16.1: Nucleophilic Addition to the Carbonyl Group

A carbonyl is any functional group that contains a C=O double bond, but we often further classify them based on what else is attached to the carbon atom. In this section of the course, we will be dealing primarily with aldehydes and ketones which contain no other heteroatoms attached to the carbonyl. Some examples are below:

The Frontier molecular orbitals we are concerned about now are the \(π\)_C-O and \(π^{*}\)_C-O. The \(π\) bond (HOMO) is polarized toward oxygen because it is more electronegative. We can draw a ketone in its dipolar resonance form to illustrate this (more \(δ^{–}\) on oxygen). There is a nodal plane that lies in the plane of the carbonyl; the phasing above and below this plane have opposite sign, a result of these orbitals originating from atomic p orbitals.

More important, however, is the \(π^{*}\) orbital (LUMO), which is strongly electrophilic at carbon (more \(δ^{+}\) due to the dipolar resonance form). Most carbonyls react via their LUMO, not their HOMO, and thus carbonyls are good electrophiles. There is one nodal plane in between the carbon and oxygen atoms now, giving rise to a \(π^{*}\) orbital in which the biggest lobes on carbon lie at an angle of 103-107° with respect to the plane of the carbonyl. This is known as the Burgi-Dünitz angle.

In nucleophilic addition reactions, a nucleophile will populate the \(π^{*}\) orbital, which breaks the \(π\) bond, resulting in rehybridization to sp³. A new \(σ\) bond is formed and electrons move onto oxygen as a lone pair. We call this species the tetrahedral intermediate, which then goes on to do others things (stay tuned).