Haloalkanes and Haloarenes

Notes

Introduction

Haloalkanes and Haloarenes are organic compounds in which one or more hydrogen atoms are replaced by halogen atoms (F, Cl, Br, I).

Haloalkanes (Alkyl Halides)

General Formula:

R – X

Example:

CH₃Cl (Methyl chloride)

C₂H₅Br (Ethyl bromide)

The halogen is attached to an sp³ hybridized carbon atom.

Haloarenes (Aryl Halides)

General Formula:

Ar – X

Example:

Chlorobenzene (C₆H₅Cl)

Bromobenzene

Here the halogen is directly attached to an sp² hybridized aromatic carbon.

Classification

According to Number of Halogen Atoms

Monohalo compounds

Dihalo compounds

Trihalo compounds

Polyhalo compounds

According to Carbon Atom

Primary (1°)

RCH₂X

Example:

CH₃CH₂Cl

Secondary (2°)

R₂CHX

Example:

CH₃CHClCH₃

Tertiary (3°)

R₃CX

Example:

(CH₃)₃CCl

Nomenclature

IUPAC Rules

Example

CH₃Cl → Chloromethane

CH₃CH₂Br → Bromoethane

C₆H₅Cl → Chlorobenzene

Methods of Preparation

Haloalkanes

1. From Alcohols

ROH + HX → RX + H₂O

Reagents:

HCl/ZnCl₂ (Lucas Reagent)

HBr

HI

2. Darzens Process

ROH + SOCl₂ → RCl + SO₂ + HCl

Why SOCl₂ is preferred?

Because gaseous by-products escape, giving pure alkyl chloride.

3. Swarts Reaction

Preparation of Alkyl Fluorides

RCl + AgF → RF

4. Finkelstein Reaction

Preparation of Alkyl Iodides

RCl + NaI (Dry Acetone)

↓

RI + NaCl↓

Preparation of Haloarenes

From Benzene

C₆H₆ + Cl₂

FeCl₃

↓

C₆H₅Cl

Physical Properties

✔ Polar molecules

✔ Insoluble in water

✔ Soluble in organic solvents

✔ Boiling point increases with molecular mass

✔ Density increases from F → I

Chemical Reactions of Haloalkanes

1. Nucleophilic Substitution

RX + OH⁻

↓

ROH

RX + CN⁻

↓

RCN

RX + NH₃

↓

RNH₂

SN1 Mechanism

Occurs in:

✔ Tertiary Alkyl Halides

Steps:

Carbocation formation

Nucleophile attack

Characteristics

Two-step reaction

Racemization occurs

First-order kinetics

SN2 Mechanism

Occurs in:

✔ Primary Alkyl Halides

Characteristics

One-step reaction

Backside attack

Inversion of configuration

Second-order kinetics

SN1 vs SN2

SN1 SN2

Two-step One-step

Carbocation formed No carbocation

Racemization Inversion

3° favoured 1° favoured

Elimination Reaction (β-Elimination)

Alcoholic KOH

↓

Alkene

Example

CH₃CH₂Br

↓

CH₂=CH₂

Reaction…