Substituent effects on the carbon-13 N.M.R. chemical shifts of side-chain carbons in aromatic systems

Abstract

Carbon-13 chemical shifts have been measured for the side-chain carbon atoms of a number of meta- and para-disubstituted benzenes. The side chains examined were CN, CF₃, Ac, CO₂Et, Me, OMe and NMe₂. The substituent-induced ¹³C shifts of these side chains were related to Hammett substituent effects by the Dual Substituent Parameter method and showed a widely varying dependence upon the blend of inductive and resonance effects. The transmission of substituent effects through aromatic systems and the factors that influence ¹³C shifts are discussed.