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Oxidation of Methylene Groups to Carbonyl Groups

Oxidation of Methylene Groups to Carbonyl Groups: Industrial Application

Of technical interest is the synthesis of acetophenone from ethylbenzene by oxidation with atmosperic oxygen. In 1949, this method replaced the Friedel-Crafts reaction which was used up to that date to prepare acetophenone.

Pressurized air is fed at 5 bar and 130°C into a reaction vessel containing ethylbenzene and manganese or cobalt salts of oil-soluble fatty or naphthalene acids as catalysts. The reaction mixture is transferred to a second vessel at which time pressurized air is introduced again. The oxidation is finished after 1.5 h. During this time, 16% of ethylbenzene is converted to acetophenone in the first vessel and 10% in the second. Since at low conversion rates 1-phenylethanol and at higher rates benzoic acid are formed as the main side products, the reaction is stopped at this point. After extracting benzoic acid with a 10% solution of sodium hydroxide, the reaction mixture is distilled. Ethylbenzene and catalyst are re-used for oxidation. The mixture of acetophenone (68%) and 1-phenylethanol (32%) is catalytically dehydrogenated at 200°C. After conversion of the alcohol is completed, acetophenone is distilled in a yield of 85%, based on converted ethylbenzene.

Today, work-up of the side products from the Hock phenol synthesis provides sufficient acetophenone to cover the demand and eliminate the need for a specific synthesis of the ketone.

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