COMMUNICATIONS TO THE EDITOR
5034
Vol. 813
The NOs- effect is attributed to a decrease in GH? with a concomitant increase in GH2O2. Quantitatively, these results are consistent with the assumption that an intermediate in the spur disappears by a firstorder process in competition with reaction with Nos-. This is a pseudo-unimolecular process since the NO3effect is temperature independent and not proportionali to Ti/v. Plausible arguments have been presented8 for the ea,-, instead of the H atom, as the main precursor of molecular Hz. Assume the eaq- to be the intermediate which reacts with X03-. Then, eq. I would yield a lifetime for the ea,- in the spur of 5 X lo-'" sec., based on a rate constantg for reaction of NOa- with ea,,- of 8.15 X 10g -\I-' set.-'. This is much longer than the lifetime of 6 X IO-" sec. to be expected for the eaClin 0.8 1V sulfuric acid owing to reaction with Ha,+ (13) W. E Harris, C a n . J C h e m . , 39, 121 (1961): alone, based on a rate constantg of 2.06 X 101" .ZIP' T. E. GILROY set.-'. This, together with absence of any marked NJC'CLBAR PHYSICS LABORATORY GEORGE MILLER influence of pH, forces the conclusion t h a t the NOaOXFORD,ESGLAND P. F. D. SHAW effect is not attributable to reaction of NOa- with RECEIVEDSEPTEMBER 16, 1964 ea,-, This is consistent with the observation of Slahlman'o that there is no effect of Haqf on the dependence of GH?on NOS- concentration. XIageeL1proposed that three consecutive elementary Kinetic Evidence for H 3 0a s the Precursor of Molecular processes yield OH and H 3 0 as the primary chemical Hydrogen in the Radiolysis of Water' intermediates in the radiolysis of water. LVith these assumptions, the results in Table I indicate that di- and tribromoethanes are more often formed with s2Br than with somBrand suggest that hydrogen atom substitution proceeds more readily through a mechanism involving Auger electron emission. Conversely the scission of carbon-carbon bonds, and the substitution of bromine atoms in heavily scavenged systems, may be caused predominantly by atoms having recoil energy. Besides the compounds listed, and carbon tetrabromide and tetrabromoethane, small yields of compounds containing more than two carbon atoms have been found in this system. Since such products may be important in determining reaction mechanisms, a fuller discussion is withheld until complete analyses can be reported.
Sir: H20---t
HtO+
+ e-
(1)
GH20:a and GH;?bfor the decomposition of water by HpOi + H10 --+ + OH (2) 6oCo y-radiation and G H for ~ ~ the decomposition of water by fission recoils are markedly dependent on H30i + e- +H30 (3) solute concentration. This is attributed to reaction of solute with free-radical precursors, e.g., reaction with We suggest t h a t H 3 0 is the main precursor of molecular OH radicals before they combine to form Hz02.2a H z and disappears by three processes Attempts to derive the observed dependence from the "diffusion-kinetic" model have resulted4 in a quantiHa0 + H2O -+- Ha,+ + eaq (4) tative inconsistency. We have found that the preHa0 f H30 --+ Hz + 2H20 (5) cursor of molecular H 2 disappears by a first-order Ha0 + OH +2H20 process. Therefore, homogeneous kinetics can be (6) substituted for diffusion kinetics to express the deAssume that H 3 0 disappears by a first-order process, pendence of GH2on solute concentration. k7[H30], in competition with reaction with NOa-, Evidence to support this suggestion resulted from ks[H30][N03-]. Then, eq. I yields a lifetime for further examination of the striking effect6 of NOaH 3 0 in the spur of 2.30,'k8 or about 4 X 10-lo sec., on C e 4 + reduction in sulfuric acid solutions. Tl+ assuming reaction with NOa- to be diffusion controlled. increases G(Ce3+) from 2 G ~ ~ 0 GH ~ - GOH to The pseudo-unimolecular rate constant, k i , in this %GH?o? GH GOH. NOa- a t concentrations greater model is taken to be k.,[H20] ks[H30] kg[OH]. than 0.01 III markedly enhances G(Ceat) both in the In the decomposition of water by fission recoils,3 presence and in the absence of T1+, the effect being GH2 is independent of UOISOl concentration up to equal in both cases. The NO3- effect is not p H deabout 0.01 M , and then i t decreases approximately pendent; no significant difference was observed belinearly with the square root of the 1;02S04 concentratween 0.08 and 0.8 A'sulfuric acid. The enhancement,6 tion. GH2,previously approximated3 as a square-root AG(Ce3+),for NOa- concentrations from 0.1 to 5.0 -If dependency, is quantitatively expressed by eq. 11. is quantitatively expressed by eq. I.
+ +
1, SG(Ce3+) = 0.185
+
+
+ o.O804/[NO3-]
(I)
(1) This research was sponsored b y t h e U. S . A t o m i c Energy Commission u n d e r contract w i t h t h e Union Carbide Corporation. ( 2 ) ( a ) T. J. S w o r s k i , J . A m . Chem. SOL., 7 6 . 4687 (1951); Radiation R e s . , 2 , 227 (195.5, ( b ) J . A . Ghormley a n d C. J. Hochanadel. ibrd., a , 227 (1955). ( 3 J . \V. Boyle, W. F. Kiefer, C. J . Hochanadel, T. J. Sworski, and J. A. Ghormley, Proc. I n t r r n . Conj. Peacefnl L-ses A t . E n e v g y , 7 , 576 (1957) (4) A . K u p p e r m a n in " T h e Chemical a n d Biological Action of Radiations," Vol. .5. h f . Haissinsky, Ed., Academic Press. N e w York. N . Y., 1 9 6 1 , Chapter 111. (.5) T. J . S w o r s k i , J . A m . Chem. S o c . , 7 7 , 4689 (1955). (6) H. A . Mahlman, J . P h y s . Chem.. 64, 1598 (1960).
1/GH2 = 0.571
+
+ 0.645[UOzS04]
(11)
This dependency is consistent with the assumptions that H 3 0 is the sole precursor of Hz and that it disappears by a first-order process in competition with reaction with uranyl sulfate, k g[ H 3 0 ][U02S04]. Equa(7) 3t. V. Smoluchowski, 2. Physik. Chem. (Leipzig), 91, 129 (1918). (8) H. A . S c h w a r z , Radialion Res. S u p p l . , 4 , 89 (1964). (9) J . H. Baxendale, el al., .Valuve, 201, 468 (1964). (10) H. A . Mahlman, Chemistry Division Annual Progress Report. Oak R i d g e Sational Laboratory. Oak Ridge, T e n n . , 1963, ORSL-3488. (11) J . L. hlagee, Radialion Res. Suppl., 4 , 20 (1964).
Nov. 20, 1964
COMMUNICATIONS TO THE EDITOR
tion I1 yields a lifetime for H 3 0in thefission recoil track of 1 .13'k9 or about sec. The dependence of GH?on the concentration of Nosfor the decomposition of water by 6oCo y-radiation, previously approximatedI2~l3 as a cube-root dependency, is quantitatively expressed by eq. 111, but only for l/GHz
=
2.93
+ 7.53[NOs-]
(111)
concentrations from 0.01 t o 1.0 &I. Equation I11 yields a lifetime for H 3 0 in the spur of 2.57/k8, in excellent agreement with eq. I. The failure of eq. I11 t o apply a t concentrations less than 0.01 leads us to propose t h a t H2 formation by 6oCoy-radiation results from two processes : intraspur and interspur processes with GH? = 0.34 for the former. I n interspur processes, intermediates from one spur interact with intermediates from an adjacent spur before they escape into the bulk of the solution. The dependence of GH? on NOS- at concentrations less than 0.01 JI is consistent with both G H ~= 0.11 and a lifetime for H30 of approximately 300/k8 for interspur processes. The new model for H2 formation leads us to propose an alternative viewpoint concerning the influence of pH on GeaQ-. Haq+ reacts with those electrons, klo[Haq+][ea,-], which would otherwise disappear in spur processes. The lifetime for the eaq- in the spur is 130/klo or 6 X sec. (originally reportedI4 as O.Ol,'klo and attributed t o H,O*).
the products. The bromides Ia-IIIa and the acetates Ib-IIIb are possible products depending on the direction and stereochemistry of the addition. No lowD. ,D
aD GD D
H
H
I
&D
I1
H
111
( a ) X = B r ; ( b ) X = OAc
field resonance is expected for Ia or I b , whereas I I a and I I b (from trans addition) and I I I a and I I I b (from cis addition) should give low-field doublets for the Cl-H resonance with expected splittings of 2-4 and 8-10 c.P.s., respectively. Complications due to coupling involving deuterium can be averted through the use of deuterium double irradiation.6 Cyclohexene-2,6-6-d3 was prepared according to the following reaction sequence
(12) H . A. Mahlman and J. W. Boyle, J . Chem. Phys., 17, 1434 (1957). (13) H. A. hlahlman, ibid., 81, 601 (1960). (14) F. S. Dainton and D. B. Peterson, Proc. R o y . SOC.(London), A167, 443 (1962).
CHEMISTRY DIVISION OAKRIDGESAT.ONAL LABORATORY OAKRIDGE,TENNESSEE
5035
vD Du OTs
THOMAS J. SWORSKI
t-BuOK
D
t-BuOH
RECEIVEDSEPTEMBER 12, 1964
The Polar Addition of Hydrogen Bromide to Cyclohexene'
Sir : In recent years a variety of examples of polar cis additions of acids and alcohols to olefins has been These include the addition of deuterium bromide to cyclohexene, which has been reported2 to give 26, 31.5, 63, and 74% of cis adduct a t 10, 25, 45, and 60°, respectively. This variation in stereochemistry with temperature seemed unusual to us and prompted us to re-examine this reaction. Our results are in substantial disagreement with the earlier findings. When cyclohexene-2,6,6-d3 is allowed to react with hydrogen bromide in acetic acid, addition of acetic acid as well as hydrogen bromide occurs, and the addition is very predominantly trans a t temperatures from 15 to 60'. The stereochemistry of addition to cyclohexene2,6,6-d3 can be established from the n.m.r. spectra of (1) This research was presented a t t h e 148th Sational Meeting of t h e American Chemical Society, Chicago, Ill., Sept., 1964. (2) J . 1'. Smirnov-Zamkov and G. A. Piskovitina, U k r . Khim. Z h . , 18, 531 (1962). (3) M. J. S. Dewar and R. C . Fahey, J . A m . Chem. Soc., 86, 2245, 2248, 364.5 (1963). (4) S. J. Cristol, L. K . Gaston, and D . W. Johnson, Telrohcdron Lelters, 185 (1963). (5) H . Kwart and J. L. Xyce, J . A m . Chem. SOL.,86, 2601 (1964).
In a typical addition experiment, 20 ml. of 0.5 3fhydrogen bromide in dry acetic acid, 0.5 ml. of cyclohexene2,6,6-d3,and 10 mg. of 2,6-di-t-butyl-p-cresol (used to inhibit free-radical addition) were sealed in an ampoule and placed in a thermostated bath for 1-4 hr. The reaction was worked up in water by extracting with pentane. The pentane fraction was washed with dilute bicarbonate, dried over anhydrous sodium carbonate, and filtered. Evaporation of the pentane gave 7 5 9 5 % crude yields of product. The polar character of the reaction was demonstrated by showing t h a t addition of hydrogen bromide to I-hexene in the presence of cyclohexene gave 2-bromohexane and cyclohexyl bromide, with no 1-bromohexane (the freeradical addition product) being formed. Analysis of the reaction products, both by n.m.r. and by v.P.c., demonstrated the presence of both acetate and bromide. In control experiments it was shown that, under the reaction conditions, cyclohexyl bromide is not converted to cyclohexyl acetate, whereas cyclohexyl acetate is very slowly converted t o the bromide. The n.m.r. spectrum of the addition product mixture showed broad signals a t 4.67 (acetate) and 4.13 p.p.m. (bromide) downfield from T M S , which sharpened into doublets with 3.3 and 3.5 C.P.S. spacings, respectively, when the samples were simultaneously irradiated a t the deuterium resonance frequency. The ob(6) A Nuclear Magnetic Resonance Specialties, I n c . , Model SD-60 spin decoupler was used in conjunction with a Varian HR-60 spectrometei- in thew studies.
