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
