Friedel Craft Acylation reaction is a type of electrophilic aromatic substitution reaction, with the acylium ion as the electrophile. In this reaction, a new Carbon-Carbon bond is formed by the replacement of the proton with the acyl group thereby introducing ketone functionality in the molecule.
The general mechanism of FC acylation shows three reagents: Benzene as the nucleophilic starting material, acyl chloride, and Lewis acid as the source of the electrophile.
The first step of the reaction is coordination of Chloro of the acyl chloride with AlCl3. AlCl3 does not coordinate with oxygen even though it is a more electronegative atom than Chlorine because, the electrons of oxygen in the carbonyl are tightly held. The lone pair of the electrons delocalizes over the π bond pushing away the less electronegative Chlorine when it forms a complex with AlCl3. The acylium ion formed is resonance stabilized and reacts with Benzene in the second step to give the carbocation intermediate that is also stabilized by resonance. The last step is the abstraction of a proton by base AlCl4, to give acylated Benzene, HCl and AlCl3 as the final products.
This regenerated AlCl3 is not available for the reaction as it co-ordinates with the oxygen of the keto hence AlCl3 is a reagent and not a catalyst. On completion of the reaction, the product is isolated by treatment with water to break the keto-AlCl3 complex resulting in aryl ketone as the desired product. AlCl3 undergoes hydrolysis to give Aluminium hydroxide that precipitates out and can be filtered.
Groups that increase the nucleophilicity of the Benzene ring that is electron donating or ring activating are preferred for electrophilic substitution reaction. Such groups can stabilize the carbocation transition state resulting in a faster reaction. The electron withdrawing substituent or ring deactivators do not undergo FC acylation reaction.
The acyl chloride if not readily available can be made from a carboxylic acid derivative on treatment with Thionyl chloride or acid anhydride. As the Lewis acid AlCl3 is a reagent, it is required in stoichiometric quantities as the first step of the reaction is the formation acylium ion as an acid-base complex. The acylium ion is not free carbocation but exists as strong ion pair complex. It does not undergo any rearrangement reaction and are stable. Other sources for generation of acylium ion are acid anhydride and AlCl3, carboxylic acid and mineral acid, carboxylic acid anhydride and mineral acid.
Several of the drawbacks of Friedel Craft Alkylation such as isomerisation and polyalkylation are overcome by FC Acylation Reaction. For Friedel Craft alkylation, the introduction of the alkyl group in the product activates the ring for more substitution resulting in polyalkylation. However, an FC acylated product is ring deactivating and does not undergo further substitution reaction.
The acyl group can be reduced to the desired mono-substituted alkyl Benzene making this reaction the preferred way for introducing a -CH2- group that is a methylene group, next to the Benzene ring to make alkyl benzenes. The reduction methods of reducing acyl to alkyl are Clemmenson, Wolff Kishner, Mozingo reduction and hydrogenation over Palladium.
A drawback of FC acylation method is alkylation. The branched alkyls favor the loss of CO on forming acylium ion to give stable alkyl cation that reacts to thereby form alkyl benzene. For, e.g., treatment of Pivaloyl chloride with AlCl3 and Benzene results in the formation of tert-butyl benzene. The loss of CO group is due to the stability of tert-alkyl carbocation.
The second drawback is that this method introduces only keto group and not formyl group. The formylation with Formyl chloride is not possible to give aldehyde product as it is an unstable reagent. Lastly, Benzene attached to electron withdrawing substituent does not undergo FC acylation reaction. E.g., nitrobenzene, aryl ketone, trifluoromethyl benzene.
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