Question

An acyl halide is formed when $PC{l_5}$ reacts with an:
A. Acid
B. Alcohol
C. Amide
D. Ester

Answer 1

Let us first understand some basic concepts before we proceed with the solution for this question:
Acyl halides can be explained as the halogen substituted hydroxyl groups in organic compounds. Acyl halides are basically formed when the hydroxy or the -OH group of the hydroxyl or -COOH group gets replaced by a halogen. This is a basic substitution reaction.

Complete Step-by-Step Answer:
In order to solve this question, we must find the products formed by $PC{l_5}$ with all the given options.
Reaction with acid: the reaction of $PC{l_5}$ with a given acid would result in the formation of acid chlorides in the products. Acid chloride is the common name for acyl halide where chlorine is the halogen.

Reaction with alcohol: alcohols react in a rather violent manner with $PC{l_5}$, resulting in the emissive release of hydrochloric acid.
$R - OH + PC{l_5} \to R - Cl + POC{l_3} + HCl$

Reaction with amide: when amides are reacted with $PC{l_5}$, it results in the dehydration of the amide. The corresponding reaction can be given as:
$R - CON{H_2} + PC{l_5} \to R - C{(Cl)_2}N{H_2} + PC{l_3}$

Reaction with esters: reaction between esters and $PC{l_5}$ results in the formation of an alkyl halide, an acyl halide and also phosphorous oxychloride. Hence, only acyl halide cannot be obtained from esters.

Hence, Option A is the correct option.

Note: In the reaction of $PC{l_5}$ with carboxylic acid, the reaction goes firstly by a cyclic transition state with the removal of HCl. Then via a nucleophilic addition of chloride where the carbonyl is simultaneously protonated.