13.10 : 红外频率区域：烯烃和羰基振动

In an IR spectrum, alkenes exhibit C=C stretching absorptions, and carbonyl compounds display C=O stretching absorptions.

Alkenes also exhibit vinylic C–H stretching absorptions and C–H out-of-plane bending absorptions. The latter is useful for identifying substitution patterns in alkenes.

Carbonyl stretching frequencies vary depending on the type of carbonyl functional group.

Unsaturated or aromatic aldehydes exhibit lower C=O stretching frequencies than saturated aldehydes. Additionally, aldehydes also exhibit the characteristic C–H absorption peak.

Aromatic or unsaturated ketones show lower C=O stretching frequencies than aliphatic or cyclic ketones.

Carbonyl groups exhibit stronger IR signals than alkenes. This is because the C=O has a larger dipole moment arising from resonance and inductive effects. In comparison, the C=C due to its smaller dipole moment, shows a weaker IR signal.

The difference in absorption efficiency is confirmed from the stronger oscillating electric field of the C=O, compared to the weaker oscillating electric field of the alkene double bond.