COMMUNICATIONS TO THE EDITOR
2584
Yol. b3
initial equilibrating solution permit the calculation of the LiCl activity in any of these mixtures. Data of this type for other cations permit the calculation of exchange selectivities in concentrated solutions. IVork is now being extended to more accurate determinations of the osmotic coefficients of mixed resin-electrolyte systems and t o measurement of selectivity as a function of mole fraction exchanged in the lower molality region (1-6 molal for NaCl and LiCl systems) where aqueous phase activity coefficients are accurately known.
doublet of 11. This probably is because oi the slight environmental non-equivalence of the tn-o types of basal borons with terminal hydrogens i n IV (4 and 3,s positions). Pure IV (0.48 millimole) was isolated after addition of excess boron trifluoride ethyl etherate to the reaction mixture and a subsequent separation of the volatiles by vapor phase chromatography. Similarly, a mixture of was rearranged t o 111. The structure of I11 was confirmed by infrared spectrophotometric and vapor pressure data. comparison of the Bll n.m.r. chemical shifts of TI, IV and pentaborane3m5reveals a shift of about (3) D e p a r t m e n t of Chemistry a n d Laboratory for Nuclear Science ol the Massachusetts I n s t i t u t e of Technology, Cambridge. Mass 13 6 units to lower field when the apex boron of the (4) T o whom correspondence should be addressed. pentaborane framework is alkyl substituted, and a CHEMISTRY DEPARTMENT similar shift of about 14 6 units when a basal boron UNIVERSITYOF BUFFALO \vALTER -4. PLATEK3 is alkyl substituted. These “alkyl shifts” are in BUFFALO,NEW YORK JACOB A. MAR IN SKY^ qualitative agreement with other studies.6 RECEIVED MARCH10, 1961 The mechanism of the reaction may involve slow “symmetrical” cleavage7 of I or TI and fast recomREARRANGEMENT OF 1-METHYL- AND bination to give I11 and 117, respectively. Among 1-ETHYLPENTABORANE-9 TO 2-METHYL- AND other possible mechanisms \Yilliams6,* has sug2-ETHYLPENTABORANE-9 gested a plausible internal rearrangement faciliSi?: tated by hydrogen tautomerism. .h extension of =In apparently quantitative conversion of a I - the present study into the mechanism of the realkylpentaborane-9 (R = methyl, T ; R = ethyl, 11) arrangement and to other related reactions will be to a 2-alkylpentaborane-9 (R = methyl, 111; reported subsequently. R = ethyl, IV) in the presence of 2,6-dimethylThe author is indebted to Dr. R. E. 1Tilliams for pyridine (Fig. 1) suggests a general reaction to use of Varian nuclear magnetic resonance equipobtain 2-substituted pentaboranes. ment.
R
(4) J H. Lamneck and S. K a y e , S a t i o n a l Advisory Committee for Aeronautics, research memorandum E5SE12, Sept. 1958. ( 5 ) U‘. D . Phillips, H . C . Miller a n d E. L . hluetterties. J . Ant. C h e m Soc., 81, 4-196 (1959). 10) R E TViliiams, private communication (7) R. W. P a r r y and I,. J Edwards. J . A m , C h r i n . Soc., 81, 32; 1 (IQ59). (8) R. E. Williams, “Tautomerism and Exchange in the Boron Hydrides; B” and HI NRIR Spectra,” J . I i i o i a . ami .Yncit‘ni. Cheiri , accepted D E P A R T M E s T O F CHEMISTRY
R = METHYL, I
R= METHYL, Ill R= ETHYL, IV
R= ETHYL, II Fig. 1.
-4 mixture of 0.30 millimole of 11’ and 0.3 ml. of 2,B-dimethylpyridine sealed in a 3-mm. tube exexhibited approximately the same B” nuclear magnetic resonance spectrum as pure IIlb (doublet 6 = +13, J = 135 c./’s.; singlet 6 = +3g3 with area ratios 4: 1, respectively). Xfter a period of four hours a t room temperature the rearrangement was essentially complete and the B” n.m.r. profile was that of IV (singlet 6 = - 2 ; doublet 6 = +16, J = 160 c./s.; doublet 6 = + 5 2 , J = 170 c.!s.; with area ratios of l : 3 : 1 respectively). B” n.m.r. spectra taken periodically during the course of the rearrangement indicated a gradual change from I1 to IV with no detectable buildup of intermediate substances. The 6 = + l f doublet of IV was not quite as sharp as the corresponding (1) (a) R. E. Williams,
U. S. P a t e n t 2,917,547, Dec. 15, 1959;
(b) B. N. Figgis a n d R. L. Williams, Speclrockimica A d a , 331, h-0. 5 (1959); (c) N. J. Blay, J. Williams, and R. L. Williams, J . Chem. Soe., 424, 430 (1960). (2) J. N. S h w l e r y , unpublished work. (3) T. P. Onrk, H. Landesman, R . E. R’illiams a n d I . Shapiro, J . Phys. Ckem., 6 3 , 1533 (1959).
THOMAS P. O S A K Los ANGELESSTATECOLLEGE Los ANGELES,CALIFORXIA RECEIVED APRIL 19, 1961
MECHANISM OF FREE RADICAL DECAY IN IRRADIATED POLYETHYLENE. EVIDENCE FROM DEUTERIUM-HYDROGEN EXCHANGE
.Civ: I n 1954’ we postulated t h a t free radicals produced in polyethylene by ionizing radiations decayed by reaction following a random walk migration of the free radical centers through the solid polyethy!ene %e‘ visualized the jumping of hydrogen atoms along or across molecular chains from a saturated CHZThis group to a free radical center -CH--. picture of free radical migration has been adopted by Voevodskii, et aL2 Evidence for the migration of free radicals also comes from the e.s.r. studies of Charlesby and Ormerod.3 In this note we wish to present a new mechanism for the migration of free radicals in solid poly(1) M. Dole, C . D. Keeling and D. G. Rose, J . A m . Chent. .So + R. +RR
(3) (4)
(5,
However, in the Russian work reaction (1) mtght have involved electronically excited or ionized methylene groups because of the existence of these species rather than the free radical R . during the irradiation. I n order to clarify this point we have irradiated Marlex-30 a t liquid nitrogen temperature and in vacuo, warmed the sample to room temperature, pumped off the evolved hydrogen gas and then introduced deuterium gas subsequent to the irradiation. During this treatment probably two thirds of the free radicals decayed6; nevertheless copious exchange up to 25yG conversion of DL to HD involving the remaining free radicals occurred over a period of about 100 hours. During this period there was less then O.2Y0 decrease in the total gas pressure; thus reactions (3) and (4) may be considered neg1igible.j Letting y represent PHJPOD?where POD,is the initial deuterium pressure, it can be shown that for various mechanisms where 2bi.a is the D-H fractionation factor, and and /3 have the values:
01
2585
existed in the polyethylene subsequent to the irradiation. I t is further evident that the R. of Eq. (2) is not a t the same location in the solid polyethylene as the R . of Eq. (1). Thus we have a n additional mechanism for free radical migration in solid polyethylene. Increase of deuterium gas pressure should not increase dy,idt, for reasons to be discussed later, but should lower the extent of D-H exchange because of acceleration of R. decay. This prediction of the mechanism postulated here is borne out by the data. There is a possibility that some of the exchange may be the result of a reaction in which the D-atom replaces the H-atom on the alkyl free radical without free radical migration as suggested by Voevodskii, et ~ l . in, ~a study involving exchange between deuterium gas and ethyl radicals. While such exchanges may occur to a small extent, evidence will be given later that this mechanism cannot account for the major effect. By R.-decay? in this note we include also the possibility of conversion of the alkyl to the allyl free r a d i ~ a l . ' , ~Because ,~ of the greater stability of the allyl free radical, reaction (1) involving it would be less likely by a factor of about lo-". This research was supported by the U. S. Atomic Energy Commission. (7) V. V . Voevodskii, G. K. Lavrovskaya a n d R. E. Mardalelshvili, Doklady Acad. N a u k , S.S.S.R., 81, 215 (1951). (8) Fulbright travel grantee f r o m t h e University of Louvain, Belgium. DEPARTMENT OF CHEMISTRY
NORTHWESTERN UNIVERSITY MALCOLM DOLE ILLINOIS FRAYCISCRACCO* EVANSTON, RECEIVED MARCH 30, 1961 THE ACID-CATALYZED ISOMERIC REARRANGEMENT OF cis AND ~ Y U ~ , ~ - ~ - M E T H Y L - ~ CY CLOHEXENOL-0 '8
Sir: Several years ago the acid-catalyzed (HClOI) decay at bt rearrangement of cis- (I) and trans-5-methyl-2First order R. cyclohexenol (11) in 35% aqueous acetone was in(a/&) (1 - g j (blkr) (1 - q ) decay q = e-kd vestigated in these laboratories and i t was found Second order R. that with both isomers the pseudo first-order rate decay (a,/Bj In (1 + Bt) (blB) In (1 + Bt) of loss of optical activity (ka) is several times The meaning of the kinetic constants a , b, B and ka larger than that of geometric isomerization ( k i ) . will be discussed in the complete paper; t is the This means that the acid-catalyzed allylic retime. Suffice i t to say that for t very small, for the arrangement is stereospecific in the sense t h a t each three sets of a and /3 values Eq. (6) reduces to y isomer is converted to its enantiomer faster than to at. A t long times, however, there is a marked its geometric isomer-in this symmetrical system, difference between the three possibilities. Our allylic rearrangement without geometric isomerizaevidence indicates that R.decays by a first order tion results in the interconversion of enantiomers. process at least a t long times (the mechanism of this I t was suggested that the racemization with preswill be considered later); the extensive and ex- ervation of geometric configuration may involve cellent work of Lawton, Balwit and Powell6 also either an SNi intramolecular rearrangement of demonstrates a first order decay of the alkyl free the conjuga.te acids of the alcohols or an interradical in irradiated Marlex-50. ' molecular stereospecific (cis) S N ~process. Referring to Eqs. (1) and ( 2 ) it is now evident that these reactions must occur provided that HX excited or ionized methylene groups no longer CY
KOfree radical
B
(4) Ya. hl. Varshavskii, G . Ya. Vasil'ev, V. I,. Karpov, Yu. S. 1,azurkin a n d I. Y a . Petrov, D o k i a d y A k a d . Ilauk. S.S.S.R., 118,315 (1958). ( 5 ) T h e possibility of a back thermal reaction between molecular hydrogen and irradiated polyethylene suggested by t h e work of hrvia and Dole. Proc. 2nd Conf. Peaceful Uses of Atomic Enerzy, United Nations, Geneva, 29, 171 (1058) is being further investigated. (6) E. J. Lawtun, J. S . Balwit and R. S. Powell, J . Chew%.Phys., 33, 395 (1960).
H36@J I
I1
I\.c have now reexamined this rearrangement and have confirmed the earlier report' that k, > ki (1) H. I,. Goering and E. F. Silversmith, J. A m . Ckent. S O L . ,79, 3-18(1057).
