Substituent effects in saturated systems. An assessment of the transmission of substituent effects by the field, σ-inductive, and π-inductive effects in 3- and 6-substituted hexahydrocarbazoles

Basicities of N-methyl-cis-1,2,3,4a,9a-hexahydrocarbazoles substituted at the 3- and 6-positions have been measured. For substituents of the type CH2X polar effects are analysed in terms of the direct field effect operating from the 3-position and the field and π-inductive effect operating in concert across the aromatic portion of the molecule (from the 6-position). Empirical correlations and the results of calculations of ΔpKa using the Kirkwood–Westheimer method show that the field effect is substantial but that relay by the π-inductive effect is greater by a factor of ca. 2.5.