6.2.1 Common Methods for Preparation Aldehydes and Ketones
6.2.1.1 Oxidation of Alcohols

Ketones may be prepared by the oxidation of secondary alcohols in the presence of acid dichromate (Scheme 1). The reaction probably proceeds via the rate limiting formation of a chromate ester followed by decomposition of the ester to give the products.

Scheme 1

In case of primary alcohol, this process may lead to oxidation of the alcohol to carboxylic acid. This problem is overcome by attenuating the oxidizing power of chromic acid based reagents to generate a milder oxidizing agent. The two most popular oxidizing agents for oxidizing primary alcohols to aldehydes are pyridinium chlorochromate (PCC) and pyridiniumdichromate (PDC) . These reagents are prepared by the treatment of chromic acid with pyridine (Scheme 2).

Scheme 2

The ketones may also be generated from the secondary alcohols using aluminium t-butoxide in the presence of acetone (Scheme 3). The secondary alcohol is refluxed with the reagent in an excess of acetone. This reagent is particularly helpful in oxidation of unsaturated alcohol as only the alcoholic OH is oxidized.

Scheme 3

6.2.1.2 Reduction of Acid Chloride

Aldehydes can be prepared by the reduction of acid chloride with hydrogen in xylene using a palladium catalyst suspended on barium sulphate (Rosenmund reduction) (Scheme 4). This process demonstrates the control of chemoselectivity by poisoning of the catalyst. The role of barium sulphate is to prevent the reaction from proceeding further to alcohol.

Scheme 4

6.2.1.3 Oxidative Cleavage of Alkenes

Both aldehydes and ketones may be formed by ozonolysis of suitable alkenes (Scheme 5). The ozonolysis of alkenes of the type RCH=CHR¹ leads to the formation of aldehydes while the tetrasubstituted alkenes will give ketones as product.

Scheme 5

6.2.1.4 Hydration of Alkynes

The hydration of alkynes by the mercuration-demercuration procedure can be utilized to prepare acetaldehyde and ketones (Scheme 6). The hydration of acetylene gives acetaldehyde while the hydration of any other alkyne leads to formation of ketones.

Scheme 6

6.2.1.5 Acylation of Arenes

Friedel Craft acylation may be used to prepare alkyl aryl ketones or diaryl ketones (Scheme 7). In this process, an arene is usually treated with an acid halide or acid anhydride in the presence of a Lewis acid catalyst. This reaction is an example of aromatic electrophilic substitution.

Scheme 7