Halogen Derivatives 

Halogen Derivatives – Introduction

Halogen derivatives are organic compounds in which one or more hydrogen atoms of hydrocarbons are replaced by halogen atoms (F, Cl, Br, I).

Methods of Preparation of Mono Haloalkanes

1. From Alkanes

CH₄ + Cl₂ → CH₃Cl + HCl (hν)

• CH₂Cl₂, CHCl₃ and CCl₄ are also formed
• Not preferred due to poor yield and difficult separation

2. From Alcohols

• R–OH + HX → R–X + H₂O (ZnCl₂)
• ROH + KBr + H₂SO₄ → RBr + H₂O
• ROH + KI + H₃PO₄ → RI + H₂O
• R–OH + PCl₅/PCl₃ → R–Cl
• R–OH + SOCl₂ (pyridine) → R–Cl + SO₂↑ + HCl↑

Note: Alkyl halide obtained using SOCl₂ is pure as gaseous by-products escape.

3. Finkelstein Reaction (Halogen Exchange)

R–Br + NaI (acetone) → R–I + NaBr

4. Swarts Reaction

R–Cl / R–Br + AgF / Hg₂F₂ → R–F

5. From Alkenes

• Markovnikov addition: forms 2° alkyl halide
• Anti-Markovnikov (peroxide effect): forms 1° alkyl halide

6. Hunsdiecker Reaction

RCOOAg + Br₂ → R–Br + CO₂ + AgBr

• Product has one carbon less than parent acid
• Yield order: 1° > 2° > 3°

Physical Properties of Alkyl Halides

• Boiling point: R–Cl < R–Br < R–I
• Insoluble in water (no H-bonding)
• Density: R–I > R–Br > R–Cl
• Most are denser than water

Chemical Properties of Alkyl Halides

1. Nucleophilic Substitution Reactions

R–X + Nu⁻ → R–Nu + X⁻

Reactivity order: R–I > R–Br > R–Cl > R–F

• Hydrolysis: R–X + aq. KOH → R–OH
• Williamson ether synthesis (1° halides preferred)
• With NaSH → R–SH (thioalcohol)
• With NaCN → R–CN (major), R–NC (minor)
• With NaN₃ → R–N₃
• With AgNO₃ → nitroalkane (major)

2. Elimination Reaction (β-elimination)

R–CH₂–CH(X)–R′ + alc. KOH → Alkene

3. Reduction

R–X + LiAlH₄ → R–H

4. Reaction with Metals

• Mg → RMgX (Grignard reagent)
• Na/dry ether → R–R (Wurtz reaction)
• Zn → R–Zn–R
• Li → R–Li

Trihaloalkanes (Haloform: CHX₃)

Preparation

• From trihalocarbonyl compounds using NaOH
• From alcohols containing –CH₃ group (Haloform reaction)
• From CCl₄ (industrial method)

Properties of Chloroform (CHCl₃)

• Colourless, sweet-smelling liquid
• Boiling point: 61°C
• Oxidises in air to form phosgene (COCl₂)

Precautions for Storage of Chloroform

• Stored in brown bottles
• Bottles filled completely (no air)
• 0.6–1% ethanol added as stabiliser

Tetrahaloalkanes (Carbon Tetrachloride – CCl₄)

Preparation

• CH₄ + Cl₂ (hν) → CCl₄
• CHCl₃ + Cl₂ → CCl₄

Uses

• Formerly used as fire extinguisher (now banned)
• Used in refrigerant preparation

Aryl Halides

Preparation

• From benzene diazonium chloride (Sandmeyer, Gattermann, Balz–Schiemann reactions)
• From benzene by halogenation (FeCl₃/FeBr₃)

Properties

• Undergo electrophilic substitution
• Halogens are ortho–para directing but deactivating
• Nucleophilic substitution possible with –NO₂ at o/p positions

Important Reactions

• Wurtz–Fittig reaction
• Fittig reaction
• Ullmann reaction
• Reaction with chloral → DDT

Uses of Chlorobenzene

• Preparation of phenol
• Manufacture of dyes, fungicides and preservatives
• Preparation of DDT