Oxidation of benzene by the OH radical. A product and pulse radiolysis study in oxygenated aqueous solution

Abstract

Hydroxyl radicals [generated radiolytically in N₂O/O₂(4:1 v/v)-saturated aqueous solutions] have been reacted with benzene. The major product is phenol. At low dose rate (γ-radiolysis) it is formed in 53% yield with respect to the OH radical yield. This value increases to 93% in alkaline solution (pH 12.3). With deuteriated benzene it is reduced to 39%. In addition, more than fifteen different, ring-opened and fragment products are formed. A good material balance (based on primary OH radical yield and oxygen consumption) was obtained.

At high dose rate (pulse radiolysis) the major products are phenol, hydroquinone and cyclohexa-2,5-diene-1,4-diol. An important intermediate is the HO₂˙/O₂˙^– radical. Its rate of formation (k_obsd= 800 s^–1) has been followed by pulse radiolysis using tetranitromethane as a scavenger as well as conductimetrically (build-up of H⁺/O₂˙^–).

The results have been interpreted as follows: in their reaction with benzene, hydroxyl radicals yield the hydroxycyclohexadienyl radical 1. In the presence of oxygen, radical 1 undergoes reversible oxygen addition yielding four different hydroxycyclohexadienylperoxyl radicals: the cis- and trans- isomers of 6-hydroxycyclohexa-2,4-dienylperoxyl radical 3 and the cis- and trans-isomers of 4-hydroxycyclohexa-2,5-dienylperoxyl radical 4. As reported previously, in the equilibrium mixture of the radicals 1, 3 and 4 the concentration of radical 3 represents only a few per cent of the total. It is suggested that 3 eliminates HO₂^. thereby yielding phenol. In basic solution deprotonation of 4 is followed by an O₂˙^–-elimination which opens up an additional route to phenol. The fact that phenol formation is not quantitative and its yield is reduced in the case of deuteriated benzene is due to another unimolecular decay route. The competing reaction is the intramolecular addition of the peroxyl radical function to a double bond (and subsequent fragmentation of the ring system). Since the HO₂˙-elimination is not very fast, bimolecular decay of the radicals 1, 3 and 4(mainly of 4, 2k= 8.9 × 10⁸ dm³ mol^–1 s^–1) plays an increasingly important role under the conditions of pulse radiolysis. As a consequence, the hydroquinone and cyclohexa-2,5-diene-1,4-diol yields increase with increasing dose rates under pulse radiolysis conditions (2–25 Gy pulse^–1) as those of phenol and HO₂˙ decrease.