Pyrolysis kinetics of 2-phenoxycarboxylic acids in the gas phase

Article abstract of DOI:10.1002/(SICI)1099-1395(199908)12:8<612::AID-POC169>3.0.CO;2-S

The gas-phase pyrolysis kinetics of primary, secondary and tertiary 2-phenoxycarboxylic acids were studied over the temperature range 240.1-409.9°C and pressure range 16.3-67.8 Torr. The elimination reactions, carried out in seasoned vessels and in the presence of the free radical chain inhibitor cyclohexene. are homogeneous, unimolecular and follow a first-order rate law. The overall rate coefficients are expressed by the following equations: for 2-phenoxyacetic acid, log k₁(s^-1)=(12.08 ± 0.54) -(190.3 ± 6.7)kJ mol^-1;(2.303RT)^-1 for 2-phenoxypropionic acid, log k₁ (s^-1) = (12.21 ± 0.31) -(172.9 ± 3.6)kJ mol^-1 (2.303RT)^-1; for 2-phenoxybutyric acid, log k₁(s^-1) = (12.29 ± 0.38) -(171.7 ± 4.3)kJ mol^-1(2.303RT)^-1; and for 2-phenoxyisobutyric acid, log k₁(s^-1) = (12.84 ± 0.36) -(155.3 ± 3.6)kJ mol^-1(2.303RT)^-1. The products of the phenoxyacids are phenol, the corresponding carbonyl compound and CO, except for 2-phenoxyisobutyric acid, which undergoes a parallel elimination into phenol and methylacrylic acid. The reaction rates increase from primary to tertiary carbon bearing the C₆,H₅O group. The mechanism of these reactions appears to proceed through a semi-polar five-membered cyclic transition state, where the acidic H of the COOH group assists the leaving C₆H₅O substituent for phenol formation, followed by the participation of the oxygen carbonyl for lactone formation. Then the unstable lactone intermediate decomposes into the corresponding carbonyl compound and CO gas. Copyright

Full text of DOI:10.1002/(SICI)1099-1395(199908)12:8<612::AID-POC169>3.0.CO;2-S

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 12, 612–618 (1999)
Pyrolysis kinetics of 2-phenoxycarboxylic acids in the gas
phase
Gabriel Chuchani,* Rosa M. Dominguez, Alexandra Rotinov and Ignacio Martin
Centro de QuõÂmica, Instituto Venezolano de Investigaciones CientÂõ®cas (IVIC), Apartado 21827, Caracas 1020-A, Venezuela
Received 28 December 1998; revised 5 March 1999; accepted 10 March 1999
ABSTRACT: The gas-phase pyrolysis kinetics of primary, secondary and tertiary 2-phenoxycarboxylic acids were
studied over the temperature range 240.1–409.9°C and pressure range 16.3–67.8 Torr. The elimination reactions,
carried out in seasoned vessels and in the presence of the free radical chain inhibitor cyclohexene, are homogeneous,
unimolecular and follow a first-order rate law. The overall rate coefficients are expressed by the following equations:
for 2-phenoxyacetic acid, log k1(s 1) = (12.08 Æ 0.54) (190.3 Æ 6.7)kJ mol 1(2.303RT) 1; for 2-phenoxypropionic
acid, log k1 (s 1) = (12.21 Æ 0.31) (172.9 Æ 3.6)kJ mol 1(2.303RT) 1; for 2-phenoxybutyric acid, log k1(s 1) =
(12.29 Æ 0.38) (171.7 Æ 4.3)kJ mol 1(2.303RT) 1; and for 2-phenoxyisobutyric acid, log k1(s 1) = (12.84 Æ 0.36)
(155.3 Æ 3.6)kJ mol 1(2.303RT) 1. The products of the phenoxyacids are phenol, the corresponding carbonyl
compound and CO, except for 2-phenoxyisobutyric acid, which undergoes a parallel elimination into phenol and
methylacrylic acid. The reaction rates increase from primary to tertiary carbon bearing the C6H5O group. The
mechanism of these reactions appears to proceed through a semi-polar five-membered cyclic transition state, where
the acidic H of the COOH group assists the leaving C6H5O substituent for phenol formation, followed by the
participation of the oxygen carbonyl for lactone formation. Then the unstable lactone intermediate decomposes into
the corresponding carbonyl compound and CO gas. Copyright 1999 John Wiley & Sons, Ltd.
KEYWORDS: 2-phenoxycarboxylic acids; pyrolysis kinetics; gas phase
INTRODUCTION
group L for a five-membered cyclic transition state, while
the hydroxy oxygen was assumed to participate in the
stabilization of the positive carbon reaction center for
lactone formation. However, recent quantum mechanical
calculations on 2-chloropropionic acids,4 glycolic, lactic
and 2-hydroxyisobutyric acids5 and mandelic acid,6 using
ab initio techniques at the MP2/6–31G** level, suggested
that mechanism (1) proceeds by the nucleophilic attack of
the carbonyl oxygen instead the hydroxylic oxygen for
the formation of the a-lactone intermediate:
In association with the elimination kinetics of 2-
halocarboxylic acids,1,2 the gas-phase pyrolyses of
2-hydroxycarboxylic acids have been examined.3
A
sequence of rate increase from primary to tertiary 2-
hydroxycarboxylic acid acid was observed. The mechan-
ism of these reactions was thought to proceed as follows:
ꢀ1†
ꢀ2†
The bond polarization of the C— L bond, in the
direction of Cꢀ‡Á Á ÁLꢀ , was believed to be rate
determining. The H of the COOH assisted the leaving
The OR group of ethers [R = CH3, CH3CH2,
(CH3)2CH] in the gas-phase pyrolyses of 2-alkoxyacetic
acids7 was shown to be faster in the elimination process
than Cl and OH and the mechanism was pictured as in
reaction (1). The alkyl group R in ROCH2COOH had no
significant effect on rates. A recent examination of
ab initio calculations of methoxy-, ethoxy- and isopro-
*Correspondence to: G. Chuchani, Centro de Qu´ımica, Instituto
Venezolano de Investigaciones Cientı´ficas (IVIC), Apartado 21827,
Caracas 1020-A, Venezuela.
Contract/grant sponsor: Consejo National de Investigaciones Cienti-
ficas
y Tecnolo´gicas (CONICIT); Contract/grant number: S1-
97000005.
Copyright 1999 John Wiley & Sons, Ltd.
CCC 0894–3230/99/080612–07 $17.50

GAS-PHASE PYROLYSIS KINETICS OF 2-PHENOXYCARBOXYLIC ACIDS
Table 1. Ratio of ®nal (Pf) to initial (P0) pressurea,b
613
Compound
Temperature (°C)
Pf (Torr)
P0 (Torr)
Pf/P0
Average
2-Phenoxyacetic acid
369.4
389.2
400.2
409.9
315.8
329.8
340.2
351.0
314.9
320.1
328.7
338.5
260.4
270.1
280.0
30.5
29.9
40.0
23.0
24.8
37.6
34.0
30.0
36.2
45.0
51.6
34.6
15.5
44.0
22.5
83.0
87.4
117.6
70.0
70.3
100.6
91.5
85.5
99.7
125.5
144.0
202.0
35.5
97.0
54.0
2.72
2.92
2.94
3.04
2.83
2.87
2.80
2.85
2.75
2.79
2.79
2.78
2.29
2.20
2.40
2.91
2.83
2.78
2.30
2-Phenoxypropionic acid
2-Phenoxybutyric acid
2-Phenoxyisobutyric acid
a
Vessel seasoned with allyl bromide.
b In the presence of the inhibitor cyclohexene.
poxyacetic acids molecular eliminations at the MP2/6–
of 2-phenoxyacetic acid, 2-phenoxypropionic acid, 2-
phenoxybutyric acid and 2-phenoxyisobutyric acid were
examined.
31G** level8 implied the mechanism to be as described
as
RESULTS AND DISCUSSION
2-Phenoxyacetic acid
ꢀ3†
The gas-phase elimination of this substrate proceeds
according to the reaction
C6H5OCH2COOH ! C6H5OH ‡ CH2O ‡ CO ꢀ4†
To increase our understanding of the nature of the
transition state in the gas-phase pyrolyses of 2-substituted
carboxylic acids, the investigation of a better leaving
group such as phenoxy, C6H5O, was thought to be of
interest. Therefore, the molecular elimination kinetics
The stoichiometry of the reaction demands Pf/P0 = 3,
where Pf and P0 are the final and initial pressure,
respectively. The average experimental results at four
different temperatures and 10 half-lives is 2.91 (Table 1).
Table 2. Stoichiometry of the reactionsa
Temperature
Substrate
(°C)
Parameter
Value
Phenylacetic acid
389
Time (min)
4
6
8
10
13
Reaction (%) (pressure)
Phenol (%) (GLC)b
Time (min)
20.5
21.1
3
33.5
31.2
5
48.2
45.7
8
56.9
54.1
12
78.2
76.2
17
2-Phenoxypropionic acid
2-Phenoxybutyric acid
2-Phenoxyisobutyric acid
320.0
320.1
250
Reaction (%) (pressure)
Acetaldehyde (%) (GLC)
Time (min)
Reaction (%) (pressure)
Propionaldehyde (%) (GLC)
Time (min)
25.0
28.5
2.5
20.4
20.9
3
39.7
42.2
4
30.4
28.6
6
44.8
47.1
6
41.8
41.3
9
59.8
61.5
8
51.5
51.0
12
72.7
75.0
10
58.8
59.4
12
65.3
64.3
Phenol (%) (GLC)
Acetone (%) (GLC)
Methylacrylic acid (%) (GLC)
35.5
19.3
15.5
57.3
33.1
24.8
72.2
40.2
31.3
83.8
46.5
34.5
a
Vessel seasoned with allyl bromide, and in the presence of cyclohexene inhibitor.
b GLC = gas-liquid chromatography.
Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 612–618 (1999)

614
G. CHUCHANI ET AL.
Table 3. Homogeneity of the reaction
1 a
1 b
1 c
Substrate
Temperature (°C)
S/V (cm
)
104k1 (s
)
104k1 (s
)
2-Phenoxyacetic acid
389.2
320.0
320.2
250.0
1
6
1
6
1
6
1
6
15.94
20.46
11.65
11.70
15.57
15.53
12.08
11.94
11.13
11.25
15.19
15.24
2-Phenoxypropionic acid
2-Phenoxybutyric acid
2-Phenoxyisobutyric acid
9.88d
4.01d
(12.30e ‡ 9.15f) = 21.45
(12.07e ‡ 9.74f) = 21.81
a
S = surface area (cm2); V = volume (cm3).
b Clean Pyrex vessel.
c
Vessel seasoned with allyl bromide.
d Average overall k values.
e
k Value of acetone formation.
f k Value of methylacrylic acid formation.
Further verification of the stoichiometry of reaction (4)
was made by comparing, up to 75% decomposition, the
pressure measurements with the results of quantitative
gas chromatographic analysis of phenol (Table 2).
The homogeneity of the reaction was examined by
carrying out several runs in a vessel with a surface-to-
volume ratio of 6.0 relative to that of the normal vessel,
which is equal to 1.0. A packed and unpacked Pyrex
vessel, seasoned with allyl bromide, had no effect on
rates. However, the packed and unpacked clean Pyrex
vessel had a significant heterogeneous effect on the rate
coefficient (Table 3). The effect of a free radical inhibitor
is shown in Table 4. The kinetic runs had to be carried
with at least a threefold excess of cyclohexene in order to
suppress any possible free radical processes of the
reactant and/or products. No induction was observed.
The rate coefficients were reproducible with a relative
standard deviation of ꢁ5% at any given temperature.
The first-order rate coefficients of phenoxyacetic acid,
calculated from k1 = (2.303/t) log[2P0/(3P0 Pt)], were
independent of the initial pressure (Table 5). A plot of log
(3P0 Pt) against time t gave a good straight line up to
75% decomposition. The variation of the rate coefficient,
in a seasoned vessel and in the presence of the inhibitor
cyclohexene, with temperature is shown in Table 6. The
data were fitted to the Arrhenius equation, where the rate
Table 4. Effect of the free radical inhibitor cyclohexene on ratesa
Temperature
1
Substrate
(°C)
Ps (Torr)
Pi (Torr)
Pi/Ps
104k1 (s
)
Phenoxyacetic acid
389.9
55.6
67.8
62.8
53.0
34.0
21.7
38.1
37.9
43.0
34.3
35.9
20.0
49.6
51.6
21.7
26.1
23.0
44.0
22.8
16.3
16.7
—
97.5
108.6
106.0
99.6
67.5
—
58.0
89.5
110.5
146.5
—
32.0
62.0
67.0
117.5
—
44.5
29.0
70.0
84.5
—
1.4
1.7
2.0
2.9
3.1
—
1.5
2.1
3.2
4.1
—
0.7
1.2
2.3
4.5
—
1.0
1.3
4.3
5.1
18.83
13.08
12.82
12.81
12.91
12.77
12.05
11.63
10.75
11.17
11.17
25.20
25.11
25.00
25.08
25.09
2-Phenoxypropionic acid
2-Phenoxybutyric acid
2-Phenoxyisobutyric acid
320.0
329.1
260.0
(24.81b ‡ 18.96c) = 43.77
(24.69b ‡ 18.12c) = 42.81
(24.88b ‡ 18.33c) = 43.21
(24.82b ‡ 18.36c) = 43.18
(24.84b ‡ 18.32c) = 43.16
a
Ps = pressure of the substrate; Pi = pressure of the inhibitor. Vessel seasoned with allyl bromide.
b k Value of acetone formation.
c
k Value of methylacrylic acid formation.
Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 612–618 (1999)

Table 5. Invariability of rate coef®cients with initial pressure
Compound
Temperature (°C)
Parameter
Values
2-Phenoxyacetic acid
389.9
P0 (Torr)
21.7
12.91
34.0
12.81
53.0
12.82
67.8
12.77
1
104 k1 (s
)
)
)
)
2-Phenoxypropionic acid
2-Phenoxybutyric acid
2-Phenoxyisobutyric acid
320.0
320.1
260.4
P0 (Torr)
22.0
34.3
43.0
50.7
1
104 k1 (s
11.53
22.1
11.17
27.4
10.75
34.6
11.33
37.6
P0 (Torr)
44.0
15.29
1
104 k1 (s
15.06
15.17
15.13
15.22
P0 (Torr)
16.3
23.0
31.0
44.0
1
104 k1 (s
(24.82a ‡ 18.36b)
(24.81a ‡ 18.96b)
(24.90a ‡ 18.83b)
(24.69a ‡ 18.12b)
a
k Value of acetone formation.
k Value of methylacrylic acid formation.
b
Table 6. Variation of rate coef®cients with temperaturea
Compound
Parameter
Values
349.8
2-Phenoxyacetic acid
Temperature (°C)
359.9
2.24
369.4
4.21
380.3
6.94
389.2
12.08
400.5
21.20
409.9
34.42
1
104 k1(s
)
1.41
1
Rate equation
Log k1 (s 1) = (12.04 Æ 0.54) (190.3 Æ 6.7)kJ mol (2.303RT) 1; r = 0.9992
2-Phenoxypropionic acid
2-Phenoxybutyric acid
2-Phenoxyisobutyric acid
Temperature (°C)
310.0
6.39
315.8
8.72
320.0
11.13
329.8
18.97
340.2
37.77
351.0
63.93
1
104 k1 (s
)
Rate equation
Log k1(s 1) = (12.28 Æ 0.31) (172.9 Æ 3.6)kJ mol 1(2.303RT) 1; r = 0.99915
Temperature (°C)
300.2
4.34
309.3
8.05
314.9
11.18
320.2
15.19
329.1
25.10
338.5
42.10
1
104 k1 (s
)
Rate equation
Log k1(s 1) = (12.29 Æ 0.38) (171.7 Æ 4.3)kJ mol 1(2.303RT) 1; r = 0.9997
Temperature (°C)
240.1
10.48
250.5
22.66
260.4
43.49
270.1
79.23
280.0
147.0
1
104 k1 (s
)
Rate equation
Log k1(s 1) = (12.84 Æ 0.36) (155.3 Æ 3.6)kJ mol 1(2.303RT) 1; r = 0.9996
a
Vessel seasoned with allyl bromide and in the presence of cyclohexene inhibitor.

616
G. CHUCHANI ET AL.
coefficient at the 90% confidence level obtained from
least-squares procedures are given.
small departure from stoichiometry in the reaction
CH3CH2CHꢀOC6H5†COOH
! C6H5OH ‡ CH3CH2CHO ‡ CO
ꢀ6†
2-Phenoxypropionic acid
was due to a parallel decomposition to give, after 68%
reaction, small amounts of phenol, vinylacetic acid and
crotonic acid. Additional examination of the stoichio-
metry of reaction (6), up to 65% reaction, showed that the
percentage decomposition of the phenoxybutyric acid
calculated from pressure measurements was in good
agreement with that obtained from the chromatographic
analysis of the product propionaldehyde (Table 2).
Reaction (6) was found to be homogeneous since no
significant variations in rates were obtained when using
both clean Pyrex and seasoned vessels with a surface-to-
volume ratio of 6.0 relative to that of the normal vessel,
which is equal to 1.0 (Table 3). The free radical inhibitor
cyclohexene had no effect on the rate of elimination
(Table 4). No induction period was observed. The rate
coefficients were reproducible with a relative standard
deviation of <5% at any given temperature.
The experimental stoichiometry for the gas-phase
elimination of 2-phenoxypropionic acid, as described in
reaction (5), implies Pf = 3P0.
CH3ꢀC6H5O†CHCOOH ! C6H5OH ‡ CH3CHO
ꢀ5†
‡ CO
The average Pf/P0 at four different temperatures and 10
half-lives was 2.83 (Table 1). The small departure from
the theoretical stoichiometry was due to a parallel path,
where less than 1% of phenol and acrylic acid are formed.
Verification of the stochiometry of reaction (5), up to
75% reaction, was possible, considering that the
acetaldehyde produced is equivalent to the pressure
increase (Table 2).
The pyrolysis reaction (5) is homogeneous, since no
significant variations in rates were obtained in these
experiments when using both clean Pyrex and seasoned
vessels with a surface-to-volume ratio of 6.0 relative to
that of the normal vessel, which is equal to 1.0 (Table 3).
This substrate was always pyrolyzed in the presence of at
least a twofold excess of the free radical chain inhibitor
cyclohexene (Table 4). No induction period was observed
and the rates were reproducible with a relative standard
deviation of ꢁ5% at any given temperature.
The rate coefficients were shown to be independent of
the initial pressure of this substrate (Table 5), and the
first-order plots are satisfactorily linear up to about 65%
reaction. The variation of the rate coefficient with
temperature is described in Table 6, which gives rate
coefficients at the 90% confidence limits obtained from a
least-squares procedure.
The rate coefficients were found to be invariant of their
initial pressure (Table 5). The logarithmic plots are linear
up to 75% decomposition. The temperature dependence
of the first-order rate coefficients is shown in Table 6,
2-Phenoxyisobutyric acid
The pyrolysis products of 2-phenoxyisobutyric acid
according to the reaction
C6H5OH ‡ CH3COCH3 ‡ CO
%
ꢀCH3†2C(C6H5O)COOH
ꢀ7†
&
C6H5OH ‡ CH2 ˆ C(CH3†COOH
where rate coefficients at the 90% confidence level
obtained with a least-squares method are given.
make it difficult to determine the stoichiometry from the
pressure increase. The average experimental Pf /P0 at
three different temperatures and 10 half-lives is 2.30
(Table 1). However, to verify the stoichiometry of
reaction (7), up to 84% decomposition, quantitative
chromatographic analyses of total phenol formation were
compared with the sum of acetone and methacrylic acid
products (Table 2). The effect of the surface on the rate in
a vessel with a surface-to-volume ratio six times greater
than that of the normal vessel showed a significant
heterogeneous effect in a clean packed and unpacked
Pyrex vessel (Table 3). This result appears to be due to
2-Phenoxybutyric acid
The stoichiometry for the gas-phase elimination of 2-
phenoxybutyric acid as represented in reaction (5)
suggests Pf = 3P0. The average Pf/P0 at four different
temperatures and 10 half-lives was 2.78 (Table 1). The
Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 612–618 (1999)

GAS-PHASE PYROLYSIS KINETICS OF 2-PHENOXYCARBOXYLIC ACIDS
617
Table 7. Variation of rate coef®cient with temperature for
formation of acetone and methylacryclic acid
According to the kinetic parameters described in Table
8, the elimination rates of the phenoxy acids increase
from primary to tertiary carbon bearing the C6H5O
susbtituent. Moreover, with the support of theoretical
studies on the reaction pathways of 2-substituted chloro4
and hydroxy5,6 carboxylic acids, the mechanism may be
rationalized in terms of a moderately polar bicyclic
(3,1,0) transition structure, where an a-lactone inter-
mediate is achieved by the assistance of acidic hydrogen
of the COOH, followed by nucleophilic attack of the
carbonyl oxygen. The unstable lactone proceeds to
decompose into the corresponding carbonyl compound
and carbon monoxide:
1
104 k1 (s
)
Temperature (°C)
Acetone
Methacrylic acid
240.1
250.5
260.4
270.1
280.0
6.12
13.03
24.94
43.03
81.17
4.36
9.63
18.55
36.20
65.87
some polymerization of methacrylic acid. However, the
rate of elimination was unaffected in a seasoned packed
and unpacked vessel. 2-Phenoxyisobutyric acid was
always pyrolyzed in seasoned vessels and in the presence
of at least a twofold excess of the free radical inhibitor
cyclohexene (Table 4). No induction period was observed
and the rates were reproducible with a relative standard
deviation of ꢁ5% at any given temperature.
The rate coefficients, in seasoned vessels and in the
presence of cyclohexene, were found to be independent
of the initial pressure, and the first-order plots are
satisfactorily linear up to about 80% reaction (Table 5).
The variation of the rate coefficients with temperature is
described in Table 6 (90% confidence coefficient).
The partial rates of primary product formation,
described in reaction (7), were determined, up to 80%
decomposition of the phenoxyisobutyric acid, by quanti-
tative gas chromatographic analyses of acetone and
methacrylic acid. The variation of the rate coefficients for
the formation of these products with temperature (Table
7) gives, by a least-squares procedure and with a 90%
confidence coefficient, the following Arrhenius equa-
tions: for acetone formation
With respect to the parallel elimination pathway of 2-
phenoxyisobutyric acid in the formation of phenol and
methacrylic acid, the mechanism may be associated with
the gas-phase pyrolysis of tert-butyl phenyl ether,9,10
where the unimolecular elimination of phenol involves a
polar four-centered cyclic transition-state reaction. Con-
sequently, reaction of this phenoxy acid appears to
proceed through a semi-polar four-membered cyclic
transition state:
1
log k1ꢀs † ˆ ꢀ12:20 Æ 0:52† ꢀ151:3
Æ 5:2†kJ mol ꢀ2:303RT† 1; r
1
ˆ 0:9996
EXPERIMENTAL
2-Phenoxyacetic acid. 2-Phenoxyacetic acid was pre-
pared by treating bromoacetic acid with phenol as
reported.11 The product was recrystallized several times
from water, m.p. 97°C. The primary product phenol
(Aldrich) was analyzed using GLC: 10% SP-1200–1%
H3PO4 on Chromosorb W AW DMCS, 80–100 mesh.
and for methacrylic acid:
1
log k1ꢀs † ˆ ꢀ12:99 Æ 0:38† ꢀ160:5
Æ 3:9†kJ mol ꢀ2:303RT† 1; r
1
ˆ 0:9998
Table 8. Kinetic parameters for the assumed a-lactone formation at 300.0°C
DS‡
DH‡
1
1
1
1
1
1
Substrate
104 k1 (s
)
Relative rate
Ea (kJ mol
)
LogA (s
)
(J mol
K
)
(kJ mol )
2-Phenoxyacetic acid
0.054
3.31
4.37
1.0
61.3
80.9
190.3 Æ 6.7
172.9 Æ 3.6
171.7 Æ 4.3
151.3 Æ 5.3
12.08 Æ 0.54
12.28 Æ 0.31
12.29 Æ 0.38
12.20 Æ 0.52
6.55
5.64
5.59
6.01
195.1
177.7
176.5
156.1
2-Phenoxypropionic acid
2-Phenoxybutyric acid
2-Phenoxyisobutyric acid
257.04
4760.0
Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 612–618 (1999)

618
G. CHUCHANI ET AL.
2-Phenoxypropionic acid. This substrate (Aldrich) was
found to be of >98.8% purity (GLC: Porapak R, 80–100
mesh). The pyrolysis products acetaldehyde (Aldrich)
and phenol (Aldrich) were quantitatively analyzed in the
same column.
platinum 13% rhodium thermocouple. The substrates 2-
phenoxyacetic acid, 2-phenoxypropionic acid and 2-
phenoxybutyric acid dissolved in dioxane (Merck) and 2-
phenoxyisobutyric acid dissolved in chlorobenzene
(Aldrich) were injected directly into the reaction vessel
through a silicone-rubber septum.
2-Phenoxybutyric acid. Phenoxybutyric acid (Aldrich) of
99.9% purity determined by GLC (10% SP-1200–1%
H3PO4 on Chromosorb W AW DMCS, 80–100 mesh)
was used. The product propionaldehyde (Aldrich) was
quantitatively analyzed in a column of Porapak Q, 60–80
mesh.
Acknowledgements
We are grateful for financial support to the Consejo
Nacional de Investigaciones Cientificas y Tecnolo´gicas
(CONICIT) of Venezuela, Project No. S1-97000005, and
to Armando Herize for technical assistance.
2-Phenoxyisobutyric acid. The synthesis of this substrate
was carried out by mixing phenol (Aldrich), 1,1,1-
trichloro-2-methyl-2-propanol (Aldrich) in NaOH and
acetone (Merck) as described12 (m.p. 99–100°C). NMR:
ꢀ 2.55 (s, 6H), 7.1 (m, 5H), 9.4 (s, 1H). MS: m/z 180
REFERENCES
‡
‡
‡
(M ), 135 [M
C6H5OC(CH3)2], 94 (M
C6H5O), 77
‡
(M
C6H5). The pyrolysis product acetone (Merck)
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was quantitatively analyzed in a column of diisodecyl
phthalate 5%–Chromosorb G AW DMCS, 60–80 mesh,
and the methacrylic acid (Aldrich) in a column of 10%
SP-1200–1% H3PO4 on Chromosorb W AW DMCS, 80–
100 mesh.
The identities of the substrates and products were
further verified by mass and NMR spectrometry.
Kinetic experiments. The phenoxy acids were pyrolyzed
in a static reaction system, seasoned with allyl bromide,
and in the presence of cyclohexene as a free-radical chain
suppressor. The rate coefficients were determined by
pressure increase and by quantitative GLC analyses of
the products. The temperature was controlled by a Shinko
DIC-PS resistance thermometer controller maintained
within Æ0.2°C and measured with a calibrated platinum–
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Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 612–618 (1999)