Methods of Preparation - From Alcohols (II)

Description:

Reaction of alcohols with Phosphorus halides (PX₅ or PX₃)

Reaction with phosphorus halides replaces the −OH group of alcohols with corresponding halides.

ROH + PCl₅ → RCl + POCL₃ + HCl

Phosphorus tribromide and triiodide are prepared in-situ by reacting red phosphorus with bromine and iodine.

3R - OH + PX₃ → 3R - X + H₃PO₃ (X = Cl, Br)

In the reaction of alcohols with PX₃, 3 molecules of alkyl halide are obtained.

Reaction of alcohols with Thionyl chloride (SOCl₂)

Reaction with SOCl₂ is preferred to prepare alkyl chlorides as the by- products obtained are gaseous in nature and escapes the reaction mixture. Thus, pure alkyl halide is obtained.

CH₃CH₂OH + SOCl₂ → CH₃CH₂Cl + SO₂ ↑ + HCl ↑

Problem − Why H₂SO₄ is not preferred for use in the reaction given below?

Solution − H₂SO₄ is a very strong acid such that it converts I⁻ → I₂. Thus, nucleophile (I⁻) required for the nucleophilic attack gets exhausted to I₂. Hence, weak acids like H₃PO₄ is preferred.