Replacement Substituent Constants for Simple Heterocycles

Abstract

13C nmr absorptions are reported for the β-vinylic carbon atoms in 15 series of ethylenes bearing heterocyclic substituents. The data are used to establish the best single-parameter substituent constants, σ 13, for the various heterocycles as replacements for para-substituted benzenes. Also reported are replacement dual substituent constants, including σ I and the various σ R scales needed in the Taft DSP treatment and those for F and R used in the Swain-Lupton treatment. Values of σ 13, F, R, σ I, and σ Ro, respectively, for the various heterocycles as replacements for para-substituted benzenes are as follows: 2-furyl, −1.01, 0.99, −2.51, 0.65, −0.60; 2-pyrryl. −2.53, 0.52, −5.09, −0.16, −0.62; 2-thienyl, −0.79, 2.49, −3.60, 1.82, −1.08; 3-thienyl, −0.40, 1.04, −1.57, 0.59, −0.39; 2-pyridyl, 0.88, 2.09, −0.24, 1.65, −0.45; 3-pyridyl, 0.60, 0.57, 0.63, 0.35, 0.04; and 4-pyridyl, 1.18, 1.22, 1.09, 0.92. −0.01. The DSP-NLR method of analysis is explored using electron demand parameters. ϵ as determined for the vinylic side chains in para-substituted styrenes.