Preparation Of Benzoic Acid By Oxidation

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Preparation of Benzoic Acid by Oxidation

Abstract

Benzoic acid was synthesized by the oxidation of ethylbenzene with potassium permanganate. “…permanganate… abstract[s] a hydrogen atom…” (Gardner, et. al.). This affords a free radical compound which catches an OH and then converts to Acetophenone. “…the oxidation of ethylbenzene [by mild Mn oxidizing agents] affords mainly acetophenone…” (Cagnina, et. al., page 221). But oxidation with concentrated KMnO4 affords the acid from an acetophenon intermediate. In the synthesis of benzoic acid from ethylbenzene there are several substances formed as intermediates. Acetophenone -the third and last of these intermediates- is made through the withdrawal of electrons. “Electron withdrawings substituents accelerate the oxidation of ethylbenzene and promote the formation of acetophenone.” (Alsters, et. al., page 7797). Acetophenon is further oxidized and generates the final product, benzoic acid.

Introduction

Generally benzene would not react in an oxidation reaction because of its high level of stability. With four unsaturations, benzene typically undergoes substitutions. Benzene possesses aromaticity, a level of stability which is greater than what expected from any of its Lewis structures. The stability of benzene is quantified by calculating its heat of hydrogenation energy. The lower the energy, the more stable the substance will be.
Reacting a single bond with hydrogen gives off 28.6 kcal/mol for cyclohexene. Benzene’s energy of hydrogenation is expected to be three times this (85.2 kcal/mol) because it has three times as many p-bonds. However, we find its actual heat of hydrogenation energy is 49.3 kcal/mol. This gives approximately 36 kcal/mol more stability than what is expected. An explanation for this high level of stability is in the resonance structures of benzene. Where the true structure of benzene is somewhere between the two.
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