G. Grabner et al., FORMATION AND REACTIVITY OF 4-OXOCYCLOHEXA-2,5-DIENYLIDENE IN THE PHOTOLYSIS OF 4-CHLOROPHENOL IN AQUEOUS-SOLUTION AT AMBIENT-TEMPERATURE, Journal of the American Chemical Society, 116(25), 1994, pp. 11470-11480
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.