Solution:Concept- Acetyl chloride is produced by heating acetic acid with phosphorus pentachloride. The COOH group is changed to the COCl group in this reaction.
CH3COOH + PCl5 = CH3COCl+HCl +POCl3
The general reaction given by carboxylic acid is nucleophilic addition elimination
also known as acyl substitution
Acyl chlorides :
most reactive of the acid derivatives
They have chloride ions with excellent leaving groups that are protonated to the carbonyl carbon atom.
The carbonyl carbon atom undergoes nucleophilic addition as the reaction's first step, followed by an elimination that regenerates the carbon-oxygen double bond.
Hence, the correct option is 3.