OXIDATION OF BENZYL RADICALS BY FE(CN)(6)(3-)

Hydroxyl radicals and their anions [O-.- radicals, pK(a)((OH)-O-.) = 1 1.9] have been generated radiolytically in N2O-saturated aqueous solut ion and reacted with 4-chlorotoluene. While the (OH)-O-. radical mainl y produces hydroxycyclohexadienyl-type radicals, the O-.- radical prac tically only abstracts H atoms from the methyl group yielding 4-chloro benzyl radicals (k = 1.9 x 10(9) dm(3) mol(-1) s(-1); determined by pu lse radiolysis). The radicals formed by (OH)-O-. radical attack at pH 7 (mainly hydroxycyclohexadienyl-type radicals) are oxidized by Fe(CN) (6)(3-) k = 1.8 x 10(7) dm(3) mol(-1) s(-1)) giving rise to the follow ing products (G values in units of 10(-7) mol J(-1) are given in paren theses): 4-chloro-2-hydroxytoluene (2.9), 4-chloro-3-hydroxytoluene (2 .4), 4-chlorobenzyl alcohol (0.05), 4-chlorobenzaldehyde (0.1) and 4-c hloro-2-hydroxybenzaldehyde (0.15). The 4-chlorobenzyl radical is the main species formed at pH 13.7 and is oxidized by Fe(CN)(6)(3-) with a similar rate constant (k = 4.2 x 10(7) dm(3) mol(-1) s(-1)), the majo r products being 4-chlorobenzaldehyde (3.7), 4-chloro-2-hydroxybenzald ehyde (0.65) and 4-chlorobenzyl alcohol (0.5). From (OH)-O-. radical a ttack (ca. 10% at this pH), 4-chloro-2-hydroxytoluene (0.4) and 4-chlo ro-3-hydroxytoluene (0.3) are also formed. It is suggested that the ox idation of the Li-chlorobenzyl radical by Fe(CN)(6)(3-) yields in the first step a carbocation which cyclizes by deprotonation. The resultin g cyclohexadienyl-type radical undergoes beta-fragmentation yielding t he 4-chlorobenzyloxyl radical. A 1,2-H shift and subsequent oxidation leads to 4-chlorobenzaldehyde. The unsubstituted benzyl radical is als o oxidized by Fe(CN)(6)(3-) yielding benzaldehyde in high yields.