Halogen Containing Compounds - Complete Summary

Introduction to Halogen Compounds

Compounds derived from hydrocarbons by the replacement of one or more hydrogen atoms by the corresponding number of halogen atoms are termed as halogen derivatives.

Classification: Halogen derivatives of saturated hydrocarbons (Alkanes) - Halo-alkanes; Unsaturated hydrocarbons (Alkenes and alkynes) - Halo-alkene or alkyne; Aromatic hydrocarbons (Arenes) - Halo-arenes.

Key Notes

Order of reactivity: F₂ > Cl₂ > Br₂ > I₂ for alkanes; 3° > 2° > 1° alkanes.

Preparation of Alkyl Halides

From Alkanes

R-H + X₂ → R-X + HX (Free radical mechanism, hv or heat)

Example: CH₃CH₂CH₃ + Cl₂ → CH₃CHClCH₃ (45%) + CH₃CH₂CH₂Cl (55%)

From Alkenes

R-CH=CH₂ + HX → R-CHX-CH₃ (Markovnikov's rule)

Order: HI > HBr > HCl; Peroxide effect with HBr.

From Alcohols

Method	Reagent	Notes
Groove’s Process	HX + ZnCl₂	Reactivity: HI > HBr > HCl > HF; 3° > 2° > 1° alcohols
With PCl₅/PCl₃	PCl₅ or PCl₃	Good for 1°; Poor for 2°/3°
Darzens Process	SOCl₂ + Pyridine	SN₂ mechanism

Hunsdiecker Reaction

R-COOH → R-Ag + Br₂ → R-Br + CO₂ + AgBr

Reactivity: 1° > 2° > 3°; Best for Br.

Finkelstein Reaction

R-Cl/Br + NaI → R-I + NaCl/Br

Other Methods

KI + H₃PO₄ + ROH

(PhO)₃P + X₂ + ROH (Rydon)

Zn/Cu + HCl + R-X (Dihalide)

RMgX + X₂

ROR + PCl₅

Properties of Alkyl Halides

Physical Properties

State & Solubility

CH₃F, CH₃Cl, CH₃Br, C₂H₅Cl: Gases

Up to C₁₈: Liquids; Higher: Solids

Insoluble in water; Soluble in organic solvents

BP & Density

BP: RI > RBr > RCl > RF

Increases with alkyl size

Density: I/Br > water; Cl/F < water

Beilstein test: Green flame on Cu wire.

Alkyl iodides turn violet/brown in light: 2RI → R₂ + I₂

Chemical Properties

Nucleophilic Substitution (SN)

Reactivity: RI > RBr > RCl; 3° > 2° > 1°

Reaction	Reagent	Product
Hydrolysis	Aq. KOH/AgOH	ROH
With Alkoxide	R'ONa	ROR' (Ether)
With Cyanide	KCN/AgCN	RCN/RNC
With Ammonia	NH₃	RNH₂ → R₄N⁺
With Nitrite	KNO₂/AgNO₂	RONO/RNO₂

Elimination Reactions

R-CH₂-CH₂X + Alc. KOH → R-CH=CH₂ + HX + KX

E1/E2 mechanisms; Order: RI > RBr > RCl; 3° > 2° > 1°

Miscellaneous

Reduction

RX + 2H → RH + HX (Zn/HCl or LiAlH₄)

Wurtz Reaction

2RX + 2Na → R-R + 2NaX

Grignard

RX + Mg → RMgX (Dry ether)

Friedel-Crafts

C₆H₆ + RX + AlCl₃ → C₆H₅R + HX

Dihalides: Preparation & Properties

Gem Dihalides

Method	Example
From Alkyne	RC≡CH + 2HX → R-CX₂-CH₃
From Carbonyl	RCHO + PCl₅ → RCHCl₂ + POCl₃

Vicinal Dihalides

Method	Example
From Alkene	R-CH=CH-R + X₂ → R-CHX-CHX-R
From Glycol	R-CH(OH)-CH(OH)-R + PCl₅ → R-CHCl-CHCl-R

Properties

Physical

Colourless liquids with pleasant smell

Insoluble in water; Soluble in organics

BP/MP ∝ Molecular mass

Vicinal > Gem reactivity

Chemical

Aq. KOH: RCHX₂ → RCHO

Alc. KOH: RCHX-CH₂X → RC≡CH

Zn dust: Alkene

KCN: R(CH₂)₂(CN)₂ → R(CH₂)₂(COOH)₂

Trihalides: Chloroform & Iodoform

Chloroform (CHCl₃)

Preparation

Lab/Industrial

Ethanol/Acetone + Bleaching powder

CCl₄ + Fe/H₂O → CHCl₃

Chloral hydrate + NaOH

Properties

Reaction	Product
Oxidation (Air/Light)	COCl₂ (Phosgene)
Reduction (Zn/HCl)	CH₄ or CH₂Cl₂
Chlorination	CCl₄
Hydrolysis	HCOOH
Nitration	CCl₃NO₂ (Chloropicrin)
With Ag	HC≡CH
With Acetone	Chloretone
Reimer-Tiemann	Salicylaldehyde
Carbylamine	RNC

Uses: Solvent, anaesthetic (old), insecticide, preservation.

Tests: Carbylamine, Tollen's (silver mirror), No ppt with AgNO₃ (pure).

Iodoform (CHI₃)

Preparation

Ethanol/Acetone + I₂ + Alkali → CHI₃ (Iodoform reaction)

Industrial: Electrolysis of KI + Ethanol + Na₂CO₃.

Properties

Similar to CHCl₃; Yellow solid, antiseptic smell.

Important Points for Exams

High Yield Concepts

Reactivity orders: Halogens, Alcohols, Alkyl halides
SN vs Elimination: Base strength, temperature
Peroxide effect: Only HBr
Hunsdiecker: Ag salt + Br₂
Chloroform oxidation to phosgene; Prevention with ethanol
Iodoform test: For CH₃CH(OH)- or CH₃CO- groups

JEE Main Weightage

Typically 2-4 questions from halo compounds. Focus on preparations, reactions, mechanisms.

Weightage High (2-4 Qs)