Substituent Effects on the Acetyl Group Conformation and the Carbonyl Frequency in Acetophenones

Abstract

The fundamental carbonyl stretching frequencies for o-, m-, and p-substituted acetophenones in dilute CCl4 solution are correlated with the electrophilic substituent constant σ+ by the expression vc=o (cm−1) = 11.53σ+ + 1691.5 (n = 27, correlation coefficient = 0.988, standard deviation = 1.2 cm−1). The frequencies of carbonyl groups trans to an o-substituent follow this correlation, assuming σo+ ≈ σp+, while those oriented cis to the substituent generally do not. Deviations from this correlation are interpreted in terms of dipole–dipole interactions, steric hindrance to planarity, intramolecular hydrogen bonding, and non-additivity of σ+ values for some combinations of substituents. The carbonyl frequencies for singly m- and p-substituted acetophenones are also correlated with the polar and resonance components of σ+ by the expression vc=o (cm−1) = 15.66σI + 10.99σR+ + 1689.8 (n = 13, correlation coefficient = 0.981, standard deviation = 0.9 cm−1). Thus the sensitivity of the carbonyl frequency to the polar effect is greater than to the resonance effect. In ω-substituted acetophenones no precise correlation with polar substituent constants is observed.