13.10: IR Frequency Region: Alkene and Carbonyl Stretching

Double bonds in alkenes and carbonyl compounds exhibit stretching frequencies in the diagnostic region of the IR spectrum. In addition, alkenes exhibit vinylic C–H stretching and C–H out-of-plane bending absorptions that are useful for identifying substitution patterns.

Stretching frequencies are affected by several factors, such as resonance, inductive effects, ring strain, dipole moment, and hydrogen bonding. Consequently, the stretching frequency of the carbonyl double bond varies in different functional groups. The C=O stretching absorptions of saturated esters and carboxylic acids appear at 1735–1750 cm^-1 and 1710–1780 cm^-1, respectively, while amide carbonyl stretching frequencies appear at 1630–1690 cm^-1. While the carbonyl stretching of aldehydes and ketones is observed in the range 1680–1750 cm^-1, aldehydes exhibit the characteristic C–H stretching absorption. Compared to saturated carbonyl compounds, delocalization lowers the C=O stretching absorptions in aromatic and unsaturated carbonyl compounds.

Carbonyl groups exhibit strong IR signals because of their large dipole moment arising from resonance and inductive effects. When the bond vibrates, the dipole oscillates, and the bond is surrounded by an oscillating electric field. This oscillating electric field interacts with the electric field of the IR radiation, increasing the efficiency of IR absorption.

Alkene double bonds, with much smaller dipole moments, have weaker oscillating electric fields that are inefficient at absorbing IR radiation. As a result, C=C bonds produce relatively weak signals, while carbonyl absorption signals are among the strongest in the IR spectrum.