Oxidation of benzyl radicals by Fe(CN)63–

Abstract

Hydroxyl radicals and their anions [O˙^– radicals, pK_a(˙OH)= 11.9] have been generated radiolytically in N₂O-saturated aqueous solution and reacted with 4-chlorotoluene. While the ˙OH 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 × 10⁹ dm³ mol^–1 s^–1; determined by pulse radiolysis). The radicals formed by ˙OH radical attack at pH 7 (mainly hydroxycyclohexadienyl-type radicals) are oxidized by Fe(CN)₆^3–(k= 1.8 × 10⁷ dm³ 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 × 10⁷ dm³ 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 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 4-chlorobenzyl radical by Fe(CN)₆^3– yields in the first step a carbocation which cyclizes by deprotonation. The resulting cyclohexadienyl-type radical undergoes β-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.