Kinetics and mechanism of thermal gas-phase elimination of α-substituted carboxylic acids: Role of relative basicity of α-substituents and acidity of incipient proton

Article abstract of DOI:10.1002/1099-1395(200009)13:9<499::AID-POC269>3.0.CO;2-0

2-Phenoxypropanoic acid together with five of its aryl derivatives, its phenylthio and its N-phenylamino analogues were pyrolyzed at 494-566 K. The reactions were homogeneous, polar and free from catalytic and radical pathways, and obeyed a first-order rate equation. The limits of the Arrhenius log A (s^-1) and E (kJ mol^-1) values obtained for these reactions averaged 11.98 ± 1.71 and 158.1 ± 17.4, respectively. Analysis of the pyrolysates showed the elimination products to be carbon monoxide, acetaldehyde and the corresponding phenol, thiophenol or aniline compounds. The pyrolysis of 2-phenoxy- and 2-(N-phenylamino)-1-propanol was also investigated over the temperature range 638-792 K. The kinetic results and products analysis lend support to a reaction pathway involving a five-membered cyclic polar transition state. Copyright

Full text of DOI:10.1002/1099-1395(200009)13:9<499::AID-POC269>3.0.CO;2-0

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2000; 13: 499–504
Kinetics and mechanism of thermal gas-phase elimination
of a-substituted carboxylic acids: role of relative basicity of
a-substituents and acidity of incipient proton
Nouria A. Al-Awadi,* Kamini Kaul and Osman M. E. El-Dusouqui
Department of Chemistry, University of Kuwait, P.O. Box 5969, Safat 13060, Kuwait
Received 20 June 1999; revised 10 February 2000; accepted 24 February 2000
ABSTRACT: 2-Phenoxypropanoic acid together with five of its aryl derivatives, its phenylthio and its N-
phenylamino analogues were pyrolyzed at 494–566 K. The reactions were homogeneous, polar and free from
catalytic and radical pathways, and obeyed a first-order rate equation. The limits of the Arrhenius log A (s 1) and E
(kJ mol 1) values obtained for these reactions averaged 11.98 Æ 1.71 and 158.1 Æ 17.4, respectively. Analysis of the
pyrolysates showed the elimination products to be carbon monoxide, acetaldehyde and the corresponding phenol,
thiophenol or aniline compounds. The pyrolysis of 2-phenoxy- and 2-(N-phenylamino)-1-propanol was also
investigated over the temperature range 638–792 K. The kinetic results and products analysis lend support to a
reaction pathway involving a five-membered cyclic polar transition state. Copyright 2000 John Wiley & Sons, Ltd.
KEYWORDS: a-substituted carboxylic acids; thermal gas-phase elimination; pyrolysis; cyclic polar transition state
INTRODUCTION
The proposed transition state (Scheme 1) helps to
explain the composition of the products of reaction: HX
in addition to CH3CHO and CO obtained from the
fragmentation of an a-lactone intermediate (Scheme 2).
The evidence given in support of the proposed mech-
anism is based on the relative acidity of the incipient
hydrogen, the basicity of X and the electronic and steric
effects of a-alkyl groups which influence the stability of
the carbocation centre of the intimate ion pair. Further,
the qualitative correlation of reactivity with structure is
consistent. However, the relative rate factors reported for
these systems are by comparison rather small (1–102)
The mechanism of pyrolysis of pyruvic and benzoyl-
formic acids, the methyl esters and related hydroxyke-
tones has been shown to involve both four and five-
membered transition states.5–7 In this paper, we report on
the thermal gas-phase elimination of a-substituted
propanoic acid and b-substituted 1-propanol, the sub-
stituent in both systems being an aryl moiety. Relative
rate factors of 102 –107 have been observed, indicative of
the major contribution to reactivity by the mesomerically
electron-accommodating phenoxy group and its thio and
amino analogues.
The overall reaction scheme for the a-substituted
carboxylic acids under discussion follows the pathway
heat
CH3CHXCO2H
!
HX ‡ CH3CHO ‡ CO
where X is either a halogen, a hydroxy or alkoxy group,
an aryloxy moiety or its amino or thio analogues.
Hydroxy, alkoxy and phenylacetic acids and acids in
which the methyl hydrogens have been replaced by alkyl
groups were also examined.
An extensive study by Chuchani and co-workers 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 transition state of the reaction was
believed to be concerted, polar, five-membered and to
involve the acid group proton rather than the aliphatic
methyl hydrogen atom (Scheme 1, I). Alternatively, the
cyclic nature of the transition state was depicted by
curved arrows (Scheme 1, II). To represent the transition
state as an intimate ion pair, the limit dipolar structure
(Scheme 1, III) was invoked, and on the basis of which
participation of the carbonyl moiety in bond formation
and bond fission would be considered feasible.
Recent theoretical calculations from a protracted study
by Andres and co-workers8–12 have confirmed the
experimental results of Chuchani and co-workers, and
have provided further insight into the nature of the
molecular transition structures along the reaction co-
ordinate which placed the mechanism of these unimol-
ecular thermal gas-phase elimination processes on a firm
theoretical and experimental basis. For an accurate
*Correspondence to: N. A. Al-Awadi, Department of Chemistry,
University of Kuwait, P.O. Box 5969, Safat 13060, Kuwait.
E-mail: nouria@kuc01.kuniv.edu.kw
Contract/grant sponsor: University of Kuwait; Contract/grant num-
ber: SC 088.
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 499–504

500
N. A. AL-AWADI, K. KAUL AND O. M. E. EL-DUSOUQUI
Scheme 1. Cyclic ®ve-membered transition state and implicit intimate ion pair
Scheme 2
Scheme 3
computation of the kinetic parameters, detailed char-
acterization using ab initio analytical gradients at the
MP2/6–31G** level of theory, optimization routines and
population analysis were found to be necessary. A
comparative study of the global PES and stationary
points on the basis of molecular geometries and intrinsic
reaction coordinates, electronic and transition structures
and theoretical rate coefficients calculated in terms of
transition-state theory suggested an irreversible two-step
reaction pathway which reconciles very well theory with
experiment and the rate constants obtained in the two
studies. The level of quantum mechanical calculations
made it possible to discriminate against a single-step
(concerted) mechanism and in favour of a transition
structure involving a distorted five-membered ring
structure leading to the elimination of HX ahead of
cyclization to give an a-lactone intermediate with pre-
organization and H-bonding being invoked.
The rate-limiting step was considered to involve the
transition structure [Scheme 3, t.s. (a)] in which
fragmentation and a-lactone formation are assisted by
the oxygen atom of the carbonyl moiety from the
carboxyl group. The reactive global PES identified and
characterized a second transition structure [t.s. (b)]
associated with the decomposition of the a-lactone
intermediate to give carbon monoxide and the aldehyde
as the other products of reaction. Accordingly, the
molecular transition structures along the reaction co-
ordinate follow Scheme 3. This mechanism is consistent
with the results of the present study.
ascertained to be homogeneous, unimolecular, non-
catalytic and non-radical processes.1,13 Arrhenius plots
of the data were strictly linear over ꢀ95% reaction, with
correlation coefficients in the 0.99 Æ 0.005 range.13–15
An example of these plots is given in Fig. 1. The log A
(s 1) and Ea (kJ mol 1) values and the first-order rate
constants of the ten compounds under investigation are
given in Table 2. The compounds investigated were 2-
phenoxypropanoic acid (1), 2-(p-nitrophenoxy)propanoic
acid (2), 2-(p-chlorophenoxy)propanoic acid (3), 2-(m-
chlorophenoxy)propanoic acid (4), 2-(p-methylphenox-
y)propanoic acid (5), 2-(p-methoxylphenoxy)propanoic
acid (6), 2-(N-phenylamino)propanoic acid (7), 2-thio-
phenoxypropanoic acid (8), 2-(N-phenylamino)-1-propa-
nol (9) and 2-phenoxy-1-propanol (10).
The products of pyrolysis were analysed using on-line
GC–MS and FT-IR pyroprobe techniques. The constitu-
ents of all pyrolysates were ascertained to be an arene
(PhOH, PhSH or PhNH2) fragment, together with
CH3CHO and CO. Mass spectrometric analysis was
particularly helpful in the identification of the aromatic
fragments and confirmation of their presence as con-
stituents of the pyrolysates.
The present and published1–4 rate constants and
relative rates thereof have been reassessed for 600 K.
This is to provide uniformity and thus allow meaningful
comparisons and structure–reactivity correlations to be
drawn. Based on these results and with reference to the
energetically more favourable molecular transition path-
way in the reactive global PES of the quantum
mechanical analysis of the elimination process, the
following mechanistic interpretations and general con-
clusions could be offered.
RESULTS AND DISCUSSION
For each substrate, first-order rate coefficients were
obtained at regular temperature intervals over the range
51 Æ 10 K. The results are given in Table 1. Each rate
constant is an average of at least three independent
measurements in agreement to within Æ2%. The reac-
tions for which the kinetic data were obtained have been
1. The reaction pathway is considered to involve the
cyclic five-membered transition state associated with
elimination of HX and formation of an a-lactone
intermediate, and the present analysis of relative
reactivities is based on this pathway. The
participation of the oxygen atom of the carbonyl
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 499–504

GAS-PHASE ELIMINATION IN a-SUBSTITUTED CARBOXYLIC ACIDS
Table 1. Rate coef®cients [104 k (s 1)] for pyrolysis of substrates 1±10
501
1
1
Compound
T (K)
104 k (s
1.04
)
Compound
T (K)
104 k (s
0.77
)
1
514.6
522.4
534.2
546.3
551.0
561.8
564.5
6
512.2
526.1
537.2
549.2
554.8
557.9
563.0
1.74
3.73
8.05
10.5
20.7
22.3
1.94
4.02
8.21
11.4
13.8
18.3
2
3
514.5
523.4
531.2
538.1
549.4
557.7
559.0
564.0
0.76
7
8
494.1
505.0
510.2
518.7
522.9
526.3
534.7
3.20
7.47
10.5
19.9
27.4
33.8
65.9
1.43
2.77
4.57
10.4
17.5
19.8
27.8
518.1
520.2
524.6
531.7
540.9
549.8
555.5
558.7
566.2
1.13
517.0
535.0
546.8
551.1
562.5
568.7
578.0
1.04
1.31
1.80
3.10
5.91
3.14
6.38
7.87
14.7
20.3
33.5
11.0
15.5
18.8
31.3
4
5
511.3
518.3
536.2
550.1
554.2
556.9
560.4
0.70
1.15
4.16
9
740.0
750.0
774.2
784.0
792.0
1.05
1.79
5.99
9.80
10.2
13.0
15.7
18.9
14.1
515.0
524.0
543.1
550.1
557.2
565.8
1.48
2.69
7.91
12.2
17.4
29.9
10
638.0
658.5
667.7
674.7
684.9
0.93
3.05
5.30
7.81
14.4
moiety of the carboxylic acid group in the
cyclization process leading to the formation of the
a-lactone intermediate together with H-bonding
between the acid proton and X offer bond-formation
processes that are expected to compensate for
energy requirements and to moderate charge
development associated with bond fission.
2. The effect of the a-substituent (X) is related to what
has been described as the basicity of X. This refers to
the capacity of the substituent (X) to accommodate the
negative charge being developed in the transition
state. The correlation of relative reactivity with group
(X) is shown below for a-substituted propanoic acid:
X :
Cl Br OH OPh HNPh SPh
ꢁ1† ꢁ7† ꢁ8†
1
104kꢁs † : 1:81 5:17 2:11 155 3150 97:4
krel
:
1
2:86 1:17 85:6 1740 53:8
Figure 1. Arrhenius plot for 2-phenoxypropanoic acid (1)
By comparison, the aryl substituents (—NHPh,
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 499–504

502
N. A. AL-AWADI, K. KAUL AND O. M. E. EL-DUSOUQUI
Table 2. Arrhenius parameters and rates [104 k (s 1)] at
600 K for substrates 1±10
panol (10) are compared. The rate factor is much
greater (3.3 Â 107) when the comparison involves the
more basic 2-(N-phenylamino) moiety of the acid (7)
and that of the corresponding alcohol (9). It should be
noted that a maximum factor of only 100 has been
reported involving the rates of 2-bromopropanoic acid
and its methyl ester.1–4
1
1
1
Compound
Log A (s
)
Ea(kJ mol
)
104 k600 (s
)
1
2
3
4
5
6
7
8
11.32 Æ 0.17
13.70 Æ 0.15
13.02 Æ 0.12
12.25 Æ 0.13
10.44 Æ 0.00
11.15 Æ 0.09
13.48 Æ 0.00
10.26 Æ 0.05
13.27 Æ 0.13
13.19 Æ 0.00
150.8 Æ 1.8
175.5 Æ 1.5
168.2 Æ 1.2
160.6 Æ 1.3
155
260
233
187
140.7 Æ 0.02
149.7 Æ 0.91
160.6 Æ 0.02
141.0 Æ 0.57
244.4 Æ 1.5
210.4 Æ 0.03
157
132
31.5 Â 102
97.4
EXPERIMENTAL
6
3
9
10
95.5 Â 10
7.47 Â 10
Kinetic measurements
The reactor, kinetic procedure and instrumentation used
for the measurement of specific first-order rate constants
at given reaction temperatures of 5–10°C intervals have
been described.7,13 Dilute (ppm) standard solutions of the
substrates in a suitable solvent (acetonitrile) were
prepared, and a suitable internal standard (chloroben-
zene) was added to each solution. Care was taken to
ensure that both the solvent and the internal standard are
not pyrolysable under the conditions of the reaction.
Further, the standard solutions used in the kinetic runs
always comprised a mixture where the substrate gave a
threshold peak which is one third higher than that of the
internal standard in the analytical high-performance
liquid chromatographic (HPLC) trace. Preparation of a
standard solution allows several kinetic runs to be
performed on the same reaction mixture.
—OPh, —SPh) show substantially larger relative rates
as a result of their pronounced resonance effects. The
trend among the aryl groups reflects the relative acid
strength of the incipient phenol, thiophene and aniline
fragments following protonation by the carboxylic
acid hydrogen, as well as the relative stability and
proton affinity of the conjugate base of each of the
departing aromatic fragments.
3. The influence of the aryl substituents on the negative
charge being developed in the transition state of the
reaction is further revealed by the electronic effects of
groups suitably substituted at the meta and para
aromatic ring positions of the phenoxy moiety.
Group :
H 4-NO2 4-Cl 3-Cl 4-CH3 4-OCH3
A sample (0.2 ml) of the standard solution was placed
in the reaction tube (9 ml) and the contents were sealed
under vacuum; a pressure of ꢂ0.3 mbar was maintained
during this process. The tube was mounted in the
pyrolyser heated to a selected temperature and for a
given length of time to allow about 10–20% threshold
pyrolysis. Pyrolysis was then followed at regular
intervals until >95% reaction was obtained. The contents
of the reaction tube were analysed using HPLC. The
configuration and model of both the pyrolysis and HPLC
units have been detailed in a previous paper.7 The
experimental and analytical techniques for evaluation of
rates have been described in detail.13 Rates were
reproduced in presence and absence of a radical
scavenger (cyclohexene); homogeneity and any possible
adverse surface effects were also tested following earlier
procedures.13 An increase of the reactor surface area of
up to about ninefold produced no detectable change in the
rate of reaction; the increase in surface area was achieved
using reaction tubes packed with glass helices. It is
noteworthy that the present values of the Arrhenius log A
(s 1) and Ea kJ mol 1 are well within the ranges expected
for unimolecular polar thermal elimination reactions, and
ꢁ1† ꢁ2† ꢁ3† ꢁ4† ꢁ5†
ꢁ6†
132
0:85
1
104kꢁs † : 155 260 233 187 157
krel
:
1
1:68 1:50 1:21 1:01
The change in relative rate, although moderate, is
nevertheless systematic and consistent with expected
electron-withdrawing and electron-donating effects of
substituents at the meta and para positions. Cycliza-
tion associated with the formation of the lactone
intermediate and H-bonding between X and the acid
proton might have combined to reduce the negative
charge being developed on the oxygen of the phenoxy
moiety and thus moderate the effects of substituents.
Structural modification of relative contribution to
molecular reactivity of a bond-breaking process,
albeit in a cyclic six-membered transition state, has
been shown to influence the magnitude of the effects
of substituents on the acidity of an incipient proton
and its contribution to bond formation.14
4. A dramatic change in reactivity is observed when the
acidity of the incipient hydrogen is reduced by
replacing the carboxylic acid proton with an alcohol
hydroxyl hydrogen. A relative rate factor of 2 Â
103 is obtained when the rates of pyrolysis of
2-phenoxypropanoic acid (1) and 2-phenoxy-1-pro-
1
are distinctly different from the values of 16 s and
1
315 kJ mol
expected for radical elimination reac-
tions.16
The rate coefficients were calculated using the
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 499–504

GAS-PHASE ELIMINATION IN a-SUBSTITUTED CARBOXYLIC ACIDS
503
expression kt = ln(a0/a) for first-order reactions.13,15 The
Arrhenius parameters were obtained from a plot of logk
vs 1/T (K), an example of which is shown in Fig. 1. The
elimination rate constant at T (K) is given by
2-(N-Phenylamino)-1-propanol (9). A solution of ethyl
2-(N-phenylamino)propionate (0.02 mol) in 50% aqu-
eous ethanol (50 ml) was added dropwise to a solution of
sodium borohydride (0.09 mol) in 50% aqueous ethanol
(500 ml). The resulting mixture was then refluxed for
4.5 h. After normal work-up, the resulting liquid was
distilled to produce 2-(N-phenylamino)-1-propanol in
70% yield.
1
1
log k ˆ log Aꢁs
†
EaꢁkJ mol †=4:574TꢁK†
Product analysis. The on-line GC-MS pyroprobe and
the flow techniques for the identification of the
constituents of the products of pyrolysis have been
described in previous reports.7,15 The products analysed
were for pyrolyses conducted at temperatures commen-
surate with those of the kinetic measurements. No
secondary decomposition was observed for any of the
compounds under study up to 98% pyrolysis.
2-Phenoxy-1-propanol (10). A solution of 2-phenoxy-
propanoic acid (0.01 mol) in dry diethyl ether (10 ml)
was added dropwise to a stirred slurry of lithium
aluminium hydride (0.01 mol) in dry diethyl ether
(100 ml) at a rate sufficient to maintain reflux (3–4 h).
The mixture was then cooled to room temperature. The
excess hydride was decomposed by dropwise addition of
water, and the resulting white suspension was filtered.
The filtrate was dried over Na2SO4 and concentrated in
vacuo, then distilled to produce 2-phenoxy-1-propanol in
75% yield.
Syntheses. 2-Aryloxypropanoic acids (1±6). Typically,
ethyl 2-phenoxypropionate was prepared by refluxing
equimolar amounts of phenol, ethyl 2-chloropropionate
and potassium carbonate in dry acetone (200 ml) for 10 h
followed by normal work-up and subsequent distillation
to produce ethyl 2-phenoxypropionate in 82% yield (b.p.
119°C at 5 mmHg; lit.17 b.p. 125°C at 6 mmHg for ester).
Of this ester, 8 g were then hydrolysed using 10%
aqueous sodium hydroxide solution (100 ml) and reflux-
ing the mixture for 3–4 h until the disappearance of the
upper ester layer was deemed complete. The contents of
the flask were then cooled and acidified by the dropwise
addition of dilute hydrochloric acid. The white crystalline
flaky solid which separated out was filtered and dried to
yield 2-phenoxypropanoic acid (1). The 2-aryloxypropa-
noic acids (2–6) were prepared from the appropriately
substituted phenols following the procedure described
above for 2-phenoxypropanoic acid (1).
Characterization. The solids were recrystallized from
diethyl ether–light petroleum and the liquids were
purified by distillation under reduced pressure. The
melting points (uncorrected) were recorded from a
Gallenkamp Sanyo apparatus, and both melting and
boiling points were compared with literature values.17–25
In addition, characterization included IR (on neat liquid
1
samples or KBr discs), H NMR (DMSO or CDCl3 as
solvents), and mass spectrometric analyses. FT-IR
spectra were recorded on a Perkin-Elmer 2000, NMR
spectra on a Bruker AC80 and mass spectra on a Finnigan
Mat INCOSXL spectrometer. Typical data were as
follows. 2-Phenoxypropanoic acid (1): m.p. 114–
2-(N-Phenylamino)propanoic acid (7). Ethyl 2-(N-phe-
nylamino)propionate required in this synthesis was
prepared by refluxing a mixture of ethyl 2-chloropropio-
nate (0.1 mol), aniline (0.1 mol) and sodium hydroxide
(0.2 mol) at 125°C for 12 h, followed by normal work-up
and subsequent distillation to produce ethyl 2-(N-
phenylamino)propionate in 45% yield. A mixture of
ethyl 2-(N-phenylamino)propionate (8 g) and 10%
aqueous sodium hydroxide solution (100 ml) was re-
fluxed for 3–4 h until the disappearance of the upper ester
layer was complete. The contents of the flask were then
cooled and acidified with the dropwise addition of dilute
hydrochloric acid. The white crystalline solid which
separated out was then filtered, dried and recrystallized
from light petroleum to obtain 2-(N-phenylamino)pro-
pionic acid in 70% yield.
1
116°C, lit.17,18 116°C; IR (KBr), ꢀmax 1718 cm
;
NMR, ꢁ (DMSO) 1.49 (d, 3H, CH3), 4.18 (q, 1H, CH),
6.80–7.01 (m, 3H, ArH), 7.18–7.37 (m, 2H, ArH). 2-
Phenoxy-1-propanol (10): b.p. 124°C/20 mmHg, lit.17,19
124°C/20 mmHg; IR (neat), ꢀmax 3608 cm 1; NMR, ꢁ
(DMSO) 1.23 (d, 3H, CH3), 2.46 (br, s, 1H, OH,
exchanges with D2O), 3.69 (br S, 2H, CH2), 4.45 (m,
1H, CH), 6.84–7.01 (m, 3H, ArH), 7.16–7.36 (m, 2H,
‡
ArH); MS, M = 152.1. 2-(p-Nitrophenoxy)propanoic
acid (2): m.p. 137–138°C, lit.20 139–140°C. 2-(p-
Chlorophenoxy)propanoic acid (3): m.p. 115–116°C,
lit.20 115–116°C. 2-(m-Chlorophenoxy)propanoic acid
(4): m.p. 113–115°C, lit.21 113–114°C. 2-(p-Methyl-
phenoxy)propanoic acid (5): m.p. 100–101°C, lit.21 101–
102°C. 2-(p-Methoxylphenoxy)propanoic acid (6): m.p.
99–101°C, lit.22 93–94°C. 2-(N-Phenylamino)propanoic
acid (7): m.p. 165°C, lit.23 165°C. 2-Thiophenoxypro-
panoic acid (8): b.p. 160°C/6 mmHg, lit.24 168–170°C/
9 mmHg. 2-(N-Phenylamino)-1-propanol (9): b.p.
115°C/3.8 mmHg, lit.25 122°C/4 mmHg.
2-Thiophenoxypropanoic acid (8). The same general
procedure as described for the aryloxy acids (1–6) but
using thiophenol produced thiophenoxy acid in 80%
yield.
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 499–504

504
N. A. AL-AWADI, K. KAUL AND O. M. E. EL-DUSOUQUI
11. Domingo LR, Picher MT, Safont VS, Andres J, Chuchani G. J.
Acknowledgements
Phys. Chem. A 1999; 103: 3935.
12. Rotinov A, Chuchani G, Andres J, Domingo LR, Safont VS. Chem.
Phys. 1999; 246: 1.
13. Al-Awadi NA, Elnagdi MH, Mathew T. Int. J. Chem. Kinet. 1995;
27: 517.
14. Al-Awadi NA, Elnagdi MH, Kaul K, Ilingovan S, El-Dusouqui
OME. Tetrahedron 1998; 54: 4633; J. Phys. Org. Chem. 1999; 12:
654.
The support of the University of Kuwait received through
research grant SC 088 and the facilities of ANALAB and
SAF are gratefully acknowledged.
15. Al-Awadi NA, Al-Bashir RF, El-Dusouqui OME. J. Chem. Soc.,
Perkin Trans. 1989; 2: 579.
16. Holbrook KA, In The Chemistry of Acid Derivatives: Supplement
B, Patai S (ed). Wiley:Chichester, 1992; Chapt. 15.
17. Lide DR. (ed). CRC Handbook of Chemistry and Physics. 1991/
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J. Phys. Org. Chem. 2000; 13: 499–504