Pulse radiolysis and election spin resonance studies of the dehydration of radicals from 1,2-diols and related compounds

Abstract

The complementary techniques of pulse-radiolysis and e.s.r. spectroscopy have been employed in a kinetic study of the dehydration of a variety of α,β-dihydroxyalkyl radicals [˙CR¹(OH)CR²R³OH] into the appropriate carbonyl-conjugated radicals [˙CR²R³C(O)R¹]. The overall rates of proton-catalysed dehydration, as revealed by steady-state (e.s.r.) and time-resolved (pulse-radiolysis) experiments, indicate the importance of the electronic effects of substituents (contrast values of 1.2 × 10⁹ and 9.8 × 10⁸ dm³ mol^–1 s^–1 for the radicals from cyclohexane-1,2-diol and butane-2,3-diol, respectively, with that for the radicals from erythritol of 4.2 × 10⁶ dm³ mol^–1 s^–1). Time-resolved experiments allow information to be obtained about the generation of the protonated species [˙CR¹(OH)CR²R³OH₂⁺] and for the loss of water from this intermediate.

For ˙CR(OH)CMe₂OH (R = H and Me) evidence is obtained for a rapid uncatalysed dehydration reaction, (k 1–2 × 10⁴ s^–1), which is believed to be assisted by the steric effect of the gem-dimethyl group as well as the polarity of the solvent. For the latter substrate, the reaction is characterized by a strongly negative activation entropy (–93 J mol^–1 K^–1).

α,β-Dihydroxyalkyl radicals are shown to reduce methyl viologen with rate constants that depend strongly on the nature of the substituents; there is a correlation between the reducing power of the radical and its rate of dehydration.