Explanation: The structure of methanol is CH₃OH. To convert methanol to ethanoic acid (the number of carbon atoms is increasing from one carbon atom to two carbon atoms) we need ethanenitrile and we can get ethanenitrile from methyl chloride.

Note: If the number of carbon atoms is increasing, we need a nitrile group and if the number of atoms is decreasing, we need an amide group (Hoffman Bromamide reaction)

Step 1: Convert methanol into methyl chloride

Methanol (CH₃OH) reacts with thionyl chloride (SOCl₂) or PCl₅ or PCl₃ to form methyl chloride (CH₃Cl) by the replacement of OH group by Cl atom.

Step 2: Convert methyl chloride into ethanenitrile

Methyl chloride (CH₃Cl) reacts with ethanolic NaCN/ KCN to form ethanenitrile (CH₃CN) by the removal of NaCl / KCl

Step 3: Convert ethanenitrile into ethanoic acid

Ethanenitrile (CH₃CN) undergoes hydrolysis to form ethanoic acid (final product) which has one carbon atom more than the ethanenitrile.