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Preparation of Haloalkanes

Synthesis from Alcohols, Hydrocarbons, and Halogen Exchange.

By chemca Team • Updated Jan 2026

Haloalkanes (Alkyl Halides) are organic compounds containing halogen atoms attached to an sp³ hybridized carbon. They are key intermediates in organic synthesis.

1. From Alcohols

Substitution of -OH Group

The hydroxyl group of an alcohol is replaced by halogen using various reagents.

A. With Hydrogen Halides (Groove's Process):

$$ R-OH + HCl \xrightarrow{ZnCl_2} R-Cl + H_2O $$

Reagent: Conc. $HCl$ + Anhydrous $ZnCl_2$ (Lucas Reagent).

B. With Thionyl Chloride (Darzen's Process): This is the best method because the byproducts ($SO_2$ and $HCl$) are gases and escape easily, leaving pure alkyl halide.

$$ R-OH + SOCl_2 \xrightarrow{Pyridine} R-Cl + SO_2 \uparrow + HCl \uparrow $$

C. With Phosphorus Halides:

$$ R-OH + PCl_5 \rightarrow R-Cl + POCl_3 + HCl $$ $$ 3R-OH + PCl_3 \rightarrow 3R-Cl + H_3PO_3 $$

2. From Hydrocarbons

A. From Alkenes (Addition of HX)

Markovnikov's Rule: The negative part of the addendum ($X^-$) attaches to the carbon with fewer hydrogen atoms.

$$ CH_3-CH=CH_2 + HBr \rightarrow \underset{\text{2-Bromopropane}}{CH_3-CH(Br)-CH_3} $$

Anti-Markovnikov (Peroxide Effect): Occurs only with HBr in the presence of peroxide.

$$ CH_3-CH=CH_2 + HBr \xrightarrow{(PhCO)_2O_2} \underset{\text{1-Bromopropane}}{CH_3-CH_2-CH_2-Br} $$

B. From Alkanes (Free Radical Halogenation)

Reaction with Chlorine or Bromine in UV light gives a mixture of isomeric mono- and poly-haloalkanes.

$$ CH_3CH_2CH_3 + Cl_2 \xrightarrow{h\nu} CH_3CH_2CH_2Cl + CH_3CHClCH_3 $$

Reactivity of H: $3^\circ > 2^\circ > 1^\circ$.

3. Halogen Exchange Reactions

Specific Synthesis

A. Finkelstein Reaction (Preparation of Alkyl Iodides): Reaction of Alkyl Chloride/Bromide with NaI in dry acetone.

$$ R-X + NaI \xrightarrow{\text{Dry Acetone}} R-I + NaX \downarrow $$

Driven by precipitation of NaCl/NaBr in acetone.

B. Swarts Reaction (Preparation of Alkyl Fluorides): Heating Alkyl Chloride/Bromide with metallic fluorides ($AgF, Hg_2F_2, CoF_2$).

$$ R-Br + AgF \rightarrow R-F + AgBr $$

4. Hunsdiecker Reaction

From Silver Salts of Acids

Reaction of silver salts of carboxylic acids with Bromine in $CCl_4$ yields alkyl bromides with one less carbon (Decarboxylation).

$$ R-COOAg + Br_2 \xrightarrow{CCl_4, \Delta} R-Br + CO_2 + AgBr $$

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