Structures of Aldehydes and Ketones

Vanillin—a flavoring agent in vanilla, cinnamaldehyde—a molecule responsible for the distinct smell of cinnamon, and acetone—a strong-smelling ingredient in nail polish removers, all belong to a class of carbonyl compounds called aldehydes and ketones (Figure 1). Although both aldehydes and ketones contain the characteristic carbonyl (C=O) bond, their chemical structures vary with respect to the groups directly attached to the carbonyl carbon.

In aldehydes (Figures 1a and 1b), the carbonyl group is directly linked to at least one hydrogen atom, and in ketones (Figure 1c), the group is connected to two carbon atoms. The carbonyl carbon in aldehydes and ketones is sp²-hybridized, and the carbonyl group has a trigonal planar geometry.

(a)	(b)	(c)

Figure 1: Structures of (a) vanillin, (b) cinnamaldehyde, and (c) acetone.

Since the oxygen atom is more electronegative than carbon, it attracts the electron pair of “C=O” towards itself, making the bond polar. Due to the polarization of the carbonyl bond, the carbonyl carbon acquires a partial positive charge, developing an electrophilic center that functions as a Lewis acid. The carbonyl oxygen acquires a partial negative charge, generating a nucleophilic center that behaves as a Lewis base.

Figure 2: Contributing structures of a ketone.