Nucleophilic Addition to the Carbonyl Group

The addition step is more important, and it forms a new C–C ? bond at the expense of the
C=O ? bond. The protonation step makes the overall reaction addition of HCN across the
C=O ? bond.
Why does cyanide, in common with many other nucleophiles, attack the carbonyl group?
And why does it attack the carbon atom of the carbonyl group? To answer these questions we
need to look in detail at the structure of carbonyl compounds in general and the orbitals of
the C=O group in particular.
The carbonyl double bond, like that found in alkenes (whose bonding we discussed in
Chapter 4), consists of two parts: one ? bond and one ? bond. The ? bond between the two sp2
hybridized atoms—carbon and oxygen—is formed from two sp2 orbitals. The other sp2 orbitals
on carbon form the two ? bonds to the substituents while those on oxygen are fi lled by the
two lone pairs. The sp2 hybridization means that the carbonyl group has to be planar, and the
angle between the substituents is close to 120°. The diagram illustrates all this for the simplest
carbonyl compound, formaldehyde (or methanal, CH2O). The ? bond then results from overlap
of the remaining p orbitals—again