Influence of substituents on the infrared stretching frequencies of carbonyl group in esters of benzoic acid

Abstract

AbstractInfrared spectra of 25 substituted phenyl esters of benzoic acid C6H5CO2C6H4‐X (XH, 3‐Cl, 3‐F, 3‐CN, 3‐NO2, 3‐CH3, 3‐OH, 3‐NH2, 4‐Cl, 4‐F, 4‐NO2, 4‐CN, 4‐OCH3, 4‐CH3, 4‐NH2, 2‐Cl, 2‐F, 2‐I, 2‐NO2, 2‐CF3, 2‐CN, 2‐CH3, 2‐OCH3, 2‐N(CH3)2, 2‐C(CH3)3), 8 alkyl benzoates C6H5CO2R (XCH3, CH2CH3, CH2Cl, CH2CN, CH2CCH, CH2CH2Cl, CH2CH2OCH3, CH2C6H5), and 22 phenyl esters of substituted benzoic acids X‐C6H4CO2C6H5 (X3‐Cl, 3‐NO2, 3‐CH3, 3‐N(CH3)2, 4‐F, 4‐Cl, 4‐Br, 4‐NO2, 4‐CH3, 4‐C(CH3)3, 4‐OCH3, 4‐NH2, 2‐Cl, 2‐F, 2‐Br, 2‐I, 2‐NO2, 2‐CN, 2‐CF3, 2‐CH3, 2‐OCH3, 2‐NH2) were recorded in tetrachloromethane in the region of 400–4000 cm−1. Carbonyl stretching frequencies νCO for meta‐ and para‐substituted phenyl esters of benzoic acid and phenyl esters of meta‐substituted benzoic acids were shown to correlate with the substituent constants σo. The influence of the through resonance effect on νCO was found to be important in the case of +R para substituents in the benzoyl part of phenyl benzoates as well. The carbonyl stretching frequencies of ortho derivatives in phenoxy part were shown to correlate with the inductive substituent constant σI only. In the benzoyl part of the esters the carbonyl stretching frequencies of cis and trans conformers (relative to the carbonyl group) of ortho derivatives were nicely described by dual parameter equations: (νCO)cis = (νCO)o + c1σI + c3 $E_{\rm s}^{\rm B} $and (νCO)trans = (νCO)o + c1σp+ + c3 $E_{\rm s}^{\rm B} $ (R = 0.99). The trans isomers of phenyl esters of ortho‐substituted benzoic acids showed direct resonance similar to that for para derivatives. The positive steric term found for both the cis and trans conformers could be considered as measure of the steric inhibition of resonance between the phenyl ring and the carboxy‐group caused by bulky ortho substituents. The existence of cis/trans conformations was supported by frequency calculations with Density Functional Theory (DFT) method at B3LYP/6‐311+G** level for the ortho‐substituted benzoates. In the case of alkyl benzoates good correlations of νCO values were obtained when both the Taft σ* and the steric $E_{\rm s}^{\rm B} $ constants were used. For meta‐ and para‐substituted phenyl benzoates s‐trans conformation where the plane of the benzene ring in the benzoyl part of the ester is coplanar with the carbonyl bond plane and the plane of the benzene ring in the phenoxy part is twisted nearly perpendicular relative to the carbonyl bond plane was supported. Copyright © 2006 John Wiley & Sons, Ltd.