Substituent effects on tautomerisation constants of alkylaryltriazenes

Abstract

Both tautomers of propyl-p-nitrophenyl-, -p-cyanophenyl-, -3,5-dichlorophenyl-, -3,4-dichlorophenyl-, and -p-chlorophenyl-triazenes are directly observable by ¹H n.m.r. at 200 MHz in CD₃CN at 25 °C. In addition, lower temperature studies enable the two tautomers of propyl-p-methylphenyl- and-p-methoxyphenyl-triazenes to be quantitated. At –45 °C rotamers about the N–N bond of the conjugated tautomers are also separately observable. ¹³C N.m.r. studies of the p-nitro- and p-chloro-compounds suggest tautomerisation involves the less abundant rotamer of the conjugated tautomer and the unconjugated tautomer. The position of equilibrium at 25 °C is described by log ([C₃H₇N : NNHAr]/[C₃H₇NHN : NAr])=(0.67 ± 0.13)–(0.31 ± 0.04)pK_a(ArNH₃ ⁺). ¹³C, as well as ¹H n.m.r. studies indicate that change of solvent to (CD₃)₂SO, CD₃CN–(CD₃)₂SO, or CD₃CN–(CD₃)₂SO–H₂O has little effect. Neither, in the p-nitrophenyl case, does change of the propyl group for benzyl.