Aldehydes and ketones are widespread in nature, often combined with other functional groups. Example are shown in the following diagram. The compounds in the top row are found chiefly in plants or microorganisms; those in the bottom row have animal origins. With the exception of the first three compounds (top row) these molecular structures are all chiral. When chiral compounds are found in nature they are usually enantiomerically pure, although different sources may yield different enantiomers. For example, carvone is found as its levorotatory (R)-enantiomer in spearmint oil, whereas, caraway seeds contain the dextrorotatory (S)-enantiomer.
Note that the aldehyde function is often written as –CHO in condensed or complex formulas.
Figure 1: Naturally occuring aldehydes and ketones
Aldehydes and ketones are obtained as products from many reactions discussed in previous sections of this text. The following diagram summarizes the most important of these.
Figure 2: Methods of synthesizing aldehydes and ketones
With the exception of Friedel-Crafts acylation, these methods do not increase the size or complexity of molecules. In the following sections we shall find that one of the most useful characteristics of aldehydes and ketones is their reactivity toward carbon nucleophiles, and the resulting elaboration of molecular structure that results. In short, aldehydes and ketones are important intermediates for the assembly or synthesis of complex organic molecules.