10.2 Preparing Aldehydes and Ketones

Preparing Aldehydes

Perhaps the best method of aldehyde synthesis is by oxidation of a primary alcohol, as we saw in Section 9.8. The reaction is usually carried out using the Dess–Martin periodinane reagent in dichloromethane solvent at room temperature:

Problem 10.3
How would you prepare pentanal from the following starting materials?

(a) CH₃CH₂CH₂CH₂CH₂OH

(b) CH₃CH₂CH₂CH₂CH=CH₂

(c) CH₃CH₂CH₂CH=CH₂

Preparing Ketones

For the most part, methods of ketone synthesis are similar to those for aldehydes. Secondary alcohols are oxidized by a variety of reagents to give ketones (Section 9.8). The choice of oxidant depends on such factors as reaction scale, cost, and acid or base sensitivity of the alcohol. The Dess–Martin periodinane is a common choice although chromic acid (H₂CrO₄) was frequently used in older work.

Other methods include the ozonolysis of alkenes in which one of the unsaturated carbon atoms is disubstituted and Friedel–Crafts acylation of an aromatic ring with an acid chloride in the presence of AlCl₃ catalyst (Section 8.5).

Problem 10.4
How would you carry out the following reactions? More than one step may be needed.