Question

The reagent which does not give acid chloride on reacting with carboxylic acid is

• A. $PC{l}_5$
• B. $SOC{l}_2$
• C. $C{l}_2$
• D. $PC{l}_3$

Answer 1

The correct option is C $C{l}_2$

The -OH of carboxylic acid is a very bad leaving group as it is a strong base, $C{l}^{-}$ on the other hand, is a weak base. We want to convert the carboxylic acid into a more reactive acid chloride via an acyl substitution reaction.

Thus, we use reagents that have a hetero atom ($PorS$, which can expand their octet by acting as a Lewis acid. The lone pair on any of the oxygen atoms of the $COOH$ group attacks the central atom of the reagent and gets a chloride to leave. This chloride, in turn, provides the nucleophile for the acyl substitution.

Except chlorine gas, the others are good at this sort of behaviour.