726
CHUCHANI AND DOMINGUEZ
Table II Effect of the Inhibitor Toluene on Rates
at 290.6ЊCa
Along this line of work, the theoretical ab initio cal-
culations for the gas phase elimination of 2-chloropro-
pionic acid at the MP2/6-31G** level [4] have sug-
gested the formation of ␣-propiolactone intermediate
through a semi-polar five-membered cyclic transition
structure by the assistance of the acidic H of the
COOH group, but followed by the participation of the
oxygen carbonyl as described in reaction (1), Path 2.
The unstable lactone then proceeded to yield CH3CHO
and CO molecules. Good agreement with the experi-
mental values were obtained.
Ϫ
1
Ps (Torr)
Pi (Torr)
Pi/Ps
104 k1 (s
)
25
61
39.5
36.5
32.5
25
—
66.5
81
159
171
201
—
13.09
11.02
9.25
8.97
8.93
8.95
1.1
2.1
4.4
5.3
8.0
a Vessel seasoned with allyl bromide, Ps ϭ pressure of the sub-
strate; Pi ϭ pressure of the toluene suppressor.
To obtain additional information about the extent
of the nature of the transition state of 2-halocarboxylic
acids decomposition, the present work aimed at study-
ing the pyrolysis kinetics of ␣-bromophenylacetic
acid. This purpose arises from the fact that the
C9Br bond polarization may be rate determining.
Therefore, the higher stabilization of the C9Br bond
by the benzylic carbocation of this molecule may
cause an effective participation of the oxygen carbonyl
for a faster elimination rate.
strate was dissolved in chlorobenzene and injected
directly into the reaction vessel through a silicon
rubber septum.
RESULTS AND DISCUSSION
CHO
ϩ HBr ϩ CO
CHCOOH
EXPERIMENTAL
Br
CH2Br
ϩ CO2
␣-Bromophenylacetic acid (Aldrich) of 98% purity (ti-
tration with 0.05N NaOH solution) was used. The
products benzaldehyde (Aldrich) and benzyl bromide
(Aldrich) were quantitatively analyzed by GLC (10%
SP 1200–1% H3PO4 , Chromosorb W AW DMCS
80–100 mesh). The identities of the substrate and
products were further verified by NMR and mass spec-
trometry.
Kinetic experiments were carried out in a static re-
action system, and in the presence of at least a large
excess of toluene inhibitor. The rate coefficients, ac-
cording to reaction (2) were determined by titrimetric
determination of HBr with a solution of 0.05N NaOH
for benzaldehyde formation, and by GLC for benzyl
bromide formation. The temperature was maintained
within Ϯ0.2ЊC with a calibrated platinum-platinum-
13% rhodium thermocouple, and no temperature gra-
dient was found along the reaction vessel. The sub-
(2)
The homogeneity of the reaction was examined, under
the free radical inhibitor toluene, by using a packed
reaction vessel 6.0 times greater surface-to-volume
ratio than the unpacked reaction vessel (Table I). The
rate coefficients were unaffected in packed and un-
packed vessel seasoned with allyl bromide. Very little
effect was found in the clean packed and unpacked
Pyrex reaction vessel. The effect of the free radical
inhibitor toluene is shown in Table II. The kinetic de-
terminations had to be carried out in the presence of
at least four times the concentration of toluene to sup-
press any possible free radical chain processes of the
substrate and/or product. No induction period was ob-
served, and the rates are reproducible with a standard
deviation not greater than Ϯ5% at a given tempera-
ture.
Table 1 Homogeneity of the Reaction at 290.6ЊCa
Ϫ
1
b
Ϫ
1
c
Ϫ1 d
104 k1 (s
)
S/V (cm
)
104 k1 (s
)
Table III Invariability of Rates Coefficients with
Initial Pressure at 297.7ЊCa
1
6
10.73
10.66
8.95
9.12
P0 (Torr)
20
13.78
32
13.86
48
13.72
52.5
14.10
70
13.68
a In the presence of toluene inhibitor.
b S ϭ Surface area; V ϭ Volume.
c Clean Pyrex vessel.
Ϫ
1
104 k1 (s
)
a Vessel seasoned with allyl bromide and in the presence of at
least fourfold of toluene inhibitor.
d Vessel seasoned with allyl bromide.