17O NMR study of ortho and alkyl substituent effects in substituted phenyl and alkyl esters of benzoic acids
17 O NMR spectra for 44 ortho -, meta - and para -substituted phenyl and alkyl benzoates (C6H 5 CO 2 C 6 H 4 -X, C 6 H 5 CO 2 R) at natural abundance in acetonitrile were recorded. Substituent effects on the 17 O NMR chemical shifts, δ( 17 O), of the carbonyl oxygen and the single-bonded phenoxy (OPh) and alkoxy (OR) oxygens have been studied. The δ( 17 O) values of the carbonyl oxygen for para derivatives showed a good correlation with the σ° constants. The δ( 17 O) values of carbonyl oxygen for ortho derivatives were found to be described well with the Charton equation containing the inductive, σ I , resonance, σ° R , and steric, E s B , substituent constants in case the data treatment was performed separately for electron-donating + R and electron-attracting – R substituents. The electron-donating + R ortho and para substituents in substituted benzoates caused shielding and the electron-withdrawing – R substituents produced deshielding of the O signal. The steric interaction of ortho substituents with the ester group decreased the electron density at the carbonyl oxygen. In alkyl benzoates the δ( 17 O) values were found to be described well with the inductive, σ I , and the steric, E s B , substituent constants.