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Oxidation of Methylene Groups to Carbonyl Groups

Oxidation of Methylene Groups to Carbonyl Groups: Introduction

The oxidation of non-activated methylene groups with oxygen gives a product mixture with carboxylic acid as the main product. Carbonyl compounds are only obtained in small amounts, rendering this method not suitable for the preparation of carbonyl compounds.

On the other hand, the oxidation of activated methylene groups is much more selective and, under certain conditions, yields ketones. However, in many cases yields and selectivity are not sufficient for the preparation of sensitive substrates. The methodology is mostly used in industry.

Starting materials

  • Alkane or alkyl group
  • Oxygen
  • Catalysts

Reaction principle

A radical mechanism is generally accepted for the oxidation of methylene groups with oxygen in the presence of manganese and cobalt salts at temperatures above 100°C. The reaction yields a product mixture consisting of alcohols, ketones, aldehydes, carboxylic acids, and other oxidation products. Initially, autoxidation of the substrates with oxygen leads to peroxides which in most cases are converted to ketones without isolation. In order to prevent further oxidation and C-C cleavage to carboxylic acids, oxidation has to be stopped at the ketone level by applying suitable reaction conditions. An alternative method involves removal of the ketones from the reaction mixture.

Example:

Synthesis of acetophenone

Application

Industrial production of acetophenone from ethyl benzene.

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