Organic Compounds Containing Halogens

Master haloalkanes, haloarenes, their preparation, reactions, and mechanisms for JEE Chemistry.

Halogen compounds are important intermediates in organic synthesis and have many industrial applications.

Overview

graph TD
    A[Halogen Compounds] --> B[Haloalkanes]
    A --> C[Haloarenes]
    B --> B1[Preparation]
    B --> B2[Reactions]
    C --> C1[Properties]
    C --> C2[Reactions]

Classification

By Halogen Position

  • Primary: Halogen on primary carbon
  • Secondary: Halogen on secondary carbon
  • Tertiary: Halogen on tertiary carbon

Nature of C-X Bond

  • Polar covalent
  • Carbon is electrophilic
  • Bond strength: C-F > C-Cl > C-Br > C-I
  • Reactivity: C-I > C-Br > C-Cl > C-F

Preparation of Haloalkanes

From Alcohols

$$\text{R-OH} + \text{HX} \rightarrow \text{R-X} + \text{H}_2\text{O}$$

Reactivity: 3° > 2° > 1°

Other reagents: PCl₃, PCl₅, SOCl₂

Halogenation of Alkanes

$$\text{R-H} + \text{X}_2 \xrightarrow{h\nu} \text{R-X} + \text{HX}$$

Free radical mechanism

From Alkenes

Addition of HX:

$$\text{R-CH=CH}_2 + \text{HX} \rightarrow \text{R-CHX-CH}_3$$

Follows Markovnikov’s rule

Reactions of Haloalkanes

Nucleophilic Substitution

$$\text{R-X} + \text{Nu}^- \rightarrow \text{R-Nu} + \text{X}^-$$

SN1 Mechanism:

  • Two steps: ionization then attack
  • Favored by: 3° substrate, polar protic solvent
  • Racemization occurs

SN2 Mechanism:

  • One step: backside attack
  • Favored by: 1° substrate, polar aprotic solvent
  • Walden inversion (stereochemistry inverted)

Elimination (E1 and E2)

Forms alkenes by removing HX.

Saytzeff’s Rule: More substituted alkene is major product.

JEE Tip
SN2 vs E2: Strong base + 1° → SN2. Bulky base + 2°/3° → E2.

Haloarenes

Preparation

Sandmeyer Reaction:

$$\text{ArN}_2^+ + \text{CuX} \rightarrow \text{ArX}$$

Properties

  • Less reactive than haloalkanes
  • Due to resonance with benzene ring
  • C-X bond has partial double bond character

Reactions

Nucleophilic Substitution: Difficult (activated by -NO₂ groups)

Electrophilic Substitution: ortho/para directing but deactivating

Polyhalogen Compounds

Chloroform (CHCl₃)

  • Anesthetic (historically)
  • Oxidizes to phosgene (toxic)

Carbon Tetrachloride (CCl₄)

  • Fire extinguisher (not used now)
  • Dry cleaning solvent

Freons

  • CFCs like CCl₂F₂
  • Ozone depletion
  • Replaced by HFCs

DDT

  • Insecticide
  • Persistent organic pollutant
  • Banned in many countries

Practice Problems

  1. Arrange in order of SN2 reactivity: 1-bromobutane, 2-bromobutane, 2-bromo-2-methylpropane

  2. Explain why chlorobenzene is less reactive than chloroethane in nucleophilic substitution.

  3. What product is obtained when 2-bromopentane reacts with alcoholic KOH?

Quick Check
Why does SN1 lead to racemization while SN2 leads to inversion?

Further Reading