The Interaction of Carbonyl Groups with Substituents

Abstract

The carbonyl group is probably the most important functional group in organic chemistry, and its properties are strongly affected by substituents. This Account will be concerned with the modes of interaction between carbonyl groups and substituents, and how these interactions are manifested in the properties of the compounds containing this group. A comparison will be made between carbonyl groups and related groups, such as thiocarbonyl. The interaction of carbonyl groups with substituents has usually been described in terms of π electron interactions such as “amide resonance”. However, substituents may affect carbonyl groups in several different ways. They may withdraw or donate electron density via the σ bond, depending on their electronegativity. One of the important features of the carbonyl group is the difference in electronegativity between carbon and oxygen, which leads to both σ and π electron transfer from C to O and to a positively charged carbon. As a result, substituents that have lone pairs may also transfer π electron density to the electron-deficient carbon. Finally, if the atom of the substituent that is attached to the carbonyl group bears a partial charge, it will lead to a Coulombic interaction with the positively charged carbon. All of these modes of interaction have been found. There are three basic ways in which the energetics of the interaction of a carbonyl group with a substituent may be examined. The first makes use of a group-transfer reaction such as