Oxidation of benzyl radicals by Fe(CN)(6)(3-)

Hydroxyl radicals and their anions [O-.- radicals, pK(a)((OH)-O-.) = 11.9] have been generated radiolytically in N2O-saturated aqueous solution and reacted with 4-chlorotoluene. While the (OH)-O-. radical mainly produces hydroxycyclohexadienyl-type radicals, the O-.- radical practically only abstracts H atoms from the methyl group yielding 4-chlorobenzyl radicals (k = 1.9 x 10(9) dm(3) mol(-1) s(-1); determined by pulse 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 following products (G values in units of 10(-7) mol J(-1) are given in parentheses): 4-chloro-2-hydroxytoluene (2.9), 4-chloro-3-hydroxytoluene (2.4), 4-chlorobenzyl alcohol (0.05), 4-chlorobenzaldehyde (0.1) and 4-chloro-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 major products being 4-chlorobenzaldehyde (3.7), 4-chloro-2-hydroxybenzaldehyde (0.65) and 4-chlorobenzyl alcohol (0.5). From (OH)-O-. radical attack (ca. 10% at this pH), 4-chloro-2-hydroxytoluene (0.4) and 4-chloro-3-hydroxytoluene (0.3) are also formed. It is suggested that the oxidation of the Li-chlorobenzyl radical by Fe(CN)(6)(3-) yields in the first step a carbocation which cyclizes by deprotonation. The resulting cyclohexadienyl-type radical undergoes beta-fragmentation yielding the 4-chlorobenzyloxyl radical. A 1,2-H shift and subsequent oxidation leads to 4-chlorobenzaldehyde. The unsubstituted benzyl radical is also oxidized by Fe(CN)(6)(3-) yielding benzaldehyde in high yields.