2074
DONNA KUTSKOBAKALE AND HUGH A. GILLIS
Free Radical Chain Reactions in the Radiolysis and Photolysis of Isobutyl Bromide' by Donna Kutsko Bakale2 and Hugh A. Gillis3 Department of Chemistry, Case Western Reserve Universitg, Cleveland, Ohio 44106 and Division of Physics, National Research Council, Ottawa 7, Canada (Received November 12, 1969)
The radiolysis or photolysis of isobutyl bromide results in isomerization to t-butyl bromide and decomposition to isobutylene and hydrogen bromide with initial yields which are characteristic of chain processes. These decomposition products reach equal plateau concentrations of about 0.05 mol % at low doses for pure isobutyl bromide. The effects of dose rate and of adding isobutylene, hydrogen bromide, or ethylene on the major products have been studied. All of the observations are consistent with a free radical chain mechanism in which the following are important reactions: Br . (CH3)&HCH2Bre HBr (CH3)2CCHZBr ( 5 ) ) Br. (CH&C=CH2 Ft (CH3j2CCH2Rr(6), Br. (CH,),C=CHz (CH3)2CBrCHt. ( 7 ) . From the data equilibrium constants are calculated for these reactions: K s = (7.9 f 2.2) X K O= 323 =t60 1. mol-'; K7 = 4 =k 2 1. mol-'. The activation energy for reaction 6 is estimated at 8.5 kcal mol-'. The production of t-butyl bromide could also be explained by an intraradical bromine atom shift or by the formation of a bridged bromoalkyl radical, instead of by reaction 7.
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Introduction During the past few years several reports on the radiation-induced isomerization of alkyl bromides and chlorides have been published. The radiation-induced isomerization of n-propyl chloride to isopropyl chloride has been particularly thoroughly studied by Willard and c o ~ o r k e r s . ~ JIt was shown that the results can be explained by an HCI-catalyzed chain mechanism involving an intraradical chlorine atom shift which is presumably reversible CH3CHCH2C1
CH3CHC1CH2 Q
n-Butyl chloride and isobutyl chloride undergo similar reaction^.^ I n contrast to these results with the chlorides, the radiation-induced isomerization of n-propyl bromide to isopropyl bromide proceeds with a G of only about 2.1,j.e which suggests that a chain mechanism is not operative, and the reaction is not catalyzed by HC1.6 Since the present investigation was begun, experiments have been reported on the radiation-induced isomerization of n-butyl bromide to sec-butyl bromide which proceeds with quite large G ' s . ~ However, a onestep intraradical shift is not invoked. Rather it is proposed that a bromoalkyl radical rearranges through steps involving elimination of a bromine atom, olefin isomerization, and readdition of a bromine atom. I n an investigation carried out in collaboration with one of us (H. A. G.), Gallivan found that upon irradiation a t low dose rates, isobutyl bromide gives large initial yields of isobutylene (G 1110), and t-butyl bromide (G _N 120).8 These yields, unfortunately rather irreproducible, are very much larger than those reported earlier by WilcoxQa t much higher doses. The present work was carried out with the hope of obtaining T h e Journal of Physical Chemistry
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further information on the mechanism of the radiation-induced chain decomposition and isomerization of isobutyl bromide.
Experimental Section Materials. Isobutyl
bromide from Matheson Coleman and Bell was washed repeatedly with distilled water to remove t-butyl bromide (by preferential hydrolysis) and isobutylene (by hydration), dried, and further purified by distillation at 8-10 mm through a 3-ft column packed with glass helices. The average mole percentages of impurities in material used in experiments were determined by gas chromatography t o be as follows: isobutane,
