Effect of ortho substituents on carbonyl carbon 13C NMR chemical shifts in substituted phenyl benzoates.

¹³C NMR spectra of 37 ortho-, meta-, and para-substituted phenyl benzoates, containing substituents in benzoyl and phenyl moiety, 4 ortho-substituted methyl and 5 ethyl benzoates as well as 9 R-substituted alkyl benzoates have been recorded. The influence of the ortho substituents on the carbonyl carbon ¹³C NMR chemical shift, δ_CO, was found to be described by a linear multiple regression equation containing the inductive, σ_I, resonance, σ°_R, and steric, E<STACK>_s^B</STACK>, or υ substituent constants. For all the ortho-substituted esters containing substituents in the acyl part as well as the phenyl part, the substituent-induced reverse inductive effect (ρ_I < 0), the normal resonance effect (ρ_R > 0), and the negative steric effect (δ_ortho < 0) with the E<STACK>_s^B</STACK> were observed. In the case of ortho substituents in the phenyl part, the resonance effect was negligible. Due to inductive effect, the ortho electron-withdrawing substituents showed an upfield shift or shielding of the carbonyl carbon, while the electron-donating substituents had an opposite effect. Because of the sterical consequences, ortho substituents revealed a deshielding effect on the ¹³C NMR chemical shift of the carbonyl carbon. For all the meta- and para-substituted esters, the reverse substituent-induced inductive and resonance effects (ρ_I < 0, ρ_R < 0) were found to be significant. In alkyl benzoates, the alkyl substituents showed the reverse inductive and steric effects. The log k values for the alkaline hydrolysis in water, aqueous 0.5 M Bu₄NBr and 2.25 M Bu₄NBr, and the IR frequencies, ν_CO, for the ortho-, meta-, and para-substituted phenyl benzoates and alkyl benzoates were correlated nicely with the corresponding ¹³C NMR substituent chemical shifts, Δδ_CO. Copyright © 2009 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]