The effects of cyclic terminal groups in 4-aminoazobenzene and related azo dyes. Part 3. Electronic absorption spectra of some monoazo dyes derived from N-phenylmorpholine, N-(phenyl)thiomorpholine, N-(phenyl)thiomorpholine 1,1-dioxide, and N-acetyl-N′-phenylpiperazine
Monoazo dyes containing a terminal morpholino group absorb hypsochromically in comparision with their piperidino counterparts as a result of electron withdrawal by the oxygen atom. Similar shifts are observed with related dyes possessing other γ-heteroatoms in the donor group. In acid solution, protonation takes place at the β-azo nitrogen atom (azonium tautomer) and at the terminal nitrogen atom (ammonium tautomer) to an extent which depends on the inductive effect of the γ-substituent.