Influence of Substituent Groups on Nuclear Reactivity in the Formation of Substituted Biphenyls by Reaction of Aromatic Diazo and Related Compounds with Aromatic Liquids. VIII. Phenylation and p-Methylphenylation. The Separation of the Polar and the Conjugative Effect

Abstract

Abstract N-Nitrosoacet(anilide-14C) and N-nitroso-(p-methyl-14C)acetanilide have been decomposed at 20.0°C in mixtures of benzene and monosubstituted benzenes, and the reaction products have been analyzed for biphenyl and substituted biphenyls by the isotope dilution method. From the results of these competitive experiments, the partial rate factors for the phenylation and p-methylphenylation have been calculated (Tables I and II). The values of the partial rate factor for the meta position give a satisfactory Hammett plot with a slope of +0.05 for the phenylation and one with a slope of +0.03 for the p-methylphenylation, suggesting that the p-methylphenyl radical has little, if any, measure of polar character. The correlation of the ρ values for various arylations with the inductive constant for the substituent group on the arylating radical indicates that the effect of the substituent group in the radical is purely inductive in nature. The partial rate factor for the para position, kp/k, is found to be related to Hammett’s substituent constant by the following equation: log(kp⁄k)=ρσp+τp where ρ is the Hammett reaction constant for this arylation as determined from the Hammett plot of the partial rate factors of the meta position. τp is found to be tolerably constant for each substituted benzene, irrespective of the nature of the attacking aryl radical, and represents that part of the conjugative effect of the substituent which is independent of the polar nature of the attacking radical. Molecular orbital calculations have given results which rationalize the trend found in the τp values.