Carbon-13 Chemical Shifts of the Carbonyl Group. III. Solvent Effects

Abstract

Carbon‐13 chemical shifts were determined for the carbonyl groups of eight carbonyl compounds in a representative variety of polar, nonpolar, protic, and aprotic solvents. Simple ketones show a large sensitivity to environment if proton donors are present and small sensitivity to aprotic solvents. The relative sensitivities of the carbonyl compounds to solvent effects on the ¹³C=O chemical shift are: acetone, 1.05; pinacolone, 1.00; fenchone, 0.90; di‐t‐butyl ketone, 0.81; ethyl acetate, 0.51; methyl formate, 0.37; dimethyl carbonate, 0.15; hexachloroacetone, 0.03; probably reflecting the effect of a reduction in carbonyl basicity by the electron‐withdrawing substituents OCH₃, OCH₂CH₃, and CCl₃. These results are interpreted in terms of carbonyl pi‐bond polarity as influenced by hydrogen‐bonding, polar and van der Waals interactions with solvent, the first being most important in protic solvents. The possible use of dimethyl carbonate as a solvent and internal reference is discussed.