Kinetics and mechanism of thermal gas-phase elimination of 2-aryloxyacetic acid

Article abstract of DOI:10.1002/kin.1058

Rates of thermal gas-phase elimination of eleven 2-aryloxyacetic acid have been measured over a 45°C temperature range for each compound. Hammett correlation of the present kinetic data with the literature σ⁰ values of the given substituents gave a reaction ρ constant of 0.69 at 600 K; this is more than that for the gas-phase elimination parameter of 2-aryloxypropanoic acid (ρ = 0.26) and consistent with a transition state with some charge separation, suggesting a partial formation of carbocation. The implications of this observation for the thermal gas-phase elimination of α-aryloxycarboxylic acids are considered.

Full text of DOI:10.1002/kin.1058

Kinetics and Mechanism
of Thermal Gas-Phase
Elimination of
2-Aryloxyacetic Acid
NOURIA A. AL-AWADI,1 AJITH KUMAR,1 GABRIEL CHUCHANI,2 ARMANDO HERIZE2
1Chemistry Department, Kuwait University, 13060 Safat, Kuwait
2Centro de Quimica, Instituto Venezolano de Investigaciiones Cientificas(IVIC), Apartado 21827,
Caracas 1020-A, Venezuela
Received 27 July 2000; accepted 8 June 2001
ABSTRACT: Rates of thermal gas-phase elimination of eleven 2-aryloxyacetic acid have been
measured over a 45 C temperature range for each compound. Hammett correlation of the
0
present kinetic data with the literature
values of the given substituents gave a reaction
constant of 0.69 at 600 K; this is more than that for the gas-phase elimination parameter
of 2-aryloxypropanoic acid ( = 0.26) and consistent with a transition state with some charge
separation, suggesting a partial formation of carbocation. The implications of this observation
C
for the thermal gas-phase elimination of -aryloxycarboxylic acids are considered. 2001 John
Wiley & Sons, Inc. Int J Chem Kinet 33: 612–616, 2001
Moreover, with the support of theoretical studies
on the reaction pathways of 2-substituted chloro [5]
and hydroxy carboxylic acids [6,7], the mechanism of
the gas-phase pyrolysis kinetics of primary, secondary,
and tertiary 2-phenoxycarboxylic acids [8] were ratio-
nalized in terms of a moderately polar bicyclic (3,1,0)
transition structure, where an -lactone intermediate
is achieved by the assistance of the acidic hydrogen of
COOH, followed by nucleophilic attack of the carbonyl
oxygen. The unstable lactone proceeds to decompose
into the corresponding carbonyl compound and carbon
monoxide [1] (see Eq. (2)).
INTRODUCTION
An extensive study of substituent effects by Chuchani
and coworkers led to the elucidation of the mechanism
of the thermal gas-phase elimination of the halo and
hydroxy acids and their alkyl esters [1–4]. The overall
reaction scheme for the -substituted carboxylic acids
follow the pathway (1):
heat
CH3CHXCO2H → HX + CH3CHO + CO (1)
Where X = Cl, Br, and OH.
Correspondence to: Nouria A. Al-Awadi; e-mail: chesc@kuc01.
kuniv.edu.kw.
Contract grant sponsor: Kuwait University.
Contract grant number: SC 098.
Contract grant sponsor: Consejo National de Investigacione
Cientificas (CONICIT).
Contract grant number: SI-97000005.
2001 John Wiley & Sons, Inc.
c

THERMAL GAS-PHASE ELIMINATION OF 2-ARYLOXYACETIC ACID
613
Recently one of us has reported [9] kinetic re-
RESULTS AND DISCUSSION
sults and proposed mechanism for the thermal gas-
phase elimination of 2-phenoxypropanoic acid, its thio-
phenyl and its N-phenylamino analogs. The effect
of the -substituent (L) was found to be related to
what has been described as the capacity of the sub-
stituent (L) to accommodate the negative charge be-
ing developed in the transition state. The correla-
tion of relative reactivity at 600 K with group (L) is
shown below in Table I for -substituted propanoic
acid.
Compounds being investigated are: 2-phenoxy- (1), 2-
( p-methyl)- (2), 2-( p-ethyl)- (3), 2-( p-tert-butyl)- (4),
2-( p-methoxy)- (5), 2-( p-formyl)- (6), 2-( p-acetyl)-
(7), 2-( p-bromo)- (8), 2-( p-flouro)- (9), 2-( p-cyano)-
(10), and 2-( p-nitro)- (11) phenoxyacetic acid.
Each of the 2-aryloxyacetic acid gave excellent re-
producible kinetic results, first-order to >98% reaction.
The kinetic data (Table III) gave excellent Arrhenius
plots with no deviant points. This is the most reliable
indicator of the absence of surface-catalyzed elimina-
tion [10].
Products of pyrolysis were analyzed using online
GC-MS and FT-IR pyroprobe. The constituents of all
pyrolysated products were ascertained to be an arene
(ArOH) fragment together with CH2O and CO. The
rate constants and relative rates have been assessed at
600 K, Table IV.
The sign and magnitude of the -factor of a reaction
is considered to be a measure of the charge separation
in the rate-determining step of the transition state, and
the value of 0.69 (Fig. 1) and 0.26 for 2-aryloxyacetic
and propanoic acids, respectively, suggest that the sub-
stituents at the aryl group of -aryloxyacetic acid have
more effect on the rate of the elimination reaction than
the substituent in the 2-aryloxypropanoic acid ( =
0.26), which indicates that the degree of charge sepa-
ration (C O) in the transition state of 2-aryloxyacetic
acid is greater than that in 2-aryloxypropanoic acid.
Kinetic data allow one to draw the following con-
clusions:
Table I Relative Reactivity for -Substituted
Propanoic Acid at 600 K
1
L
k (10 4 s
)
krel
Cl
1.81
1
Br
OH
OPh
HNPh
SPh
5.17
2.11
155
3150
97.4
2.86
1.17
85.6
1740
53.8
The influence of the aryl substituents on the nega-
tive charge being developed in the transition state of
the reaction is further revealed by the electronic ef-
fects of groups suitably substituted at the meta and
para aromatic ring positions of the phenoxymoiety
(Table II).
The change in relative rate, though moderate,
is nevertheless systematic and consistent with ex-
pected electron-withdrawing and electron-donating
effects of substituents at the meta and para positions.
To determine the relative abilities of the various ary-
loxy groups to transmit electronic effects, in the gas-
phase elimination of -aryloxylcarboxylic acids, we
have prepared a similar series of more than 10 com-
pounds of -aryloxyacetic acid and measured their
rates of thermal elimination. The Hammett -factor for
a reaction is a function of both the ability of the system
to transmit electronic effect and the amount of charge
to be stabilized.
1. 2-Aryloxypropanoic acid are consistently more
reactive than 2-aryloxyacetic acid (Table V).
2. The greater reactivity of 2-aryloxypropanoic acid
is attributed to replacing a hydrogen atom adja-
cent to the carbocation center in 2-aryloxyacetic
acids by a methyl group in 2-aryloxypropanoic
acids, which will help to neutralize the + charge
on -carbon atom, which inturn increases the sta-
bilization of the carbocation.
3. The relative rates at 600 K gave a satisfactory
Hammett correlation with 0 value, and -factor
of 0.69 was obtained.
Table II Relative Reactivity of -Aryloxy Propanoic
Acid at 600 K
4. The elimination of substituted phenols from
2-aryloxyacetic acid and propanoic acid takes
place via a concerted mechanism, with some
charge separation resulting in the partial
formation of a carbocation. This carbocation is
stabilized by electron-release or-withdrawal sub-
stituents attached to the -carbon atoms, as ob-
served in the present study.
1
Group
k (10 4 s
)
krel
H
4-NO2
4-Cl
3-Cl
4-CH3
4-OCH3
155
260
233
187
157
132
1
1.68
1.50
1.21
1.01
0.85

Table III Kinetic Data and Arrhenius Parameters for Gas-Phase Pyrolysis of 2-Aryloxyacetic Acid
4
1
1
1
Compound
1
Ar
T (K)
k (10
s
)
log A (s
)
Ea (kJ mol
)
R2
C6H5
582.15
594.45
603.25
613.45
623.15
575.2
589.4
600.8
609.9
619.0
626.8
636.0
578.6
591.4
603.4
612.3
622.7
630.8
576.85
588.55
600.25
609.15
618.85
628.25
583.6
595.8
605.9
619.5
629.05
637.2
551.35
566.25
576.25
585.95
596.15
574.4
586.4
597.8
606.0
613.8
621.2
562.0
572.2
583.4
594.5
603.3
612.2
560.3
571.2
579.2
589.2
598.0
608.4
617.5
546.5
561.4
572.2
583.4
592.1
601.6
564.2
577.4
585.3
593.9
602.8
611.5
3.13
5.98
9.49
10.01 0.10
150.58 1.19
0.996
17.00
23.35
1.01
2.94
4.81
2
p-CH3C6H4
10.28 0.37
156.27 4.31
0.997
8.02
13.59
17.48
27.20
1.69
4.29
7.42
12.74
18.10
32.32
1.83
4.40
8.45
12.57
17.59
33.19
1.04
2.24
4.90
11.23
20.29
36.01
2.25
5.01
7.71
12.53
29.12
2.43
4.65
6.82
12.10
18.23
26.22
2.05
3.51
6.26
12.41
18.47
28.80
1.80
3.16
4.79
3
4
5
p-C2H5C6H4
p-(CH3)3CC6H4
p-CH3OC6H4
11.03 0.40
10.96 0.55
14.23 0.22
163.54 4.67
161.95 3.43
203.73 2.60
0.993
0.994
0.999
6
7
p-OHCC6H4
10.32 0.67
10.21 0.43
147.58 7.57
152.33 4.94
0.992
0.998
p-CH3OCC6H4
8
9
p-BrC6H4
10.49 0.21
152.69 2.32
0.999
0.997
p-FC6H4
8.21 0.19
128.22 2.14
6.84
10.69
14.86
24.82
1.51
4.75
7.62
13.61
21.62
32.39
2.18
4.82
7.60
10
11
p-NCC6H4
10.44 0.41
148.58 4.54
0.993
0.997
p-O2NC6H4
10.02 0.35
147.42 4.01
11.46
16.29
26.75

THERMAL GAS-PHASE ELIMINATION OF 2-ARYLOXYACETIC ACID
615
Table IV Rate Coefficients k (10 4s 1) and log k/k0 at
600 K for 2-Aryloxyacetic acid
Table V Rate Constants for the Gas-Phase Pyrolysis of
2-Aryloxypropanoic and Acetic Acids at 600 K
1
Ar
k (10 4 s
)
Log k/k0
C6H5
7.95
4.73
6.20
7.26
3.11
29.63
8.88
15.74
11.15
31.97
15.34
0
p-CH3C6H4
p-C2H5C6H4
p-(CH3)3CC6H4
p-CH3OC6H4
p-OHCC6H4
p-CH3OCC6H4
p-BrC6H4
p-FC6H4
p-NCC6H4
p-O2NC6H4
0.225
0.108
0.039
0.408
0.571
0.048
0.296
0.147
0.604
0.285
X
krel
H
155
260
157
132
7.96
15.32
4.75
19.47
16.47
33.05
42.3
4-NO2
4-Me
OMe
3.12
The overall yield was about 60%.
4-Bromo-phenoxyacetic acid, m.p. 157 C.
H
EXPERIMENTAL
(CDCl3), 4.58 (2H, s, CH2), 6.82–6.79 (2H, d, ArH),
7.35–7.32 (2H, d, ArH), 11.22 (1H, s, COOH).
4-Fluoro-phenoxyacetic acid, 4-formyl-phenoxyacetic
acid, and 4-methyl-phenoxy-acetic acid were acquired
from Aldrich, and their purity proved to be better than
98% (GLC: 10% SP 1200, 1% H3PO4, Chromosorb
WAW DMCS 80-100 mesh).
4-Methoxy-phenoxyacetic acid, m.p. 112 C.
(CDCl3), 3.6 (3H, s, OCH3), 4.4 (2H, s, CH2), 6.71–
6.73 (4H, m, ArH), 11.52 (1H, s, COOH).
H
4-Ethyl-phenoxyacetic acid, m.p. 165 C.
H
(CDCl3), 1.08–1.03 (3H, t, CH3), 2.46–2.43 (2H,
m, CH2CH3), 4.48 (2H, s, CH2).
General Procedure of Synthesis [12]
In a three-necked flask, with a reflux condenser and a
mechanical stirrer, 0.1 mol of bromoacetic acid was
added to a refluxing solution of NaOH (0.2 mol in
40-ml H2O) and the corresponding substituted phe-
nol (0.1 mol). After 6 h of heating in a boiling water
bath, the reaction mixture was cooled and acidified,
extracted with ether, and washed with a 10% Na2CO3
solution. The alkaline water solution was acidified with
10% HCl solution. The precipitated product was crys-
tallized from water to better than 98% purity by GLC.
4-Cyano-phenoxyacetic acid, m.p. 119 C.
(CDCl3), 4.77 (2H, s, CH2), 7.96–7.64 (4H, m,
ArH), 10.00 (1H, s, COOH).
H
4-Formyl-phenoxyacetic acid, m.p. 115 C,
H
(CDCl3), 4.53 (2H, s, CH2), 6.81–6.78 (2H, d, ArH),
7.23–7.20 (2H, d, ArH), 10.00 (1H, s, CHO), 11.57
(1H, s, COOH).
4-Nitro-phenoxyacetic acid, m.p. 122 C.
(CDCl3), 4.69 (2H, s, CH2), 6.98–6.95 (2H, d, ArH),
8.10–8.07 (2H, d, ArH), 11.60 (1H, s, COOH)
H
4-t-Butyl-phenoxyacetic acid, m.p. 145 C.
H
(CDCl3), 1.08 (9H, s, CH3), 4.69 (2H, s, CH2), 6.98–
6.95 (2H, d, ArH), 8.10–8.07 (2H, d, ArH), 11.00
(1H, s, COOH).
Kinetic and product analysis has been described in an
earlier communication [13].
The facilities provided by Analab and SAF are gratefully
acknowledged.
BIBLIOGRAPHY
0
Figure 1 Correlation of log k/k0 with Hammett ␴ values
for gas-phase elimination of 2-aryloxyacetic acid.
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367.

616
AL-AWADI ET AL.
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