FORMATION AND REACTIVITY OF 4-OXOCYCLOHEXA-2,5-DIENYLIDENE IN THE PHOTOLYSIS OF 4-CHLOROPHENOL IN AQUEOUS-SOLUTION AT AMBIENT-TEMPERATURE

Nanosecond laser flash photolysis of an aqueous solution of 4-chloroph enol (lambda(exc) = 266 nm) produces, at pulse end, a transient with a bsorption maxima at 384, 370, and ca. 250 nm; upon addition of an H-do nor such as 2-propanol, this spectrum is converted into that of the ph enoxyl radical (lambda(max) = 400 and 385 nm), and in presence of O-2, it is converted into a transient with a broad absorption band peaking at 460 nm. This reaction behavior can be understood by assuming forma tion of the carbene, 4-oxocyclohexa-2,5-dienylidene, by elimination of HCl from excited 4-chlorophenol; the pulse end transient spectrum is assigned to this species, while the 460 nm band is assigned to benzoqu inone O-oxide formed by addition of O-2 to the carbene. Both phenoxyl radical and benzoquinone O-oxide are produced upon photolysis of 4-chl orophenol in neat alkanols as well. On the other hand, photolysis in n -hexane yields the triplet-triplet absorption, which is absent in pola r solvents, and no indication of carbene formation. It can be conclude d that the primary step of 4-chlorophenol photolysis in aqueous or alc oholic solution is heterolytic C-Cl bond scission; a quantum yield of 0.75 is determined for it in neutral or acid aqueous medium upon excit ation at 266 nm. Photolysis of chlorophenolate produces the same trans ients, but with a markedly lower yield, and, in addition, e(aq)(-) and 4-chlorophenoxyl radicals. The proposed reaction mechanism provides a straightforward explanation of the results of photoproduct analysis, published by previous authors as well as contributed in the present wo rk. In particular, formation of p-benzoquinone in the presence of O-2 can be accounted for by intermediate formation of benzoquinone O-oxide . Production of 4-oxocyclohexa-2,5-dienylidene with high yield allows, for the first time, extensive investigation of the kinetics and mecha nism of the reactions of a carbene in an aqueous environment. In the p resent work, we have studied (a) the addition reaction with O-2 on the one hand and with halides on the other; (b) H abstraction reactions w ith alkanols; (c) reaction with 4-chlorophenol itself; and (d) reactio n with H2O. The rate constants for reaction with O-2 (3.5 x 10(9) M(-1 ) s(-1)) and with I- (4.6 x 10(9) M(-1) s(-1)) are close to the diffus ion-controlled limit, whereas reactions with Br- (6.8 x 10(7) M(-1) s( -1)) and Cl- (<3 x 10(5) M(-1) s(-1)) are slower. Rate constants for r eaction with alkanols follow the pattern known for their reactions wit h radicals, with values ranging from 5 x 10(5) M(-1) s(-1) for tert-bu tyl alcohol to 1.9 x 10(7) M(-1) s(-1) for 2-butanol. All these observ ations are consistent with the triplet character of the carbene. A rat e constant of 1.5 x 10(3) M(-1) s(-1) has been determined for reaction with H2O. This reaction is not accompanied by formation of OH radical s; it is concluded that it proceeds by insertion into the O-H bond rat her than by O-H cleavage. The exceptional stability of the carbene in aqueous solution is thus mainly attributed to the high barrier for O-H rupture in the water molecule. Additionally, a specific carbene-H2O i nteraction is revealed by semiempirical calculations, which could cont ribute to energetic and orientational hindrance of the reaction. Furth er theoretical results support the interpretation of both spectroscopi c and kinetic properties of the carbene.