effect is relatively weak and gets increasingly weaker as one moves further away within the molecule
from the more electronegative atom. This effect is responsible for the dipole character of the
carbonyl group, as well as the increased dipole character (and therefore susceptibility to
nucleophilic attack) of carboxylic acids—which contain an additional oxygen atom in their leaving
group. This also explains the overall relative reactivity of anhydrides, esters, and amides toward
nucleophilic attack. Anhydrides have two electron-withdrawing groups, which leaves a significant
partial positive charge on the electrophilic carbon. This effect is smaller in amides because nitrogen
is less electronegative than oxygen, and the dipole is not as strong.
Resonance and conjugation also affect the reactivity of a molecule. Conjugation refers to the
presence of alternating single and multiple bonds. This setup implies that all of the atoms involved
in these bonds are either sp^2 - or sp-hybridized—and therefore have unhybridized p-orbitals. When
these p-orbitals align, they can delocalize π electrons through resonance, forming clouds of electron
density above and below the plane of the molecule. This type of electron sharing is most commonly
demonstrated using benzene, as shown in Figure 9.10.
Figure 9.10. Conjugation in Benzene
Parallel unhybridized p-orbitals combine to form delocalized electron clouds above and below the
plane of the molecule.In carbonyl-containing compounds, conjugation can be established with the carbonyl group itself.
α,β-unsaturated carbonyls (enones) are common examples, as shown in Figure 9.11.
Figure 9.11. Conjugation in a Carbonyl-Containing Compound