Aldehydes and ketones are among the most important of all functional groups, both in the chemical industry and in biological pathways. In this chapter, we’ve looked at some of their typical reactions. Aldehydes are normally prepared in the laboratory by oxidation of primary alcohols or by partial reduction of esters. Ketones are similarly prepared by oxidation of secondary alcohols.
The nucleophilic addition reaction is the most common general reaction type for aldehydes and ketones. Many different kinds of products can be prepared by nucleophilic additions. Aldehydes and ketones are reduced by NaBH4 or LiAlH4 to yield primary and secondary alcohols, respectively. Addition of Grignard reagents to aldehydes and ketones also gives alcohols (secondary and tertiary, respectively), and addition of HCN yields cyanohydrins. Primary amines add to carbonyl compounds yielding imines, or Schiff bases, and secondary amines yield enamines. Reaction of an aldehyde or ketone with hydrazine and base gives an alkane (the Wolff–Kishner reaction). Alcohols add to carbonyl groups to yield acetals, which are valuable as protecting groups. Phosphorus ylides add to aldehydes and ketones in the Wittig reaction to give alkenes.
α,β-Unsaturated aldehydes and ketones often react with nucleophiles to give the product of conjugate addition, or 1,4-addition. Particularly useful are the conjugate addition of an amine and the conjugate addition of an organic group by reaction with a diorganocopper reagent.
IR spectroscopy is helpful for identifying aldehydes and ketones. Carbonyl groups absorb in the IR range 1660 to 1770 cm–1, with the exact position highly diagnostic of the kind of carbonyl group present in the molecule. 13C NMR spectroscopy is also useful for aldehydes and ketones because their carbonyl carbons show resonances in the 190 to 215 δ range. 1H NMR is useful for aldehyde –CHO protons, which absorb near 10 δ. Aldehydes and ketones undergo two characteristic kinds of fragmentation in the mass spectrometer: α cleavage and McLafferty rearrangement.
