Question

Account for the following:
Aromatic carboxylic acids do not undergo Friedel-Crafts reaction.

Answer 1

Hint:Friedel Crafts reaction is of two types-alkylation and acylation. Both are types of electrophilic aromatic substitution reaction. Alkylation reaction introduces alkyl group to the aryl ring which produces alkyl arenes. The acylation reaction introduces acyl groups to the aryl ring which gives aryl ketones as products.

Complete step by step solution:
The reacting species will be mostly substituted or unsubstituted benzene and acyl halide in the presence of a Lewis acid as catalyst. The general reaction of Friedel-Crafts reaction is given below:
${{Ar}} - {{H}} + {{RCOCl}}\xrightarrow{{{{AlC}}{{{l}}_3}}}{{ArCOR}}$
Friedel-Crafts acylation reaction requires electron donating groups or ring activating groups as reactants. The electron density in $ - {{COOH}}$ group increases since it is an electron-withdrawing group. Some of the groups which are favored for the acylation reaction are alkyl $ - {{R}}$, alkoxy $ - {{OR}}$, halogen ${{X}}$, acetamido groups ${{C}}{{{H}}_3}{{CONH - }}$ etc.
In alkylation reactions, the reacting species are mostly substituted or unsubstituted benzene, alkyl halide in the presence of a Lewis acid catalyst. There is a negative resonance effect of electrons from the benzene ring. The general reaction of Friedel-Crafts reaction is given below:
${{Ar}} - {{H}} + {{RCl}}\xrightarrow{{{{AlC}}{{{l}}_3}}}{{ArH}}$
When $ - {{COOH}}$ group is attached to an aryl ring, it gets polymerized and degraded. Since it is very less reactive, it becomes unavailable for the reaction.
Similarly, Friedel-Crafts reaction does not take place in the case of phenol and aniline. There is a chance of forming a coordinate bond between the catalyst and the lone pair of corresponding functional groups.

Note:
If the carboxylic group is not directly attached to the aryl group, but bonded to an alkyl group which is attached to the aryl ring, then it undergoes Friedel-Crafts reaction. This undergoes intramolecular cyclization. But it is influenced by the chain length of the alkyl group.