The effects of cyclic terminal groups in 4-aminoazobenzene and related azo dyes. Part 1. Electronic absorption spectra of some monoazo dyes derived from N-phenylpyrrolidine and N-phenylpiperidine

Abstract

Monoazo dyes containing a terminal piperidino group absorb hypsochromically when compared with their pyrrolidinyl counterparts. In acid solution, the pyrrolidinyl dyes protonate almost exclusively at the β-azo nitrogen atom (azonium tautomer), whereas the analogous piperidino compounds protonate very largely at the terminal nitrogen atom (ammonium tautomer). These effects are related to differences in the conjugative capacity of the lone pair of electrons on the terminal nitrogen atom brought about by a change in size of the saturated heterocyclic ring.