Formation and reactivity of 4-oxocyclohexa-2,5-dienylidene in the photolysis of 4-chlorophenol in aqueous solution at ambient temperature

Article abstract of DOI:10.1021/ja00104a029

Nanosecond laser flash photolysis of an aqueous solution of 4-chlorophenol (λ_exc = 266 nm) produces, at pulse end, a transient with absorption maxima at 384, 370, and ca. 250 nm; upon addition of an H-donor such as 2-propanol, this spectrum is converted into that of the phenoxyl radical (λ_max = 400 and 385 nm), and in presence of O₂, it is converted into a transient with a broad absorption band peaking at 460 nm. This reaction behavior can be understood by assuming formation 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 benzoquinone O-oxide formed by addition of O₂ to the carbene. Both phenoxyl radical and benzoquinone O-oxide are produced upon photolysis of 4-chlorophenol in neat alkanols as well. On the other hand, photolysis in n-hexane yields the triplet-triplet absorption, which is absent in polar solvents, and no indication of carbene formation. It can be concluded that the primary step of 4-chlorophenol photolysis in aqueous or alcoholic solution is heterolytic C-Cl bond scission; a quantum yield of 0.75 is determined for it in neutral or acid aqueous medium upon excitation at 266 nm. Photolysis of chlorophenolate produces the same transients, 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 work. In particular, formation of p-benzoquinone in the presence of O₂ 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 mechanism of the reactions of a carbene in an aqueous environment. In the present work, we have studied (a) the addition reaction with O₂ on the one hand and with halides on the other; (b) H abstraction reactions with alkanols; (c) reaction with 4-chlorophenol itself; and (d) reaction with H₂O. The rate constants for reaction with O₂ (3.5 × 10⁹ M^-1 s^-1) and with I^- (4.6 × 10⁹ M^-1 s^-1) are close to the diffusion-controlled limit, whereas reactions with Br^- (6.8 × 10⁷ M^-1 s^-1) and Cl^- (<3 × 10⁵ M^-1 s^-1) are slower. Rate constants for reaction with alkanols follow the pattern known for their reactions with radicals, with values ranging from 5 × 10⁵ M^-1 s^-1 for tert-butyl alcohol to 1.9 × 10⁷ M^-1 s^-1 for 2-butanol. All these observations are consistent with the triplet character of the carbene. A rate constant of 1.5 × 10³ M^-1 s^-1 has been determined for reaction with H₂O. This reaction is not accompanied by formation of OH radicals; it is concluded that it proceeds by insertion into the O-H bond rather 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 interaction is revealed by semiempirical calculations, which could contribute to energetic and orientational hindrance of the reaction. Further theoretical results support the interpretation of both spectroscopic and kinetic properties of the carbene.