2.1. (Very) Brief Refresher of the Basics

Recall that carbonyls are not functional groups on their own but are part of many functional groups. For example, aldehydes, ketones, carboxylic acids, esters, acid halides, and amides all contain carbonyls (Figure 2.1).

Figure 2.1 – Examples of Carbonyls in Functional Groups.

Oxygen is more electronegative than carbon, so all carbonyl-containing functional groups have permanent dipoles (Figure 2.2). This becomes even more apparent when the resonance forms of carbonyls are considered. It is important to remember that depending on what the two other groups are, additional resonance forms may be possible. Based on these it should be clear that there is a permanent excess of electron density on the oxygen and a permanent deficiency of electron density on the carbon.

Figure 2.2 – Dipole and Resonance Forms of a Generic Carbonyl Group.

Because this is common to many functional groups, it imparts similar reactivity to all groups with it. Introductory organic chemistry focuses on carbonyls as electrophiles: the carbon of the carbonyl has a permanent deficiency of electron density and is electrophilic. However, the opposite type of reactivity is also feasible: the oxygen of the carbonyl has a permanent excess of electron density and is nucleophilic.

It is possible for the oxygen of the carbonyl to act as a nucleophile directly. However, it is much more common to take advantage of these features to make a carbon NEAR a carbonyl nucleophilic. In these cases the carbonyl itself is not the nucleophile; the nucleophile is made from a carbonyl.