7 - ALDEHYDES

Abstract

This chapter discusses the laboratory preparations of aldehydes that involve oxidation, reduction, condensation, and elimination reactions. The oxidation of primary alcohols using chromic acid, chromic oxide-pyridine complex, and manganese dioxide gives aldehydes in good yields. Allylic alcohols are readily oxidized at room temperature with active manganese dioxide. Olefins, alkyl groups, and alkyl halides may be oxidized to aldehydes. The chapter presents several reduction methods that are available to convert nitriles or acyl chlorides or amides to aldehydes. It also discusses Gattermann condensation reaction and its modifications, Vilsmeier reaction, and the use of modified Friedel–Craftsand Grignard reactions that offer useful methods for the laboratory synthesis of aromatic and heterocyclic aldehydes. Darzens reaction and McFadyen–Stevens reaction are also useful aldehyde syntheses involving elimination reactions. The use of phase transfer catalysts has facilitated the oxidation of alcohols and primary alkyl halides to aldehydes. Benzal halides are hydrolyzed to aldehydes via water and an iron catalyst. Aromatic aldehydes are also obtained by the partial oxidation of an aromatic side chain using chromic acid and acetic anhydride followed by hydrolysis of the diacetate. Industrially, olefins are converted to aldehydes via the OXO process. In addition, olefins can be oxidized to give the aldehydes.