Preparation of ketones:
Ketones are the organic compound containing carbonyl groups (C=O). The general formula for a ketone is R(C=O)R’, where R and R’ can be alkyl or aryl groups. They are classified into two categories by their substituents: symmetrical ketones (when two identical groups are attached to the carbonyl group) and asymmetrical ketones (when two different groups are appended to the carbonyl group). Many methods exist for the preparation of ketones at industrial scale and in laboratories. Standard methods include oxidation of alcohol, hydrocarbons, etc. Some general methods for the preparation of ketones are explained below:
Preparation of ketones from acyl chlorides: Acyl chlorides upon treatment with Grignard reagent and a metal halide, yields ketones. For example: when cadmium chloride is reacted with the Grignard reagent, dialkyl cadmium is formed. Dialkylcadmium thus formed is further reacted with acyl chlorides to form ketones.
Preparation of ketones from nitriles: Treatment of nitriles with Grignard reagent upon further hydrolysis yields ketones.
Preparation of ketones from benzenes or substituted benzenes: Electrophilic aromatic substitution of a benzene ring with acid chlorides in the presence of a Lewis acid such as AlCl3 results in the formation of ketones. This reaction is popularly known as Friedel Craft’s acylation reaction.
Preparation of ketones by dehydrogenation of alcohols: Dehydrogenation of alcohol is a reaction in which two hydrogen molecules are removed from an alcohol molecule upon oxidation. During oxidation of alcohol both C-O and O-H bonds are broken for the formation of C=O bonds. Secondary alcohols in the presence of strong oxidizing agents undergo dehydrogenation to produce ketones. For example: when vapours of secondary alcohols are passed over heated copper at 573 K, ketones are produced due to the dehydrogenation.Tertiary alcohols upon oxidation undergo dehydration in place of dehydrogenation. Hence alkenes are manufactured in the case of tertiary alcohols.
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