Synthesis, characterization of phenyl (2,4-dinitro-1-naphthyl) thioethers, kinetic and mechanistic study of their hydrazinolysis in methanol

The rate of reactions of aryl-2,4-dinitro-1-naphthyl sulfides with hydrazine in large excess in methanol were followed spectrophotometrically at λ max = 515 nm corresponding to the formation of 1-hydrazino-2,4-dinitronaphthalene anion. The rate of reaction is measured at different temperatures under pseudo first-order condition. The reaction rate is not subject to catalysis by the hydrazine. The second-order rate constant (k N ) at 25 °C depended marginally on the nature of the 4′-substituent. Low enthalpy and negative entropy of activation values suggested a second-order nucleophilic substitution mechanism for the reaction, further supported by a linear plot ( r = 0.927) with the isokinetic temperature of the reaction “β” predicted to be 1382 K. The isokinetic temperature significantly exceeds the experimental range (298–318 K), indicating enthalpy-controlled reaction kinetics. The correlations of σ-Hammett constants with k N values gave good correlation coefficients ( r = 0.9939–0.9571) with ρ values ranged between 0.35 and 0.25 depending on temperatures. The plot of the logarithmic of the rate and the p K a of the nucleophile gave value of β LG value (−0.14, r = 0.97) suggested that the transition state involving in the fast step is consistent with substrates having good leaving groups.