No Friedel-Crafts reaction of benzoic acid

Cause Analysis of No Friedel-Crafts Reaction of Benzoic Acid

in organic chemical reactions, Friedel-Crafts reaction is one of the important alkylation and aromatization reactions. It is usually by Lewis acid catalyst (such as AlCl3) to introduce alkyl or acyl groups into aromatic compounds. As an aromatic compound, benzoic acid is not easy to participate in Friedel-Crafts reactions. This article will explore the reasons for the non-Friedel-Crafts reaction of benzoic acid and analyze the mechanism behind it.

Benzoic acid structure characteristics

We need to understand the molecular structure of benzoic acid. The structure of benzoic acid contains a benzene ring and a carboxyl (-COOH) substituent. The benzene ring has a high electron density, so it is an ideal electrophile recipient and exhibits strong aromaticity in many reactions. The carboxyl substituent of benzoic acid is an important factor, which affects the electronic effect of the benzene ring.

EFFECT OF CARBOXYL ON BENZENE RING

One of the main reasons why benzoic acid does not participate in the Friedel-Crafts reaction is the electronic effect of its carboxyl group. Carboxyl group is an electron attracting group, which reduces the electron density of the benzene ring, especially the para and ortho carbon atoms, by resonance effect and induction effect. Therefore, the benzene ring in benzoic acid is not as easy to accept the attack of electrophiles as other aromatic compounds without substituents. In the Friedel-Crafts reaction, the aromatic compound needs to provide electron density to react with the electrophile of the alkyl or acyl group, and the carboxyl group of benzoic acid significantly inhibits this electron supply.

carboxyl group induced effect and resonance effect

Specifically, the carboxyl group in benzoic acid draws electrons from the benzene ring by its inductive effect (-I effect), so that the electrophilicity of the benzene ring decreases. The resonance effect of the carboxyl group (-M effect) will also push the electron cloud to the carboxyl group, thus further weakening the electron supply ability of the benzene ring. The Friedel-Crafts reaction requires the aromatic ring to provide sufficient electron density to react with the electrophile, while the carboxyl group of benzoic acid greatly reduces its electron donating ability.

Lewis acids and benzoic acid reactivity

Friedel-Crafts reactions usually require Lewis acid catalysts, such as AlCl3, which polarize alkyl halides or acyl halides to produce strong electrophiles. In most aromatic compounds, the production of electrophiles is the key to the smooth progress of the reaction. The carboxyl group of benzoic acid will complex with Lewis acid to form a stable complex, which inhibits the reaction. Even though benzoic acid is capable of interacting with Lewis acids, due to its low reactivity, this complex is not sufficient to drive Friedel-Crafts reactions to occur.

Other Aromatic Compounds vs. Benzoic Acid

In contrast, some aromatic compounds without electron-attracting groups, such as benzene, can participate in Friedel-Crafts reactions smoothly. An aromatic ring without an electron-attracting group such as a carboxyl group provides sufficient electron density to allow the electrophilic reagent to attack smoothly to complete the alkylation or acylation reaction. Benzoic acid, on the other hand, cannot participate in the reaction as effectively as these aromatic compounds due to the electronic effect of the carboxyl group.

Summary

The fundamental reason for the non-Friedel-Crafts reaction of benzoic acid lies in the electronic effect of the carboxyl group on the benzene ring in its molecular structure. The carboxyl group, as an electron attracting group, reduces the electron density of the benzene ring through the induction effect and resonance effect, thus inhibiting the ability of benzoic acid to participate in the electrophilic Friedel-Crafts reaction. Complexation between the carboxyl group of benzoic acid and the Lewis acid catalyst also further hinders the reaction. Therefore, understanding the structure and reactivity of benzoic acid is the key to explain its non-participation in Friedel-Crafts reactions.