1911
Reactivity of the Carbonate Radical in Aqueous Solution ratory, Report No. ORNL-TM-2257 (1968). (35) L. G. Christophorou, "Atomic and Molecular Radiation Physics", WileyInterscience, New York, N.Y.. 1971. (36) S. Altshuier. Phys. Rev., 107, 114 (1957). (37) A. A. Christodoulides. R. Schumacher, and R. N. Schindler, manuscript in preparation.
(38)
E. Chen, R . D. George, and W. E. Wentworth, J. Chem. Phys.,
49, 1973 (1968). (39) K. Takayanagi and Y. Itlkawa, J. Phys. SOC.,24, 160 (1968). (40) For convenience, the units for k values and for concentrations will be given in the text as cm3 sec-l and cm-3 rather than cm3 molecule-' sec-' and molecule respectively.
Reactivity of the Carbonate Radical toward Aromatic Compounds in Aqueous Solution' Schoen-nan Chen, Morton 2. Hoffman;
and George H. Parsons, Jr.
Department of Chemistry, Boston University, Boston, Massachusetts 022 15 (Received March 14, 1975)
The rate constants for the reaction of COS- and CO3H radicals, generated in the flash photolysis of aqueous solutions of Co(NH3)4C03+,with a series of substituted benzene derivatives have been measured. The rate constants at pH 7 , ranging from 3 X IO3M-' sec-l for benzene to 1.8 X lo9 A4-l sec-' for dimethylanaline, correlate very well with the Hammett equation using upara+values for electrophilic substitution yielding p = -3.6. The COS- radical can be viewed as a selective electrophilic reagent.
Introduction The carbonate radical (COS-) and its conjugate acid (C03H; pK, = 9.6 f 0.3)*v3show a wide range of reactivity with aromatic compounds with the rate constants depending upon the nature of the substituents and the aromatic ring ~ y s t e m .Thus, ~ benzene reacts very slowly (k < lo4 M-l sec-l) while phenol reacts a factor of -lo3 more rapidl ~Very . ~high rates are shown by indole and its derivatives5 including tryptophan-containing enzymes such as lysozyme and c h y m o t r y p h 6 Although the exact mode of reaction of CO3- with aromatic compounds is not known, there is evidences to indicate that radical addition to the aromatic system, similar to that shown by OH radical^,^ is the operative mechanism. In that case, the mechanism of COS- addition is analogous to electrophilic substitution and should give rise to a linear free energy correlation between the rate constant for the reaction and a substituent parameter. The reactivity of eaq-,l0 OH radicals," H atoms,'* and, most recently, positronium atoms13 with aromatic compounds has been successfully correlated with appropriate u constants for electrophilic and nucleophilic substitutions by some form of the Hammett equation: log k = pu log ko. For OH radicals," a good agreement was seen using upara and urnetavalues with p = -0.41 although the range of rate constants measured varied over less than a factor of 4. The small absolute value of p in this case is indicative of the high reactivity of OH radicals (It lo9 M-l sec-l) and resulting low selectivity. The lower reactivity of COB- radicals should result in a high degree of selectivity in its reaction with benzene and its derivatives.
+
-
Experimental Section The generation of COS- radicals from the flash photolysis of Co(NH3)&03+ has already been described in detail.14 Solutions were prepared from triply distilled water containing 5 X M Co(NH3)&03+ and -0.02 M phosphate buffer. The organic solutes were of the purest grade
commercially available and were used without further purification. The flash photolysis apparatus dissipated up to 250 J of energy with l/e time of 30 bsec. The solution was contained in a 22-cm quartz optical cell and the COS- absorption a t 600 nm was monitored by a 75-W Osram Xe lamp, a RCA 4832 PM tube, a Hilger-Engis 0.6 M spectrometer, and associated fast detection electronics. An Oriel wedge interference monochromator, set at 600 nm (band width = 28 nm) was always in place between the analyzing lamp and the cell and an electric shutter restricted exposure of the solution to the light. The solutions were flashed without deoxygenation inasmuch as c o s - radicals do not react with 0 2 . The pseudofirst-order rate constant for the decay of COS- was determined a t a single scavenger concentration; the general linear dependence of k on [scavenger] had already been establ i ~ h e d .The ~ , ~ values of the second-order rate constants for the reaction of the radical with the solutes were obtained from a t least four individual decays and are known to an accuracy of f10% with the exception of benzene. Here the decay of the radical was only modestly faster than that in the absence of benzene so that the uncertainty in k may be as high as 50-100%.
Results and Discussion Table I gives the values of the rate constants for the reaction of COS- radicals with substituted benzene derivatives at pH 7.0. Those compounds which react slowly with the radical require relatively high concentrations in order for scavenging to be observed and we recognize that the presence of an adventitious impurity in the soldte, although at a low concentration, may react with the radical a t relatively high rates. For example, the rate constant for cyclohexene, which may be an impurity in benzene or toluene, is 2.5 X lo6 M-' sec-'. Therefore, rate constants
