Let us consider benzoic acid as the substrate and as a reagent in both Fridel Craft Acylation and Friedel Craft Alkylation Reactions to see for any favorable reaction conditions.
Friedel-Craft Acylation Reaction
A type of electrophilic aromatic substitution reaction to introduce an acyl group (R-C=O) in the aromatic ring resulting in aryl ketones.The reacting species are usually but not always -benzene (substituted or unsubstituted), an acyl halide and a Lewis Acid catalyst. In short, it can be stated as the acylation of the aromatic rings by the Friedel-Craft reaction method.
When Benzoic acid is the substrate for Friedel-Craft Acylation Reaction
Benzoic acid being electron withdrawing deactivates the ring and does not undergo Friedel-Craft Acylation Reaction. Friedel-Craft Acylation Reaction prefers electron donating or ring activating groups in the substrate. Remember, Aniline and Phenol does not take part in Friedel-Craft Acylation reaction.Groups or substitutions attached to the benzene ring that favors Friedel-Craft Acylation Reaction are- alkyl, alkoxy, halogen, acetamido groups. Meta directing groups usually do not undergo Friedel-Craft Acylation Reaction.
When Benzoic acid is the reagent (source of the electrophile) for Friedel-Craft Acylation Reaction
Benzoic acid in the presence of strong acid -PPA (polyphosphoric acid), H2SO4 (sulfuric acid), TFA (trifluoroacetic acid), generate acylium ion (Ph-C≡O +) that is resonance stabilized and therefore will undergo Friedel-Craft Acylation reaction.
The other method commonly employed is to convert the carboxylic acid to more reactive acid chloride or acid anhydride. Remember- If the carboxylic group (-COOH) is not directly attached to the phenyl ring but is a part of the alkyl functional group, it can then undergo intramolecular cyclization reaction by the FC acylation method. Intramolecular cyclization would, of course, depend on the length of the alkyl chain.
Friedel-Craft Alkylation Reaction
A type of electrophilic aromatic substitution reaction to introduce an alkyl group (-R) in the arene ring giving alkyl arenes. The reacting species are usually but not always- benzene ring (substituted or unsubstituted), an alkyl halide and a Lewis Acid catalyst. In short, it can be stated as the alkylation of the aromatic rings by the Friedel-Craft reaction method.
When Benzoic acid is the substrate for Friedel-Craft Alkylation Reaction
The -COOH gr (carboxylic group) attached to the benzene ring is electron withdrawing and ring deactivating. Benzoic acid does not undergo Friedel-Craft Alkylation Reaction. The reagent then undergoes polymerization, degradation and is not available for the reaction with the substrates of low reactivity.
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