Oxidation of Methylene Groups to Carbonyl Groups
Oxidation of Methylene Groups to Carbonyl Groups: Selection of Reaction Conditions
Oxidation of a methylene group activated by an adjacent functional group proceeds easier and with higher selectivity. Activating groups in decreasing order of activity are shown below.
Oxidation to carboxylic acids can be interrupted by the proper selection of oxidizing agents and reaction conditions. The oxidizing agent has to react selectively because a carbonyl group is oxidized faster than a methylene group, for example. The catalyst of choice is selenium dioxide. Cobalt or manganese salts are preferred for the oxidation of aryl alkanes. Since the resulting carbonyl compounds are oxidized faster than the corresponding alkyl residues, they have to be removed from the reaction mixture.
Linear alkanes do not react with the usual oxidizing agents, such as potassium permanganate, but have to be oxidized with either oxygen in the presence of manganese or cobalt salts at temperatures above 100 or with hot sulfuric acid. The product mixture consists of alcohols, ketones, aldehydes, and other oxidation products. Additionally, C-C cleavage and rearrangements contribute to an even larger number of side products. Since the oxidation of saturated hydrocarbons cannot be applied to the preparation of specific compounds, the method has no preparative value.
Example: The catalytic oxidation of butane with oxygen and cobalt acetate as catalyst at 165 yields mainly a mixture of ethyl methyl ketone, acetic acid and methyl- and ethyl acetate in the product ratio of 1:15:3.