Ac. Bhasikuttan et al., Study of spectral characteristics, kinetics, and equilibria of radicals derived from hydroxy benzophenones, J PHYS CH A, 103(24), 1999, pp. 4703-4711
Studies on radical equilibria and the spectral evaluation have been carried
out for three monohydroxy-substituted benzophenones (HOBP's) in aqueous so
lution. The transient ketyl or anion radicals were generated via hydrated e
lectron (e(aq)(-)) reaction or dimethyl ketyl radical reaction in a pulse r
adiolysis experiment. The reactivity of both the undissociated (HOBP) as we
ll as dissociated (-OBP) forms toward these reducing agents were studied. e
(aq)(-) was found to react with the HOBP's and -OBP's with diffusion-contro
lled rates which are of the order 3 x 10(10) dm(3) mol(-1) s(-1). In the ca
se of the para derivative, two acid-base equilibria were identified (pK(a)
9 and 11) in the pH range 6-13, one corresponding to the protonation-deprot
onation at the OH site of the ketyl radical and the other for the protonati
on-deprotonation at the carbonyl site. For the ortho derivative, the intram
olecularly hydrogen bonded structure of the radical anion has been found to
be stable even in strongly alkaline solution (pH 13). Three different radi
cal forms for the meta derivative have been identified at different pH cond
itions. The spectral evaluation of these radical species at different pH co
nditions was carried out. Reactions of dimethyl ketyl radicals with HOBP's
and -OBP's were found to be different at different pH conditions. At acidic
pH, it forms an adduct with the HOBP's, whereas at alkaline pH, the reacti
on with the ortho and meta derivatives occurs by H atom transfer. However,
no reaction is seen with the para derivative in alkaline solution. Detailed
spectral and kinetic data on the formation and decay of the various transi
ent intermediates have been obtained.