2878
B. D. MICHAEL AND EDWIN J. HART
The Rate Constants of Hydrated Electron, Hydrogen Atom, and Hydroxyl Radical Reactions with Benzene, 1,5-Cyclohexadiene, 1,4-Cyclohexadiene, and Cyclohexene' by B. D. Michael2 and Edwin J. Hart Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received October 8 , 1969)
The hydrated electron, H atom, and OH radical rate constants of reaction with benzene, 1,3-~yclohexadiene (1,3-C&,),1,ii-cyclohexadiene (1,4-Ce&), and cyclohexene (C6HIO) in aqueous solutions were determined by pulse radiolysis. In the order, e,,-, H, and OH, these rate constants in units of M-l sec-l are: C6Hs,(1.2 f 0.2) X lo', (5.3 f 1.0) X los, (7.6 f 1.9) X lo9; 1,3-CsHs, (1.0 f 0.15) X loQ,(9.8 f 2.0) X log, (9.8 f 2.5) X loQ; 1,4-c&j,6HBOH',2000 psi H2 (0.10 M ) was used to convert OH into H, enhancing H-atom and lowering OH radical effects. At 0.10 mM CeH6,we estimate our C6H7 spectrum contains 15% CaHeOH , an amount that should have a relatively small effect on the extinction coefficient as well as on its second-order decay constant. From our data we conclude that the properties of (&&* are Xmax a t 310 nm; E310 = 3300 f 200 M-' cm-'; a second-order decay constant, 2k = (1.6 f 0.15) X lO9M-' sec-l. The spectrum of the C6H7* radical has also been obtained by a completely different reaction, namely, the oxidation of 1,4-C6H8by OH radicals. Below 100" alkyl radicals generate CsH7. by the established gas phase reaction22
+
+
-
0
-1000 M-l cm-'. Support for the view that the delocalized allyl radical would have a higher extinction coefficient than the alkyl radical comes from studies on irradiated polyethylene25 where the following absorption maxima were reported: alkyl, c216 = 1600 M-' cm-l; allyl, E268 = 6800 M-' cm-l; dienyl, 6268 = 18,000 M-l cm-l. Products of C&,3 Radiolysis. It is clear from the extensive studies made on the deaerated aqueous system that the radiolysis products are indeed complexa3 Phenol, diphenyl, and a group of hydrogenated diphenyl derivatives are the common products reported. When eaq- is converted to OH by N20,the proportion of phenol and diphenyl rises in the expected manner." And when the OH and H atoms are converted to eaq-, then the reduced products, 1,4-C~&, C&0, C6H12, and the group of dimeric products ranging from ClZH14 to C12Hz2appear.'O In acid Hz solutions where OH and eaq- are converted to H atoms, the reduction proceeds a t a lower rate but the nature of the monomeric products changes. Now 1,3-C6H8is the dominant C6H8isomer and the proportion of dimeric products increases appreciably. The present results with emphasis primarily on the reduction products of C6H6indicate the types of compounds to be looked for in the radiolysis of C6H6-3-7,11,12 Generally accepted now are the primary reactions enq-
We find that the OH radical extracts H from both 1,3- and l,4-C6He, since C6H7' forms in each case. (See Figure 5.) However, this is not an exclusive reaction since the expected amount of C&7* is not obtained. I n these cases the OH reacts partly by H abstraction and partly by addition. From these data the relative proportions of H abstraction to OH addition are 45% with 1,4-CeHs and 30% with l13-CeH8. Because of the high H-atom rate constants of these hydrocarbons, the H atom appears to add exclusively. Cyclohexene, too, forms a transient absorption upon reaction with OH, but with Amax S 240 nm. It was not possible to determine the extent to which each of the four possible modes of the reaction contributed to this absorption; however, to explain the absorbance obtained a t 230-240 nm the extinction coefficient of one of the products must be greater than 2400 M-' cm-l. We tentatively assign the absorption to the allylic isomer of CeHg. We exclude C6HloOH. as a major contributor although isomers of this radical have an absorption maximum a t 230 nm in aqueous because their extinction coefficients are only about half the value we observe at this wavelength. Similarly we exclude nonallylic C ~ H Ssince this radical would be expected to display a p-alkyl radical type of absorption spectrum. I n aqueous solution ,&alkylz1 and p-carboxyalkylZ4 radicals have absorption maxima 5 240 nm; however, their peak extinction coefficients are +
2883
+ C & ,% CeH7- + OH-
(1)
H f CsH6 4C&7*
(2,
OH
+ CeH6 +OHC&&,.
(3)
followed by C6&.
+ CeH7.
4products
(4)
C6H7. 4- OHC6He.-+products
(5)
OHCOHG. OHC6H6.+products
(6)
OHCaHe. +CeH5.
+ H2O
(7)
These reactions and their rate constants have all been established by pulse radiolysis techniques. When any appreciable concentration of CBHBis present, eaq-, H, and OH react exclusively with benzene. At low pH, Hatom reactions are dominant. In neutral and alkaline solutions eaq- reactions become important. The radicals C6H7' and OHC6H6-form phenol and diphenyl through reactions 4, 5, and 6. However, in many studies in which phenol and diphenyl only were rea lack of material balance is found. Reduced dimers are suggested as the missing products. I n view (22) D. G. L. James and R. D. Suart, Trans. Faraday SOC.,64, 2762 (1968). (23) M. Simic, P. Neta, and E. Hayon, J. Phys. Chem., 73, 3794 (1909).
(24) P.Neta, M.Simic, and E. Hayon, ibid., in press. (26) D. C.Waterman and M. Dole, ibid., in press. T h e Journal of Physical Chemistry, Vol. 74, No. 16,1070
2884
B. D. MICHAEL AND EDWIN J. HART
+
of the nearly equal yields of (H eaq-) and OH and the similar rates of reactions 4, 5, and 6, 1,4-CeHs should form in neutral and alkaline solutions, whereas 1,4- and 1,3-CeH8should be present in acid solutions. Dimerization products of C6?&', CeHeOH., and C&. should also be found. Previous worklo indicates that CeH7 * disproportionates in aqueous solution to produce about 25% CaH8 and 75% dimers. This is close to the fraction reported in the gas phasez0where 2CeH7. +CeHrCaH7
--.)
1,4-CeHs
+ CaHa
where [HI, is the initial concentration of H atoms formed during the pulse and t is the time after the pulse. The solution for [XI is
The reciprocal relative concentration of X is given by
69%
+ C&
41,3-C&3
The solution for H-atom concentration [HI is [HI = [HIoe- (kl[PNBAI+kz[Sl)t
11% 20%
According to this distribution, some 1,4-CeHs should be found in neutral and alkaline solutions. The absence of 1,3-CeHs in our previous worklo is probably due to its high eaq- rate constant which is 100-fold greater than that of CsHa and >lOOO-fold higher than that of 1,4-CsH8. This large divergence in eaq- rate constant between 1,3- and 1,PC& explains the absence of the former in metal-NH3 reductions of CeH6. Acknowledgments. We wish to thank Mr. E. E. Klocek for the design of the optical high-pressure cell, Miss P. Walsh and Mr. R. M. Clarke for technical assistance, and the accelerator crew, particularly Mr. B. Naderer, for their cooperation.
Appendix Kinetic Treatment of H-A tom Competition Results. The competing reactions of H atoms with a solute, S, and with PNBA to form an absorbing product, X, which subsequently undergoes a first-order decay, follow the reaction scheme
+ PNBAAX H + S -% products H
X -% products
The Journal of Phwlsical Chemistry, Vol. 74, No. 16,1970
where [XI = [XI, when [SI = 0, thus 1 - ,-(ki[PNBA]-ka)t F = 1 - e-(k~[PNBAl+kz~Sl-ka)t The expression in brackets in eq 1 leads to a linear competition plot of [X],/[X] vs. [S]/[PNBA] of slope [PNBA]kz/[PNBA]kl - k3 if F is close to unity. However, under the experimental conditions used (kl[PNBA] = 2 X 106 sec-', k3 = 4.1 X lo4 sec-' and t = 12 rsec), F varies from 1.OOO for kz[S] = 0 to 0.851 for k2[S] = 5.6 X lo6. Under these experimental conditions, a theoretical plot of [X]O/[X]vs. kz[S]may be drawn. The dependence of F on kz[S] introduces a slight curvature, although this was not observed experimentally in Figure 4 because of random errors. The best fitting straight line on the theoretical plot for the range of values of [Xlo/ [XI used experimentally passes through the points [Xl0/[X] = 2.5 where kz[S] = 3.05 X lo6 sec-'. Accordingly, values of ( [S]/PNBA]),xp have been determined from the best fitting straight lines in Figure 4 such that Ao/A = 2.5. For each solute kz,the H-atom rate constant is calculated from the equation
kz
=
3.05 X lo6 [PNBA] [PNBAI [SI
(
>,
