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the values listed in the NBS Table.2a However, the
NBS values wl3re rejected by Furuyama, Golden, and Benson4 on the basiE of a large scatter in the secondorder differences, A [ A ( A H f o ) ] in the bromomethane series. Bernstein's interaction scheme appears to work fairly weU, not only for estimating heats of formation, but for other molecular properties as wellq3 Nevertheless, ti consideration of the bond dissociation energy6 and hoat of formation datazb used by Bernstein to arrive at bHfozg8(CBr4) = 38 kcal/moF (used for calculating bond energy and interaction parameters) indicates that the estimates of the heats of formation of CH2Br2,CHIBr3,and CBr4 could be in error by as much as f3 4 kcal/niol. The initial rate measurements were interpreted in accordance with the following mechanism, which is similar to that for other thermal bromination system^^*^ 2Br; KBrz
Brz
1
r3
+ HBr CBr4 + Br
eBrg 2
3
eBr3 -t Brz 4
A steady-state treatment gives - (d [BrL]/dt) =
kLK1'zBrz[Brz]''z [CHBra]
The reproducibilit,y of kl for a threefold change in Br2 pressure anid a twofold change in CHBr3 pressure suggests that the mechanism is correct. The A factors for bromine atom reactions show consistent trends with the type of hydrogen atom being abstracted and the structure of the substrate.$ Moreover, they are well predicted by transition-state theory calculations.5~8~10 Therefore, a consideration of the experimental A factors for similar bromine atom reac-
t i o n ~combined ~ , ~ ~ with a transition-state theory calculation leads to an assignment for A1 of 1010.0k0.51. mol-l sec-l. Thus, taking the mean value of kl from Table I at T , = 588°K) it may be calculated that El = 10.8 f 1.2 kcal/mol. Previous considerations5 have shown that, in general, reactions such as (2) have E2 = 2 f 1 kcal/mol, and therefore, dH1,~(588"K)= 8.7 f 1.5 kcal/mol. Combined with an estimated E,= 1.0 f 1gibbs/mol, this leads to AH1,2(29801
