / ArBr
KI	ArI
Balz–Schiemann	ArF

Physical Properties

Property	Trend
Water solubility	Very low
Organic solvent solubility	High
Boiling point	Increases with molecular mass
Branching	Decreases boiling point
Density	RF < RCl < RBr < RI
Colour	Mostly colourless

Part 4 – Chemical Reactions of Haloalkanes (Most Important Section)

1. Why are Haloalkanes Reactive?

The carbon atom bonded to halogen is electron deficient (δ⁺) because the C–X bond is polar.

Therefore:

Nucleophiles attack the carbon atom.
Halogen leaves as a halide ion (X⁻).

Thus, haloalkanes mainly undergo:

Nucleophilic substitution reactions
Elimination reactions

2. Nucleophilic Substitution Reactions

What is a Nucleophile?

A nucleophile is an electron-rich species that donates an electron pair.

Common Nucleophiles

Nucleophile	Product Formed
OH⁻	Alcohol
CN⁻	Nitrile
NH₃	Amine
RO⁻	Ether
I⁻	Alkyl iodide
SH⁻	Thiol

General Reaction

R–X + Nu⁻  →  R–Nu + X⁻

Here,

X⁻ = Leaving group
Nu⁻ = Nucleophile

3. Factors Affecting Reactivity

Reactivity depends on:

Strength of C–X bond
Stability of intermediate (if formed)
Nature of nucleophile
Structure of alkyl halide

4. SN2 Reaction (Bimolecular Nucleophilic Substitution)

Meaning

S = Substitution
N = Nucleophilic
2 = Two species take part in the rate-determining step.

Mechanism

Nucleophile attacks from the back side of the carbon atom.
Carbon–halogen bond breaks at the same time.
Only one step is involved.
No carbocation is formed.

Characteristics

One-step reaction
Backside attack
Simultaneous bond making and bond breaking
No intermediate

Order of Reactivity

CH₃X > 1° > 2° >> 3°

Reason

Less crowding around the carbon allows easier attack by the nucleophile.

Hence,

Methyl halides react fastest.
Tertiary halides react slowest.

Stereochemistry

SN2 causes inversion of configuration (Walden inversion).

This happens because the nucleophile attacks from the opposite side of the leaving group.

5. SN1 Reaction (Unimolecular Nucleophilic Substitution)

Meaning

S = Substitution
N = Nucleophilic
1 = One species is involved in the rate-determining step.

Mechanism

Step 1 (Slow)

The C–X bond breaks.

R–X

↓

R⁺ + X⁻

A carbocation is formed.

Step 2 (Fast)

The nucleophile attacks the carbocation.

R⁺ + OH⁻

↓

ROH

Characteristics

Two-step reaction
Carbocation intermediate formed
First step is slow
Second step is fast

Reactivity Order

3° > 2° > 1° > CH₃

Reason

More stable carbocations form more easily.

Tertiary carbocation → Most stable
Primary carbocation → Least stable

Stereochemistry

Since the carbocation is planar, the nucleophile can attack from either side.

Hence, both configurations are formed.

Result:

Racemization (mixture of enantiomers).

6. SN1 vs SN2 (Very Important)

Property	SN1	SN2
Steps	Two	One
Intermediate	Carbocation	None
Rate depends on	Haloalkane only	Haloalkane + nucleophile
Reactivity order	3° > 2° > 1°	CH₃ > 1° > 2° > 3°
Stereochemistry	Racemization	Inversion
Favoured by	3° halides	Methyl & 1° halides

7. Elimination Reaction (β-Elimination)

When haloalkanes are heated with alcoholic KOH, a hydrogen atom and the halogen atom are removed from adjacent carbon atoms.

An alkene is formed.

General Reaction

R–CH₂–CH₂–X
      Alcoholic KOH
Heat
↓

R–CH=CH₂ + KX + H₂O

Example

CH₃CH₂Br

Alcoholic KOH

↓

CH₂=CH₂

Why is it Called β-Elimination?

Halogen leaves from the α-carbon.
Hydrogen is removed from the β-carbon.

8. Competing Reactions

Reagent	Major Product
Aqueous KOH	Alcohol
Alcoholic KOH	Alkene

Easy Memory

Water → Substitution
Alcohol → Elimination

9. Reaction with Magnesium

Haloalkanes react with magnesium in dry ether to form Grignard reagents.

R–X + Mg
Dry ether
↓

R–MgX

Importance

Grignard reagents are extremely useful for preparing:

Alcohols
Carboxylic acids
Hydrocarbons
Higher organic compounds

Exam Point: Grignard reagents react immediately with water, so they must be prepared in absolutely dry ether.

10. Wurtz Reaction ⭐

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

2R–X + 2Na
Dry ether
↓

R–R + 2NaX

Product

Higher alkane

Example

2CH₃Br + 2Na

↓

C₂H₆ + 2NaBr

Uses

Preparation of symmetrical alkanes.
Not suitable for unsymmetrical alkanes because a mixture of products is formed.

11. NCERT Reaction Summary

Reagent	Product
Aqueous KOH	Alcohol
Alcoholic KOH	Alkene
NH₃	Amine
KCN	Nitrile
AgCN	Isocyanide
NaI (dry acetone)	Alkyl iodide
Mg/Dry ether	Grignard reagent
Na/Dry ether	Higher alkane (Wurtz reaction)

Board Exam Highlights

Must Remember

SN1 → Carbocation intermediate.
SN2 → Backside attack.
SN2 gives Walden inversion.
SN1 gives racemization.
Aqueous KOH → Alcohol.
Alcoholic KOH → Alkene.
Grignard reagent must be prepared in dry ether.
Wurtz reaction prepares symmetrical alkanes.

One-Page Revision

Reactivity Order

SN1

3° > 2° > 1° > CH₃

SN2

CH₃ > 1° > 2° > 3°

Key Named Reactions

Reaction	Product
Wurtz	Higher alkane
Grignard	R–MgX
Aqueous KOH	Alcohol
Alcoholic KOH	Alkene
KCN	Nitrile
NH₃	Amine

Part 5 – Chemical Reactions of Haloarenes

1. Why are Haloarenes Less Reactive than Haloalkanes?

Haloarenes (aryl halides) are much less reactive towards nucleophilic substitution reactions compared to haloalkanes.

Example:

Chlorobenzene reacts much slower than chloroethane.

Reasons for Low Reactivity of Haloarenes

(i) Resonance Effect

In chlorobenzene, the lone pair of chlorine participates in resonance with the benzene ring.

This gives the C–Cl bond a partial double bond character.

Therefore:

Bond becomes shorter.
Bond becomes stronger.
Breaking the bond becomes difficult.

(ii) Hybridisation of Carbon

In haloalkanes:

Carbon attached to halogen is sp³ hybridised.

In haloarenes:

Carbon attached to halogen is sp² hybridised.

The sp² carbon has more s-character, making it more electronegative and holding the C–X bond more strongly.

(iii) Instability of Phenyl Cation

SN1 reaction requires formation of a carbocation.

Formation of phenyl cation is highly unstable.

Therefore, haloarenes do not easily undergo SN1 reactions.

(iv) Difficulty in Backside Attack

SN2 reaction requires attack from the backside.

In haloarenes:

Benzene ring blocks the backside attack.
Therefore SN2 reaction becomes difficult.

2. Nucleophilic Substitution in Haloarenes

Although haloarenes are less reactive, under severe conditions they can undergo nucleophilic substitution.

Example: Chlorobenzene to Phenol

Reaction:

C₆H₅Cl + NaOH

High temperature and pressure

↓

C₆H₅OH + NaCl

Conditions:

Temperature: about 623 K
Pressure: about 300 atm

Product:

Phenol

3. Effect of Electron Withdrawing Groups

Electron withdrawing groups increase the reactivity of haloarenes towards nucleophilic substitution.

Examples:

–NO₂
–CN
–COOH

Position Effect of Nitro Group

Nitro group increases reaction speed when present at:

Ortho position
Para position

It decreases when present at meta position.

Example:

p-Nitrochlorobenzene

reacts faster than chlorobenzene.

Reason:

The nitro group stabilises the intermediate formed during substitution.

4. Electrophilic Substitution Reactions of Haloarenes

Haloarenes contain a benzene ring, therefore they undergo electrophilic substitution reactions.

Common reactions:

Halogenation
Nitration
Sulphonation
Friedel-Crafts reaction

(A) Halogenation

Chlorobenzene reacts with chlorine in presence of FeCl₃.

Reaction:

C₆H₅Cl + Cl₂

FeCl₃

↓

o-Dichlorobenzene + p-Dichlorobenzene

Products:

Ortho isomer
Para isomer

(B) Nitration

Reaction:

Chlorobenzene + HNO₃

H₂SO₄

↓

o-Nitrochlorobenzene + p-Nitrochlorobenzene

(C) Sulphonation

Reaction:

Chlorobenzene + H₂SO₄

↓

o-Chlorobenzenesulphonic acid
+
p-Chlorobenzenesulphonic acid

(D) Friedel-Crafts Reaction

Haloarenes generally do not undergo Friedel-Crafts alkylation and acylation.

Reason:

The halogen atom reduces the reactivity of the benzene ring.

5. Directive Influence of Halogen Atom

Halogens show:

−I effect (electron withdrawing)
+R effect (resonance donating)

Because of these opposite effects:

Overall effect:

Ring becomes less reactive towards electrophilic substitution.
Halogen directs incoming groups to ortho and para positions.

Important Point

Halogens are:

Deactivating groups
Ortho-para directing groups

This is an important board exam concept.

6. Reactions with Metals

Haloarenes react with metals to form useful products.

(A) Fittig Reaction ⭐

Two molecules of aryl halides react with sodium metal in dry ether.

General reaction:

2Ar–X + 2Na

Dry ether

↓

Ar–Ar + 2NaX

Example:

2C₆H₅Cl + 2Na

↓

Biphenyl + 2NaCl

Product:

Biphenyl

(B) Wurtz–Fittig Reaction ⭐

Aryl halide reacts with alkyl halide in presence of sodium metal.

Reaction:

Ar–X + R–X + 2Na

Dry ether

↓

Ar–R + 2NaX

Example:

C₆H₅Cl + CH₃Cl + 2Na

↓

Toluene + 2NaCl

Product:

Alkyl benzene

Fittig vs Wurtz–Fittig

Reaction	Reactants	Product
Fittig	Two aryl halides	Biaryl
Wurtz	Two alkyl halides	Alkane
Wurtz–Fittig	Alkyl + aryl halide	Alkyl benzene

7. Important Comparison: Haloalkanes vs Haloarenes

Property	Haloalkanes	Haloarenes
Carbon type	sp³	sp²
C–X bond	Weaker	Stronger
Reactivity	Higher	Lower
SN1 reaction	Easy (3°)	Difficult
SN2 reaction	Common	Difficult
Example	CH₃Cl	C₆H₅Cl

NCERT Important Facts

Haloarenes are less reactive because the C–X bond has partial double bond character.
SN1 reaction is difficult due to unstable phenyl carbocation.
SN2 reaction is difficult due to steric hindrance and resonance.
Electron withdrawing groups increase nucleophilic substitution.
Nitro group at ortho and para positions increases reactivity.
Halogens are deactivating but ortho-para directing.
Fittig reaction gives biphenyl.
Wurtz–Fittig reaction gives alkyl benzene.
Chlorobenzene can be converted to phenol under severe conditions.

Quick Revision Sheet

Haloarenes

Low reactivity because:

Resonance strengthens C–X bond.
sp² carbon forms stronger bond.
Phenyl cation is unstable.
Backside attack is difficult.

Named Reactions

Reaction	Product
Fittig	Biphenyl
Wurtz–Fittig	Alkyl benzene
Chlorobenzene + NaOH	Phenol
Nitration	Nitro derivatives
Halogenation	Ortho/para halo products

Part 6 – Polyhalogen Compounds & Complete Chapter Revision

1. Polyhalogen Compounds

Organic compounds containing more than one halogen atom are called polyhalogen compounds.

Examples:

Dichloromethane (CH₂Cl₂)
Chloroform (CHCl₃)
Carbon tetrachloride (CCl₄)
Freons
DDT

Many of these compounds have important industrial applications.

2. Dichloromethane (Methylene Chloride)

Formula:

CH₂Cl₂

Preparation:

Prepared by chlorination of methane.

CH₄ + Cl₂

hv

↓

CH₃Cl

↓

CH₂Cl₂

Properties

Colourless liquid
Sweet smell
Volatile
Slightly soluble in water
Soluble in organic solvents

Uses

Dichloromethane is used as:

Solvent for extraction processes
Solvent in paints and varnishes
Cleaning agent
Used in aerosol formulations

3. Chloroform (Trichloromethane)

Formula:

CHCl₃

Preparation

Chloroform is prepared by chlorination of methane.

CH₄ + Cl₂

↓

CH₃Cl

↓

CH₂Cl₂

↓

CHCl₃

Important Reaction

Chloroform gets oxidised in air and sunlight to form poisonous phosgene gas (COCl₂).

Reaction:

CHCl₃ + O₂

↓

COCl₂ + HCl

Therefore:

Chloroform is stored in dark-coloured bottles.
A small amount of ethanol is added as a stabiliser.

Ethanol converts phosgene into harmless compounds.

Uses

Earlier:

Used as an anaesthetic.

Present uses:

Solvent for organic compounds
Chemical preparation

4. Carbon Tetrachloride

Formula:

CCl₄

Properties

Colourless liquid
Non-flammable
Toxic
Insoluble in water

Uses

Previously used as:

Fire extinguishing agent
Solvent

However, its use has reduced because it damages the ozone layer.

5. Freons (Chlorofluorocarbons)

Freons are compounds containing:

Carbon
Chlorine
Fluorine

General formula:

CCl₂F₂

(Example: Freon-12)

Uses

Freons are used as:

Refrigerants in refrigerators and air conditioners
Propellants in aerosol sprays

Environmental Effect

Freons release chlorine radicals in the atmosphere.

These radicals destroy ozone molecules.

Reaction:

Cl· + O₃ → ClO· + O₂

Ozone layer depletion increases harmful UV radiation reaching Earth.

6. DDT (Dichlorodiphenyltrichloroethane)

Formula:

C₁₄H₉Cl₅

DDT is a powerful insecticide.

Uses

Used for controlling:

Mosquitoes
Agricultural pests

Problems

DDT is:

Non-biodegradable
Persistent in the environment
Accumulates in living organisms

It enters food chains and causes harmful effects.

Important Polyhalogen Compounds Table

Compound	Formula	Important Use
Dichloromethane	CH₂Cl₂	Solvent
Chloroform	CHCl₃	Solvent, anaesthetic (earlier)
Carbon tetrachloride	CCl₄	Fire extinguisher, solvent
Freons	CCl₂F₂	Refrigerants
DDT	C₁₄H₉Cl₅	Insecticide

COMPLETE CHAPTER QUICK REVISION

1. Classification

Haloalkanes

Halogen attached to sp³ carbon

Example:

CH₃Cl

Haloarenes

Halogen attached directly to benzene ring.

Example:

C₆H₅Cl

2. Important Orders

Bond Length

C–F < C–Cl < C–Br < C–I

Bond Strength

C–F > C–Cl > C–Br > C–I

Reactivity of Haloalkanes

RI > RBr > RCl >> RF

3. Preparation Reactions

Reaction	Important Point
Alcohol + SOCl₂	Best method for alkyl chloride
Finkelstein	Preparation of alkyl iodide
Swarts	Preparation of alkyl fluoride
Sandmeyer	ArCl/ArBr preparation
Balz–Schiemann	ArF preparation

4. SN1 and SN2 Summary

Feature	SN1	SN2
Steps	Two	One
Intermediate	Carbocation	None
Rate	Depends on haloalkane	Depends on haloalkane + nucleophile
Favoured	3° halides	Methyl and 1° halides
Stereochemistry	Racemisation	Inversion

5. Reactions with KOH

Aqueous KOH

Haloalkane → Alcohol

Alcoholic KOH

Haloalkane → Alkene

6. Important Named Reactions

Wurtz Reaction

2R–X + 2Na → R–R + 2NaX

Product:

Symmetrical alkane

Fittig Reaction

2Ar–X + 2Na → Ar–Ar + 2NaX

Product:

Biphenyl

Wurtz–Fittig Reaction

Ar–X + R–X + 2Na → Ar–R + 2NaX

Product:

Alkyl benzene

Sandmeyer Reaction

Diazonium salt + CuCl/CuBr

→ Haloarene

Balz–Schiemann Reaction

Diazonium salt

→ Fluorobenzene

MOST IMPORTANT BOARD QUESTIONS

Q1. Why is chlorobenzene less reactive than chloroethane?

Answer:

C–Cl bond in chlorobenzene has partial double bond character due to resonance.
Carbon is sp² hybridised.
Bond is stronger and difficult to break.

Q2. Why is SOCl₂ preferred for preparing alkyl chlorides?

Answer:

SOCl₂ produces gaseous SO₂ and HCl which escape, leaving pure alkyl chloride.

Q3. Give reason:

Haloalkanes are insoluble in water.

Answer:

They cannot form strong hydrogen bonds with water molecules.

Q4. Why do tertiary haloalkanes undergo SN1 reaction faster?

Answer:

They form more stable tertiary carbocations.

Q5. Why are halogens ortho-para directing but deactivating?

Answer:

−I effect decreases ring activity.
+R effect directs substitution to ortho and para positions.

Final 10-Minute Revision

Remember these keywords:

C–X bond polarity → Nucleophilic attack
SOCl₂ → Pure alkyl chloride
Finkelstein → Iodide
Swarts → Fluoride
SN1 → Carbocation → Racemisation
SN2 → Backside attack → Inversion
Alcoholic KOH → Elimination
Mg + dry ether → Grignard reagent
Wurtz → Alkane
Fittig → Biphenyl
Wurtz–Fittig → Alkyl benzene
Freons → Ozone depletion
DDT → Non-biodegradable insecticide