HALOALKANES & HALOARENES

0. Overview

PART 1 – INTRODUCTION & CLASSIFICATION

What are Haloalkanes?

• Haloalkanes (also called alkyl halides) are organic compounds containing a carbon-halogen (C-X) bond
• The halogen atom (X) can be fluorine (F), chlorine (Cl), bromine (Br), or iodine (I)
• General formula: CₙH₂ₙ₊₁X for monohaloalkanes
• Example: CH₃Cl (chloromethane), C₂H₅Br (bromoethane)

What are Haloarenes?

• Haloarenes (also called aryl halides) have the halogen atom directly attached to an aromatic ring
• The halogen is bonded to an sp² hybridized carbon of the benzene ring
• Example: C₆H₅Cl (chlorobenzene), C₆H₅Br (bromobenzene)

Functional Group

• Functional group: -X (halogen atom)
• The halogen is the characteristic functional group in both haloalkanes and haloarenes

General Formula

• Monohaloalkanes: CₙH₂ₙ₊₁X
• Dihaloalkanes: CₙH₂ₙX₂
• Trihaloalkanes: CₙH₂ₙ₋₁X₃
• Polyhaloalkanes: CₙH₂ₙ₊₁₋ₓXₓ

Classification of Haloalkanes

Based on Number of Halogen Atoms

Based on Carbon Atom Bearing Halogen

Classification of Haloarenes

• Classified based on the position of halogen on the benzene ring
• Ortho: Halogens at positions 1 and 2
• Meta: Halogens at positions 1 and 3
• Para: Halogens at positions 1 and 4

NOMENCLATURE (IUPAC)

Rules for Haloalkanes

1. Select the longest continuous carbon chain containing the halogen
2. Number the chain to give the halogen the lowest possible number
3. Name the halogen as a prefix: fluoro, chloro, bromo, iodo
4. For multiple halogens, use prefixes: di-, tri-, tetra-
5. List substituents alphabetically (ignoring prefixes like di-, tri-)

Examples

Board Naming Questions

PART 2 – PREPARATION OF HALOALKANES FROM ALCOHOLS

1. Reaction with HX (Hydrogen Halides)

Conditions:

• HCl, HBr, or HI
• Anhydrous ZnCl₂ (Lucas Reagent) for HCl
• Conc. H₂SO₄ for HBr and HI

Lucas Test

Mechanism:

• 3° alcohols: SN1 mechanism (carbocation formation)
• 1° and 2° alcohols: SN2 mechanism

2. Reaction with SOCl₂ (Thionyl Chloride)

Conditions:

• Anhydrous conditions
• Pyridine can be used as base

3. Reaction with PCl₃ (Phosphorus Trichloride)

Conditions:

• Heat
• Anhydrous conditions

4. Reaction with PCl₅ (Phosphorus Pentachloride)

Conditions:

• Heat
• Anhydrous conditions

5. Reaction with PBr₃ (Phosphorus Tribromide)

Conditions:

• Heat
• Anhydrous conditions

6. Reaction with PI₃ (Phosphorus Triiodide)

PART 2 (CONTINUED) – PREPARATION FROM ALKANES (FREE RADICAL HALOGENATION)

General Reaction

Where: RH = Alkane, X₂ = Cl₂ or Br₂, RX = Haloalkane, HX = HCl or HBr

Why is Sunlight Required?

• Sunlight (hν) or heat (Δ) provides energy to break the X-X bond
• This bond breaks homolytically (each atom gets one electron)
• Free radicals are formed: Cl• or Br•

What is a Free Radical?

• A neutral species containing an unpaired electron
• Highly reactive because it tries to pair its unpaired electron
• Examples: Cl•, CH₃•, Br•

Mechanism of Free Radical Halogenation

Stage 1 – Initiation

Stage 2 – Propagation

Stage 3 – Termination

Chlorination of Methane

CH₄
 ↓
CH₃Cl (Chloromethane)
 ↓
CH₂Cl₂ (Dichloromethane)
 ↓
CHCl₃ (Trichloromethane/Chloroform)
 ↓
CCl₄ (Tetrachloromethane)

Reactions:

1. CH₄ + Cl₂ ──hν──► CH₃Cl + HCl
2. CH₃Cl + Cl₂ ──hν──► CH₂Cl₂ + HCl
3. CH₂Cl₂ + Cl₂ ──hν──► CHCl₃ + HCl
4. CHCl₃ + Cl₂ ──hν──► CCl₄ + HCl

Bromination of Alkanes

Chlorination vs Bromination

Why is Bromination More Selective?

• Bromination has a higher activation energy
• Therefore, it reacts mainly with the most stable free radical
• Major product predominates
• Chlorination reacts rapidly and forms many products

Stability of Free Radicals

Example: Monochlorination of Propane

Possible Products:

1. 1-Chloropropane: CH₃CH₂CH₂Cl (Primary)
2. 2-Chloropropane: CH₃CHClCH₃ (Secondary)

Conditions Required

Without light, no reaction occurs.

NCERT Points to Remember

• Chlorination and bromination proceed by a free radical chain mechanism
• UV light or heat is essential to generate free radicals
• Homolytic bond cleavage occurs in the initiation step
• Bromination is more selective than chlorination
• Radical stability order: 3° > 2° > 1° > CH₃•

Board PYQs

PART 2 (CONTINUED) – PREPARATION FROM ALKENES

Introduction

• Alkenes contain a carbon-carbon double bond (C=C)
• The π-bond is weaker than the σ-bond
• Therefore, the π-bond breaks easily
• Halogens or hydrogen halides add across the double bond
• This is called an Addition Reaction

General Reaction

where HX = HCl, HBr, or HI

Why are Alkenes More Reactive?

Double bond
 ↓
Contains π bond
 ↓
π bond is weak
 ↓
Breaks easily
 ↓
Electrophilic addition occurs

Addition of Hydrogen Halides (HX)

HX may be: HCl, HBr, or HI

Examples:

1. Ethene + HCl: CH₂=CH₂ + HCl ──► CH₃CH₂Cl (Chloroethane)
2. Ethene + HBr: CH₂=CH₂ + HBr ──► CH₃CH₂Br (Bromoethane)
3. Ethene + HI: CH₂=CH₂ + HI ──► CH₃CH₂I (Iodoethane)

Unsymmetrical Alkenes

This is where Markovnikov Rule comes into the picture.

🔴 MARKOVNIKOV RULE ⭐⭐⭐⭐⭐

Most important theory of this chapter.

Definition

When an unsymmetrical reagent (HX) adds to an unsymmetrical alkene:

• Hydrogen goes to the carbon already containing more hydrogen atoms
• Halogen goes to the carbon having fewer hydrogen atoms

Example: Propene

Hydrogen count:

• CH₃ = 3 H
• CH = 1 H
• CH₂ = 2 H

Carbon with more hydrogen: CH₂

Therefore:

• H goes to CH₂
• Br goes to middle carbon

Mechanism of Markovnikov Addition

Why is Markovnikov Product Major?

Because reaction proceeds through the most stable carbocation

Another Example: But-1-ene

Board Question

🔴 ANTI-MARKOVNIKOV RULE (PEROXIDE EFFECT) ⭐⭐⭐⭐⭐

Also called Peroxide Effect

Definition

In the presence of Organic Peroxides (ROOR), the addition of HBr takes place opposite to Markovnikov Rule.

Therefore:

• Br attaches to carbon having more hydrogen atoms

Important Condition

Board Favourite Question

Example: Propene

Comparison

Why only HBr?

Mechanism of Peroxide Effect

Three steps (exactly like free radical chlorination):

Initiation

Propagation

Termination

Addition of Halogens

Very Important

Alkenes react with Cl₂ or Br₂ to form Vicinal Dihalides.

Examples:

1. Ethene + Br₂: CH₂=CH₂ + Br₂ ──► CH₂BrCH₂Br (1,2-Dibromoethane)
2. Ethene + Cl₂: CH₂=CH₂ + Cl₂ ──► CH₂ClCH₂Cl (1,2-Dichloroethane)

Test for Unsaturation

This reaction is used in laboratory.

NCERT Important Notes

• Addition to alkenes is an electrophilic addition reaction
• Markovnikov's rule applies to addition of HX to unsymmetrical alkenes
• Peroxide effect is shown only by HBr
• Addition of Cl₂ or Br₂ forms vicinal dihalides
• Bromine water is decolourised by alkenes because bromine adds across the double bond

Comparison Table

Board PYQs

PART 3 – PREPARATION OF HALOARENES

Introduction

Haloarenes cannot be prepared by free radical halogenation like alkanes because:

• The benzene ring is highly stable due to resonance
• Addition reactions would destroy aromaticity
• Therefore, benzene undergoes electrophilic substitution instead of addition

METHOD 1 – DIRECT HALOGENATION OF BENZENE

General Reaction

where X = Cl or Br

This is an Electrophilic Aromatic Substitution (EAS) reaction.

A. Chlorination of Benzene ⭐⭐⭐⭐⭐

Product: Chlorobenzene

Catalyst Used

• FeCl₃
• AlCl₃

Both are Lewis acids.

Role of FeCl₃

FeCl₃ reacts with Cl₂ to generate the electrophile (Cl⁺).

The Cl⁺ ion attacks the benzene ring.

Why is FeCl₃ Needed?

Mechanism of Chlorination

B. Bromination of Benzene ⭐⭐⭐⭐⭐

Product: Bromobenzene

Catalyst: FeBr₃

Mechanism is exactly the same as chlorination.

C. Iodination of Benzene

Why oxidising agent?

Because HI formed during the reaction reverses the reaction.

Common oxidising agents:

• HNO₃
• HIO₃

They remove HI, shifting the reaction forward.

D. Fluorination

Direct fluorination is not preferred.

Reason:

• Fluorine is extremely reactive
• Reaction becomes violent and difficult to control
• Therefore, fluorobenzene is generally prepared by the Balz–Schiemann reaction

PREPARATION FROM DIAZONIUM SALTS

Very Important for Board Exams

What is a Diazonium Salt?

Example: C₆H₅N₂⁺Cl⁻ (Benzenediazonium chloride)

Preparation of Benzenediazonium Chloride

🔴 SANDMEYER REACTION ⭐⭐⭐⭐⭐

Definition

The Sandmeyer reaction is used to replace the diazonium group with various halogens using copper salts as catalysts.

General Reaction

Specific Reactions

1. Sandmeyer Chlorination: C₆H₅N₂⁺Cl⁻ + CuCl ──► C₆H₅Cl + N₂↑
2. Sandmeyer Bromination: C₆H₅N₂⁺Cl⁻ + CuBr ──► C₆H₅Br + N₂↑
3. Sandmeyer Cyanation: C₆H₅N₂⁺Cl⁻ + CuCN ──► C₆H₅CN + N₂↑

Mechanism

1. Diazonium salt reacts with Cu(I) salt
2. Free radical mechanism
3. Nitrogen gas is evolved
4. Aryl halide is formed

Board Question

🔴 GATTERMANN REACTION ⭐⭐⭐⭐

Definition

The Gattermann reaction is similar to the Sandmeyer reaction but uses Cu powder and HX instead of CuX.

General Reaction

Specific Reactions

1. Gattermann Chlorination: C₆H₅N₂⁺Cl⁻ + Cu + HCl ──► C₆H₅Cl + N₂↑
2. Gattermann Bromination: C₆H₅N₂⁺Cl⁻ + Cu + HBr ──► C₆H₅Br + N₂↑

🔴 BALZ–SCHIEMANN REACTION ⭐⭐⭐⭐⭐

Definition

The Balz–Schiemann reaction is used to prepare aryl fluorides from diazonium salts.

General Reaction

Specific Reaction

Why is this important?

• Direct fluorination of benzene is not possible
• This is the only practical method to prepare fluorobenzene

Board Question

NCERT Points to Remember

• Diazonium salts are prepared at 0-5°C
• Sandmeyer reaction uses CuX (X = Cl, Br, CN)
• Gattermann reaction uses Cu powder + HX
• Balz–Schiemann reaction is used for fluorination
• All these reactions evolve nitrogen gas

PART 4 – PHYSICAL PROPERTIES

State

• Lower alkanes (C₁-C₄): Gases
• C₅-C₁₀: Liquids
• Higher: Solids

Colour

• Colourless

Odour

• Sweetish characteristic odour

Solubility

• Insoluble in water
• Soluble in organic solvents (ether, alcohol, chloroform)

Boiling Point

• Increases with molecular weight
• For isomeric haloalkanes: 1° < 2° < 3° (due to surface area)
• Order: RI > RBr > RCl > RF (for same alkyl group)

Melting Point

• Generally increases with molecular weight
• Symmetrical molecules have higher melting points

Density

• Most haloalkanes are denser than water
• Order: RI > RBr > RCl > RF

Dipole Moment

• CH₃F > CH₃Cl > CH₃Br > CH₃I (due to electronegativity difference)

Comparison Table

Physical Properties of Haloarenes

• Generally liquids (C₆-C₁₀) or solids (higher)
• Higher boiling points than corresponding alkanes due to polarity

PART 5 – CHEMICAL REACTIONS OF HALOALKANES

Nucleophilic Substitution Reactions

Comparison: SN1 vs SN2

Stereochemical Aspects of Nucleophilic Substitution

Optical Activity

Compounds that rotate the plane of plane-polarised light are called optically active compounds.

• Dextrorotatory (+): Rotates plane to the right (clockwise)
• Laevorotatory (-): Rotates plane to the left (anticlockwise)

Chirality and Enantiomers

Chiral molecules: Molecules that are non-superimposable on their mirror images (like left and right hands).

Achiral molecules: Molecules that are superimposable on their mirror images.

Enantiomers: Stereoisomers that are non-superimposable mirror images of each other.

Racemic mixture: Equal mixture of two enantiomers (optically inactive, denoted as dl or ±).

Asymmetric Carbon (Stereocentre)

A carbon atom attached to four different groups is called asymmetric carbon or stereocentre.

Retention, Inversion and Racemisation

When a bond to an asymmetric carbon is broken during a reaction, three outcomes are possible:

• Retention of configuration: Spatial arrangement remains the same
• Inversion of configuration: Spatial arrangement is inverted (like umbrella turning inside out)
• Racemisation: 50:50 mixture of both configurations (optically inactive)

Stereochemistry of SN1 and SN2 Reactions

Elimination Reactions

β-Elimination (Dehydrohalogenation)

• Reagent: Alcoholic KOH
• Product: Alkene

Zaitsev Rule

The more substituted alkene is the major product.

Hoffmann Rule

With bulky bases, the less substituted alkene is the major product.

Reactions with Metals

Wurtz Reaction

Fittig Reaction

Wurtz-Fittig Reaction

Organometallic Compounds

Grignard Reagent

• Preparation: Alkyl halide + magnesium in dry ether
• Importance: Used in organic synthesis to form C-C bonds

Reduction

Reducing Agents

• Zn + HCl
• LiAlH₄
• NaBH₄
• H₂/Pd

Complete Nucleophilic Substitution Reactions

Important reagents and their substitution products:

PART 6 – CHEMICAL REACTIONS OF HALOARENES

Why Haloarenes are Less Reactive

Reasons:

1. Resonance: The lone pair on halogen participates in resonance with the benzene ring
2. Partial double bond character: C-X bond acquires partial double bond character
3. sp² carbon: The carbon is sp² hybridized (more electronegative than sp³)
4. Bond length: C-X bond in haloarenes is shorter than in haloalkanes

Electrophilic Substitution

Haloarenes undergo electrophilic substitution reactions, but more slowly than benzene because halogens withdraw electron density by the -I effect.

Why halogen is ortho/para directing?

Although halogen is electron-withdrawing by inductive effect, it donates electron density to the ring by resonance through its lone pair. This resonance stabilizes the sigma complex formed during attack at ortho and para positions more than at meta position. Thus, haloarenes are deactivated yet ortho/para directing.

NCERT reactions to remember:

1. Halogenation: C₆H₅Cl + Cl₂ ──anhyd. FeCl₃──► o-dichlorobenzene + p-dichlorobenzene + HCl
2. Nitration: C₆H₅Cl + HNO₃ ──conc. H₂SO₄──► o-nitrochlorobenzene + p-nitrochlorobenzene + H₂O
3. Sulphonation: C₆H₅Cl + H₂SO₄ ──heat──► o-chlorobenzenesulphonic acid + p-chlorobenzenesulphonic acid + H₂O

Nucleophilic Substitution

Haloarenes are less reactive towards nucleophilic substitution.

Reasons for low reactivity:

1. Resonance effect: Lone pair on halogen conjugates with benzene ring, giving C-X bond partial double bond character
2. Hybridisation difference: Carbon is sp² (more electronegative) in haloarenes vs sp³ in haloalkanes
3. Bond length: C-X bond is shorter in haloarenes (169 pm vs 177 pm), harder to break
4. Instability of phenyl cation: Phenyl cation is not stabilized by resonance, SN1 ruled out
5. Electron repulsion: Nucleophile repelled by electron-rich arene ring

Conditions required:

• High temperature and pressure
• Strong electron-withdrawing groups (NO₂) ortho/para to halogen

Dow's Process (Replacement by hydroxyl group)

Chlorobenzene can be converted to phenol by heating with aqueous NaOH at 623K and 300 atm pressure.

Effect of NO₂ group

The presence of electron-withdrawing NO₂ group at ortho and para positions increases reactivity of haloarenes towards nucleophilic substitution. The NO₂ group withdraws electron density from the ring and stabilizes the carbanion intermediate through resonance. No effect is observed at meta position.

Benzyne Mechanism

Under extreme conditions, elimination-addition occurs via benzyne intermediate.

Reaction with Metals

Haloarenes undergo sodium-metal coupling reactions in dry ether to form larger hydrocarbons.

Wurtz-Fittig Reaction

A mixture of an alkyl halide and an aryl halide reacts with sodium in dry ether to give an alkylarene.

Example: CH₃Cl + C₆H₅Cl + 2Na ──dry ether──► C₆H₅CH₃ + 2NaCl

Fittig Reaction

Two molecules of an aryl halide react with sodium in dry ether to give a biaryl compound.

Example: 2C₆H₅Cl + 2Na ──dry ether──► C₆H₅-C₆H₅ + 2NaCl

PART 7 – POLYHALOGEN COMPOUNDS

Chloroform (CHCl₃)

• Preparation: Chlorination of methane or by haloform reaction
• Properties: Sweet-smelling liquid, heavy
• Uses: Previously used as anaesthetic, now limited
• Harmful effects: Carcinogenic, liver damage

Carbon Tetrachloride (CCl₄)

• Preparation: Exhaustive chlorination of methane
• Properties: Colourless liquid, non-flammable
• Uses: Solvent, fire extinguisher (now limited)
• Harmful effects: Liver and kidney damage, ozone depletion

Freons (CCl₂F₂, CCl₃F)

• Preparation: Manufactured via halogen exchange
• Properties: Colourless gases, non-toxic
• Uses: Refrigerants, propellants
• Harmful effects: Ozone-depleting (banned by Montreal Protocol)

DDT (C₁₄H₉Cl₅)

• Preparation: Reaction of chloral with chlorobenzene
• Properties: White crystalline solid
• Uses: Potent insecticide (banned)
• Harmful effects: Bioaccumulation, environmental persistence

Iodoform (CHI₃)

• Preparation: Haloform reaction of methyl ketones with I₂/NaOH
• Properties: Yellow crystalline solid, characteristic odour
• Uses: Antiseptic

📚 FINAL REVISION SHEET

Key Reactions to Memorise

Memory Tricks Summary

Board Exam Tips

🔄 IMPORTANT CONVERSIONS (Maximum 2-Step Reactions)