LESLIEBATTAND FRANK R. CRUICKSHANK
1836
in NOz pressure. However, it is necessary to consider reactions 8-10 to explain the behavior at the end of the process when (NO2) is approaching a final steady state.
Further investigation of this system a t low partial pressures of NOz and HN03 should prove of interest in atmospheric photochemistry.
Complex Formation in the Gas-Phase Reaction of Hydrogen Bromide with Di-t-butyl Peroxide
by Leslie Batt and Frank R. Cruickshank Department of Chemistry, Uniuersity of Aberdeen, Old Aberdeen, Scotland Accepted and T~ansmittedby the Faraday Society
(August 10, 1966)
A mechanism for the decomposition of di-t-butyl peroxide (dtBP) in the presence of HBr, HBr + t-BuOH Br (2), t-BuO + 140" and 100 mm, is dtBP + 2t-BuO (l), 1-BuO Me&O Me (3), Me HBr CH, Br (4), Me Br2+ MeBr Br (5), Me Qe --t C2Hs (6), Br Br M --t Br2 M (7), Me Br (M?) --t MeBr (M?) (8). Here reaction 7 is the main termination process with reactions 6 and 8 playing minor roles, in contrast to the normal decomposition of dtBP where reaction 6 is the main termination process. Formation of isobutene oxide (IBO) indicates a catalyzed decomposition of dtBP according to Br dtBP + dtBP-H HBr (9), dtBP-H + IBO t-BuO (10). Alternatively, complex formation occurs between dtBP and Brz or HBr, IBO being subsequently produced as a result of this. The variation of pressure with time and the very low experimental value of Jcg provide some evidence for complex formation. The following Arrhenius parameters have been estimated. EZ = 15.5-17.5 kcal/mole (assuming A2 = 1013.5sec-l, AB = lo9 l./mole sec, E2 = 0-2 kcal/mole); A4 = 108.g5l./mole sec, E4 = 2.9 kcal/mole, E5 = 0.9 kcal/mole; E , = 17 kcal/mole, E-, = 5 kcal/mole.
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Introduction
Experimental Section
Di-t-butyl peroxide is a very convenient thermal source of t-butoxy radicals (t-BuO) in the gas phase over the temperature range 120-180O (for a static system). This allows a study to be made of the pressure-dependent decomposition of t-BuO,' provided that an efficient radical trap is used to measure its concentration. Raley, Rust, and Vaughan have shown that HBr does not catalyze the decomposition of dtBPz and therefore seems to be a suitable radical trap for studying the decomposition of t-BuO.
The dtBP was purified as before.' The HBr (Matheson) was dried by passing it through a Dry Ice-acetone trap several times, which also removed traces of bromide, bulb-to-bulb distilled, and stored in a 3-1. bulb at -80' in the dark. The apparatus and essential experimental technique have been described in detail el~ewhere.~Auramine
The Journal of Physical Chemistry
(1) L. Batt and 9. W. Benson, J . Chem. Phgs., 3 6 , 895 (1962). (2) J. H . Raley, F. F. Rust, and W. E. Vaughan, J . A m . Chem. Soc., 7 0 , 2767 (1948).
REACTION OF HYDROGEN BROMIDE WITH DI-GBUTYL PEROXIDE
1837
Table I: Products Formed in the Decomposition of dtBP in the Presence of HBr at 140’. All Quantities Expressed in #moles Length of run.
Run
min
2
13.3
3
25.9
Initial dtBP
Final dtBP
CHd
1204 1283
1157 1203
62.2 71.5
CnHa
0.141 99.8
CHaBr
MerCO
-3 16.2
65 129.2
was used to absorb the HBr4 prior to condensation of the products and the presence of t-BuOBr was inferred from the green coloration produced on passing the products and undecomposed reactants from the reaction vessel through the auramine.6
Resnlts and Discussion Three runs were carried out a t 140’ and a total pressure of 100 mm. The products and the details of the individual concentrations of dtBP and HBr are shown for two of the runs in Table I. In contrast to no t-BuBr was found. Its formation is Tipper, et probably due to a condensed phase reaction between HBr and t-BuOH. A mechanism for the formation of the products of the reaction other than isobutylene oxide (IBO) is dtBP +2t-BuO t-BuO
+ HBr +t-BuOH + Br
+ M +Me2CO + Me + M Me + HBr CH4 + Br Me + Brz +MeBr + Br Me + Me CZH6 Br + Br + M Br2 + M Me + Br MeBr
t-BuO
4
(1)
(3) (4)
(5) (6)
--j
(7) (8)
The high yield of CHI compared to CzHa (see Table I, run 2) demonstrates the high efficiency of HBr as a radical trap. The high yield of C2H6 in run 3 is due to the disappearance of HBr before the end of the run. Using a value of 0.5 pmole for C2Ha and an average value of HBr of 37 pmole, the relationship RcH,/ R’/’C,H, = k,(HBr)/ka’” produces a value for k4 of 2.6 X lo7 l./mole sec at 140°, assuming Shepp’s value for ka? The standard entropy change computed from listed standard entropiess.9 is 8.0 eu for the equilibrium Br CHh Me HBr. A-4 has been measured to be 1010.7l./mole secl0 and hence A4 is 108*96I./mole sec. Thus our data produce a value for E4 of 2.9 kcal/mole which is in reasonable agreement with 4.5 kcal/mole found by O’Nealll and with 1.7
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