12.1:
Structures of Aldehydes and Ketones
The distinct smell of vanilla and nail polish remover is derived from a class of carbonyl compounds called aldehydes and ketones.
In aldehydes, the carbonyl group is connected to one side group and one hydrogen atom; in ketones, it is bonded to two side groups and no hydrogen atoms.
The carbonyl group is sp2-hybridized, and the sigma bonds are about 120° apart.
Carbon’s unhybridized p orbital and oxygen’s 2p orbital overlap to give the π bond in the carbonyl group.
The electronegativity of the attached oxygen atom makes the carbonyl bond significantly polar, with the positive charge centered around the carbon atom and the negative charge around the oxygen atom.
The resonance structure further confirms the charge separation in the carbonyl bond. Polarization of the bond makes the carbonyl carbon electrophilic and the carbonyl oxygen nucleophilic.
12.1:
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.