Kinetics of the reactions between 1,2-dinitrobenzene and aliphatic primary amines in benzene. A probable mechanism for the observed mild acceleration

Abstract

The kinetics of the aromatic nucleophilic substitution (S_NAr) between 1,2-dinitrobenzene (1,2-DNB) and n-(BuⁿNH₂) and s-butylamine (Bu^sNH₂) have been investigated in benzene and hexane-benzene. The second-order rate coefficient for these reactions varies linearly with the amine concentration. However, the ratio of the slope and intercept of the straight lines (k″/k′) are in the range for Bunnett's mild acceleration of unclear origin. This is in contrast to the genuine catalysis previously observed for the same reactions in n-hexane. The effect of additives such as pyridine, triethylamine, α-pyridone, and dimethyl sulphoxide was examined. It was found that the catalytic power of the additives qualitatively correlates with Kamlet–Taft's π* and β parameters rather than with basicity constants of the additives in benzene. On other hand, when the steric requirements for the reactions of Bu_sNH₂ are compared with those for BuⁿNH₂, for the reaction with 1,2-DNB in benzene it seems that mild acceleration in benzene takes place for the decomposition of the S_NAr intermediate. Studies of the reaction of 1,2-DNB with BuⁿNH₂ in hexane–benzene give good evidence that benzene forms an electron-donor–acceptor (EDA) complex with 1,2-DNB. It is concluded that, in pure benzene, 1,2-DNB is preferentially solvated by EDA complex formation and consequently the proximity effects either of the nucleophile or any additive will be quite different than in n-hexane. Hence benzene cannot be considered a typical non-polar inert solvent for S_NAr reactions.