36
FARKAS ET AL.
[21] provides ꢀ f H2o98(CH3C(O)CH2) = (−28.1 3.1)
kJ mol−1. That is, a CH3C(O)CH2 heat of formation
which is in between the previous “high” values and the
more recent “low” values reported in the literature (see
Introduction).
That is, no conclusive answer can be offered to
support either the “high” or “low” ꢀ f H2o98 values for
CH3C(O)CH2 by the kinetic information available for
the bromination equilibrium reaction of acetone at the
present stage. Clearly, a more accurate CH3C(O)CH2
heat of formation might be expected from temperature-
dependent kinetic studies of both reaction (1) and
its reverse (−1) by using preferably direct kinetic
methods in a wide and at least partially overlapping
temperature ranges.
Enthalpy of Formation of CH3C(O)CH2
The rate constant determined for reaction (1) in the cur-
rent work enables the estimation of the heat of forma-
tion for the acetonyl radical by employing the so-called
“third-law analysis” procedure [13,14]. To accomplish
this procedure in the present case, the rate constant for
the “reverse” reaction Br + CH3C(O)CH3 (–1) at T =
298 K must also be known.
We are aware of only two kinetic studies of the reac-
tion of Br atoms with acetone [9,21]. King et al. inves-
tigated the gas-phase thermal bromination of acetone
over the temperature range 494–618 K. From measure-
ments of initial rates of Br2 consumption, they deter-
mined a rate constant expression in the Arrhenius form.
An extrapolation of the rate constant expression to
room temperature yields k1 (298 K) = (6.34 7.18) ×
103 cm3 mol−1 s−1 (with 1σ propagated error).
Very recently, we have performed photobromina-
tion kinetic study of acetone employing the relative-
rate method coupled with gas-chromatographic prod-
uct analysis [21]. The rate constant ratio k−1(Br +
acetone)/k (Br + neo-C5H12) = 0.50 0.31 was de-
termined at room temperature. This was converted to
The authors are indebted to Prof. J. Espinosa-Garc´ıa for help-
ful discussions.
BIBLIOGRAPHY
k
−1(298 K) = (2.73 1.71) × 104 cm3 mol−1 s−1 by
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making use of the rate constant expression for the refer-
ence reaction Br + neo-C5H12 that was determined in
a previous direct kinetic study in the temperature range
688–775 K [22]. The two literature rate constants avail-
able for reaction (−1) differ by a factor of 4. This large
disparity may be due, at least in part, to the long-range
extrapolation of the rate constant expressions to room
temperature.
An estimation of the CH3C(O)CH2 heat of forma-
tion by the kinetic data requires auxiliary thermochem-
ical quantities as well. Most of them have been taken
from [20], which are the following: ꢀ f H2o98(HBr) =
(−36.29 0.16), ꢀ f H2o98(acetone) = (217.1 0.7),
and ꢀ f H2o98(Br) = (111.87 0.12) kJ mol−1 as well
as S2o98(HBr) = (198.7
0.01), S2o98(acetone) =
(295.46
0.01), and S2o98(Br) = (175.02
0.01)
J mol−1 K−1. The ab initio computational result of
S2o98(CH3C(O)CH2) = (298
2) J mol−1 K−1 by
Bouchoux et al. [5] was accepted for the entropy
value of the acetonyl radical (the error given is our
estimation).
A combination of k1(298 K) from the current work
with the extrapolated k−1(298 K) from King et al. [9] in
a third law procedure gives ꢀ f H2o98(CH3C(O)CH2) =
(−24.3 5.8) kJ mol−1. This datum is close to the
“high” heat of formation value long in use for the
acetonyl radical (see Introduction).
11. Holmes, J. L.; Lossing, F. P.; Terlouw, J. K. J Am Chem
Soc 1986, 108, 1086.
12. Hoyermann, K. H. In Physical Chemistry—An
Advanced Treatise; Jost, W. (Ed.); Academic: New York,
1975; Vol. VI B, p. 931.
13. Tsang, W. In Energetics of Organic Free Radicals;
Simo˜es, J.; Greenberg, A. M. A.; Liebman, J. F. (Eds.);
Blackie: London, 1996; p. 22.
In contrast with the above result, the third law
analysis of k1(298 K) (this work) and k−1(298 K)