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

Ilmar A Koppel (1) , Mare Piirsalu (2) , Vilve Nummert (2) , Vahur Mäemets (2) , Signe Vahur (2)

1 Institute of Chemical Physics
2 Institute of Chemistry

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 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, resonance and steric substituent constants. For all the ortho-substituted esters containing substituents in acyl part as well as 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_s^B were observed. In the case of ortho substituents in phenyl part the resonance effect was negligible. Due to inductive effect the ortho electron-withdrawing substituents showed 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 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 and 2.25 M Bu₄NBr and the IR frequencies, ν_CO, for ortho-, meta- and para-substituted phenyl benzoates and alkyl benzoates were correlated nicely with the corresponding ¹³C NMR substituent chemical shifts, Δδ_CO.

Mots clés: Physical Sciences

Ilmar A Koppel, Mare Piirsalu, Vilve Nummert, Vahur Mäemets, Signe Vahur. Effect of ortho substituents on carbonyl carbon ¹³C NMR chemical shifts in substituted phenyl benzoates. Journal of Physical Organic Chemistry, 2009, 22 (12), pp.1155. ⟨10.1002/poc.1569⟩. ⟨hal-00495777⟩