Kinetic, Electron Spin Resonance, and Product Studies of the Reaction Between 2,2-Diphenyl-1-picrylhydrazyl and 2,4,6- Tri-t-butylbenzenethiol

Abstract

The reaction of 2,2-diphenyl-1-picrylhydrazyl (DPPH) with 2,4,6-tri-t-butylbenzenethiol (TBBT) in benzene solution has been studied. The two major products, 2,2-diphenyl-1-picrylhydrazine (DPPH-H) and di-(2,4,6-tri-t-butylphenyl) disulfide, are those anticipated for a reaction in which DPPH abstracts hydrogen from the thiol yielding thiyl radicals which undergo coupling to produce the disulfide. About 30% of the DPPH is converted into a derivative in which a nitro group has been displaced by a 2,4,6-tri-t-butylphenylthiyl group. Displacement of NO2 from DPPH by the tri-t-butylphenylthiyl radical is thought to constitute an important secondary reaction.In the presence of excess thiol, the rate constant for disappearance of DPPH, determined spectrophotometrically, is 1.23 ± 0.04 M−1 s−1 at 25 °C, and the activation energy is 5.8 ± 0.5 kcal mol−1. The rate of reaction is halved when the thiol is deuterated at the SH group, confirming that the primary step involves hydrogen transfer. Addition of DPPH-H retards the overall reaction indicating that this primary step is reversible.An e.s.r. study of the reacting solutions shows that the concentration of thiyl radical is relatively low until most of the DPPH disappears; it also shows the slow buildup of the 2,4,6-tri-t-butylphenylthiyl derivative of DPPH. The e.s.r. spectrum of the latter derivative has been analyzed and shown to be consistent with nitrogen coupling constants A1 = 10.4 ± 0.2 G and A2 = 6.45 ± 0.2 G.