Structure and reactivity in the pyridine series
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
In order to study the effects of substituents on the pyridine ring, the thermodynamic acid dissociation constants of the pyridinium ion and thirty three 3- and 4- substituted pyridiniunl ions have been measured in water at 25°, together with the rate constants for the reactions of ethyl iodide with pyridine and with twenty 3- and 4- substituted pyridines in nitrobenzene at 60°. The results, which have been analysed using the Hammett equation, show that reactions on the pyridine nitrogen are very sensitive to substituent effects. Para substituents with +R characteristics are involved in direct conjugative interaction with the reactive centre. In contrast, -R substituents appear to exert inductive effects only. Meta amino and methoxy substituents show exalted substituent constants, which are a consequence of the very strong polarisation effects of the positively charged nuclear nitrogen atom. All other meta groups show little or no deviation from the Taft σ° values. The ultra-violet spectra of the thirty four pyridines in acidic and alkaline media are given, in the appendix, for the range 230 - 360 mμ.