that are observed when comparing similar but not identical compounds. Predictable shifts occur on
altering the substituents or structure adjacent to a group giving rise to a particular absorption. These can
be explained in terms of an inductive effect, a mesomeric or resonance effect and steric factors such as
ring strain. The effects are typified by the carbonyl group, whose stretching frequency occurs within the
range 1650–1850 cm–^1. Shifts are usually related to the position of the carbonyl band in pure acetone
(1715 cm–^1 ). The proximity of electron withdrawing substituents causes an inductive withdrawal of
electron density from around the oxygen atom thereby shortening the bond (Figure 9.15). This increases
the force constant and hence the wavenumber of the vibration (Table 9.7). If the adjacent group
contains a weakly electronegative atom with lone-pair electrons or electrons occupying π orbitals,
resonance interaction with electrons in the carbonyl group leads to a reduction in bond order (Figure
9.15). The correspondingly smaller force constant results in a lower value for the wavenumber of the
vibration, (Table 9.7). Steric hindrance may inhibit the resonance effect by reducing the coplanarity of a
conjugated system whilst carbonyl groups which form part of a strained ring system absorb at high
wavenumbers because of the constraint imposed upon the carbon atom (Table 9.7).
Figure 9.15
Effect of substituents of the carbonyl bond.
(a) Inductive effect, X = H, OH,
OR, halogen.
(b) Resonance effect, Y = NH 2 , NHR,^
NR 2 , phenyl.Table 9.7 Effect of substituents on the wavenumber of the carbonyl stretching vibration
Compound C=O Stretch/cm–^1
saturated ketones 1710 – 1720
carboxylic acids, monomer 1750 – 1770
carboxylic acids, dimer 1710 – 1720
saturated esters 1725 – 1745
acid chlorides 1790 – 1815
aliphatic amides 1630 – 1700
aromatic ketones 1680 – 1700
aromatic and α, β-unsaturated esters^1715 –^1730
γ-lactones^1760 –^1780