T H E
J O U R N A L
O F
PHYSICAL CHEMISTRY Registered
in U.S. Patent
Ofice
@ CopyTight, 1984, by the American Chemical Society
VOLUME 68, NUMBER 6 J U S E 15, 1964:
Rate Constants of Hydrated Electron Reactions with Organic Compounds'
by Edwin J. Hart, Sheffield Gordon, and J. K. Thomas Argonne Yational Laboratory, Argonne, Illinois
(Received February 1 , 1964)
The second-order rate constants of reaction of some aliphatic, aromatic, and heterocyclic compounds with the hydrated electron are reported. These constants were iiieasured in unbuffered and in alkaline solutions by following the decay of the hydrated electron absorption band produced by a single 0.2- or 0.4-~sec.pulse of 15 MeV. electrons. Unreactive compounds with second-order rate constants less than lo8M-' sec. -' include saturated organic compounds containing carbon, hydrogen, and oxygen. Representative compounds are hydrocarbons, alcohols, fatty acids, esters, and amino acids. Unreactive also are unsaturated compounds including aromatic derivatives such as ethylene, benzene, aniline, and hydroquinone. Reactive compounds with rate constants in the range of 109 to 3 X 1O1o M-' sec. are: unhydrated aldehydes, ketones, conipounds containing C=C or C=N, disulfides, peroxides, and purine and pyrimidine heterocyclic derivatives.
-'
Introduction Previous work has shown that the hydrated electron, demonstrates high selectivity in its reactions with organic compound^.^^^ Second-order rate constants, in general, vary from the diffusion controlled rates of 10'0 M-' set.-' for very reactive compounds to lo6 J4-l sec. - for unreactive compounds. In the present paper we report absollute rate constants for reaction of some aliphatic, aromatic, and heterocyclic compounds with the hydrated electron. Experimental The technique of pulsed radiolysis has been described el~ewhere.~-*Hydrated electrons are generated in aqueous solution by a single 0.2- or 0.4-p sec. pulse of 15 MeV. electrons at a concentration of the order of 10-6 M . Decay of the hydrated electron spectrum is followed at 5780 A. at an appropriate concentration of solute where pseudo-first-order kinetics prevail.
Great care inust be paid to the removal of oxygen and carbon dioxide from the irradiated solutions because of their high reactivity with the hydrated electron.2 Therefore, special consideration was given to the preparation of the solutions. The method of evacuation and filling of the syringes with the solutions to be (1) Based on work performed under the auspices of the U . S.Atomic Energy Commission. (2) S. Gordon, E. J. Hart, 1%.S. Matheson, J. Rabani, and J. K . Thornas, Disczissions Faraday SOC.,36, 193 (1963). (3) E. J. Hart, S.Gordon, and J. K . Thomas, Radiation Res. S u p p l . , 4 , 74 (1964). (4) S. Gordon, E. J. Hart, M .S. Matheson, J. Rabani, and J. I(. Thomas, J . Am. Chem. Soe., 85, 1375 (1963). (5) E. J. Hart and J. W. Boap, ibid., 84, 4090 (1962). (6) M.S.Matheson and L. 11.Dorfman, J . Chem. Phgs., 32, 1870 (1960). (7) L. M. Dorfman. I. A. Taub, and R. E. Biihler. ibid., 36, 3051 (1962). (8) S.Gordon, E. J. Hart, and .J K. Thomas, J . P h y s . Chem., 613, 1262 (1964).
1271
1272
E. J. HART,S. GORDON, AND J. K. THOMAS
I n general, reagent-quality chemicals were used irradiated consists of a preliminary pumping of 600 ml. without further purification. The gases were conof solution in a 1000-ml. evacuation chamber. This densed in a liquid nitrogen or carbon dioxide trap, solution is then saturated with helium (or argon) and evacuated further, and then the solution was prepared re-evacuated. By this procedure, the oxygen and by saturation of the solution a t a pressure of 1 atm. carbon dioxide concentrations are reduced to less than or less. For compounds with rate constants of the M and M , respectively. Before the syringes are filled with the deaerated solution, they are purged order of 10'O M-l set.-', purity is of lesser importance with helium several times in order to remove air.5~g~10than for compounds with rate constants in the range below lo9 set.-'. Consequently, the rate conSince a small amount of helium remains in the stants given in Table I are more reliable for the group syringe, the initial 10 ml. of solution introduced into the syringes is expelled with the remaining bubble of above lo9 iW-l sec.-l. Methanol was used as a solvent in many experiments in order to prepare stock helium. I n this way, completely filled syringes of the solutions, particularly of the more volatile and waterstock solution, usually triply distilled water or 0.01 insoluble chemicals. Solutions of volatile chemicals N sodium hydroxide, are prepared. Solutions at the such as chloroform, carbon tetrachloride, carbon didesired concentration are obtained by adding heliumsulfide, benzene, styrene, and nitrobenzene were prepurged concentrated solutions to the oxygen-free pared in the following way : helium-purged solutions of stock solutions described above. For added volumes in the range from 0.05 to 1.0 ml., the helium-purged these pure chemicals were injected directly into 10solution is injected through the capillary tip of the ml. syringes of deaerated methanol. The final solution was then made by the injection of the required syringe shown in Fig. l a by means of a microsyringe amount of this solution into a syringe of previously deaerated water. For the more insoluble and less reactive compounds, this procedure frequently gave a solution high in methanol concentration; and for this a reason methanol solutions above 0.01 M were occasionally used. However, methanol at this concentration and even in concentrations up to 1 M has little or no effect on rate-constant measurements. This low reactivity of methanol is further supported by the results of Taub, Sauer, and Dorfman, who showed that the electron solvates in pure methanol. l1 Figure 1. Syringea for sample preparation and dilution.
provided with a very fine Teflon capillary tube. The technique used for larger volumes, particularly for volatile solutes, is shown in Fig. lb. The solution in the smaller syringe is purged by adding helium to the solution in the syringe, then shaking and expelling the gases. This procedure, repeated four times, reduces the oxygen concentration in the solution to less than 10-6 M . The solution was then transferred through a capillary tube (see Fig. lb) into the larger syringe. Flat glass plates of about 1-ml. volume were present in the large syringes and permitted thorough mixing of the solution by repeated inversions of the syringe. Dilutions, too, were carried out by transferring the degassed solution into the appropriate volume of the concentrated solution in 50- or 100-nil. syringes used to supply solution to the irradiation cell. The technique of introducing and removing these solutions from the irradiation cell is identical with the method described elsewhere.5 The Journal of Physical Chemistry
Discussion of Results Our hydrated electron rate constants were measured either in neutral solution containing methanol or in alkaline solutions a t pH 12 or greater. The results are presented in Table I. I n both cases methanol was usually added in order to eliminate the reactions of the hydrated electron with the hydroxyl radical and hydrogen atoms. These free radicals are very reactive with eaq- while the resulting CHzOH radical is relatively unreactive. Alkaline solutions were used in order to neutralize hydrogen ions formed in the ('spur'' and thereby eliminate the reaction eaq-
+ Hf -+ H
The effect of added methanol and alkali can be deduced from the slope of the hydrated electron decay curves (9) C. Senvar and E. J. H a r t , Proc. 2nd Intern. Conf. Peaceful Uses A t . Energy, Geneva, 29, 19 (1959). (10) A. R. Anderson and E. J. H a r t , J. Phw. Chem., 66, 70 (1962). (11) I. A. T a u b , M. C. Sauer, Jr., a n d L. M. Dorfman, Discussions Faraday SOC.,36, 1 (1963).
RATECONSTANTS
OF
HYDRATED ELECTRON REACTIONS WITH ORGANIC COMPOUNDS
-Table I : Hydrated Electron Rate Constants of Organic Compounds CHsCompound Aliphatic Acetaldehyde Acetone Acrylamide Butadiene Carbon disulfide Carbon tetrachloride Chloroform ~-Cyetine Fumarate ion Maleate ion Maleic acid Methacrylate ion
Conon., OH. mM mM
0.10 0.20 0.06 0.10 0.144 0.20 0.05 0.060 0.066 0.033 0.10 0.133 0.455 0,020 0.021 0.2
0.1 Pyruvate ion Tetracyanoethylene Thiourea Trichloroacetate Aromatic Aniline Benzene Benzoquinone Hydroquinone Naphthalene &Naphthol
Nitrobenzene Phenylalanine Phthalate ion Picric acid
Styrene Heterocyclic Adenosine Cytidine Hypoxanthine 5-Meth ylcytoeine Orotic acid Purine Pyridine Thymine Uracil
0.2 0.19 0.10 0.111 0.037
10 20 0.50 2.0 0.5 1.0 1.0 1.0 0.02 0.01
... ,..
... ... 1.0 6.0 2.25 16.2 1.0 1.0 1.0 1.0 1.0 1.0
... 1.0 1.0 1.0
...
... ...
... 1.0 .
I
.
1.0
1.0
... 1.0 1.0 1.0
...
...
1.0
...
0.50 0.10 0.333 0.01 0.13 0.089
1.0 1.0 1.0
0.02 0.02 0.02 0.02 0.02 0.33 1.0 0.33 0.020 0.200 0.100 0.033
1.0 1.0
... 1.0
...
... ...
... ...
... I
.
6.55
1.5
3 5 X IO-@*
7.0 7' 7.7
7.84 5.0
1 . 8 X 10'0 8 X 10s
;: ;:;;[
5 . 9 X 108
...
7c 7c 7c 12.0 13
8.45 12.7 6.5 10.1 10.1 12.7 7= 6.41 10.7 -10
3 . 1 >c 10'0
:::
3 . 0 X 1010
4.38 1.48 4.65 3.35 1.9 1.0 7.4 3.2 6.95 4.8 1.25
3.0 X 3.4 x 7.5 X 1.7 X 2.2 X 1.2 X 8.4 X 7.2 X 6.8 X 1.5 X 2.9 X
10'0 109 10s 109
109 1010 108 109 109 10'0 lo8
11.94 7c 6.6 13 7c
0.80 : 10' 0.6 < 7 X 108 2.73 1 . 2 5 X 109 0.08
