Gas-phase pyrolytic reactions of esters of 2-pyridine and 8-quinoline sulfonic acid

Article abstract of DOI:10.1002/1097-4601(2000)32:12<771::AID-KIN4>3.0.CO;2-0

The rates of gas-phase thermal elimination reaction of esters of 2-pyridine sulfonic acid and 8-quinoline sulfonic acid have been measured over at least 50° for each compound. The relative rates of the first-order unimolecular decomposition at 500 K for the primary:secondary esters suggest that C-O bond breaking is kinetically more significant than C-H bond breaking, leading to a transition state of carbocationic character. This is consistent with the electron-donating effect provided by the methoxy substituent in 2-p-methoxyphenylethyl of 2-pyridine and 8-quinoline sulfonate esters stabilizing the carbocation center.

Full text of DOI:10.1002/1097-4601(2000)32:12<771::AID-KIN4>3.0.CO;2-0

Gas-Phase Pyrolytic
Reactions of Esters of
2-Pyridine and 8-Quinoline
Sulfonic Acid
NOURIA A. AL-AWADI, KAMINI KAUL, HICHAM H. DIB
Chemistry Department, Kuwait University, P.O. Box 5969, 13060 Safat, Kuwait
Received 15 February 2000; accepted 20 June 2000
ABSTRACT: The rates of gas-phase thermal elimination reaction of esters of 2-pyridine sul-
fonic acid and 8-quinoline sulfonic acid have been measured over at least 50Њ for each com-
pound. The relative rates of the first-order unimolecular decomposition at 500 K for the
primary:secondary esters suggest that C9O bond breaking is kinetically more significant than
C9H bond breaking, leading to a transition state of carbocationic character. This is consistent
with the electron-donating effect provided by the methoxy substituent in 2-p-methoxyphen-
ylethyl of 2-pyridine and 8-quinoline sulfonate esters stabilizing the carbocation center. ᭧ 2000
John Wiley & Sons, Inc. Int J Chem Kinet 32: 771–775, 2000
INTRODUCTION
pyridine sulfonic acid are clearly consistent with a car-
It has been emphasized by the work of Corey et al. [1]
that (based on product analysis only) results of the
pyrolysis of esters of 8-quinoline sulfonic acid and 2-
bocation intermediate formation and abstraction of (H)
from the carbocation by a ring nitrogen to give olefin,
Scheme I.
Scheme I
The pyrolysis of trans-4-tert-butylcyclohexyl-3-
pyridinesulfonates was considered by the same group,
and found to decompose at the same temperature range
as the corresponding 2-pyridinesulfonates, and based
on the fact that a 6-membered cyclic transition state is
geometrically impossible for the 3-pyridinesulfonate,
they have ruled out a concerted mechanism, shown in
Scheme II.
Correspondence to: N. A. Al-Awadi (chesc@kuc01.kuniv.edu.kw)
Contract grant sponsor: University of Kuwait
Contract grant number: SC 091
᭧ 2000 John Wiley & Sons, Inc.

772
AL-AWADI ET AL.
Scheme II
They also noted that only one study of the thermal gas-
phase 1,3-elimination of 2-admantyl methanesulfonate
[2] predates their investigation.
In an earlier study, we have analyzed the structural
factors that influence the thermal gas-phase reactivity
of sulfonyl hydrazones [3] and oximes [4], Scheme
III. It was shown that within the semiconcerted
R ϭ ethyl, iso-propyl, 2-phenylethyl, and 2-p-
methoxyphenylethyl
EXPERIMENTAL
Materials
Characterization. The route to the synthesis of the 2-
pyridine and 8-quinoline sulfonate esters has been de-
1
scribed in (1). Element analysis and H NMR have
characterized esters of the present study.
Ethyl-2-Pyridine Sulfonate. ␦(CDCl3), 1.4 (3H, t,
CH2CH3), 4.5 (2H, q, CH2CH3), 7.6 (1H, d, H-3), 8.0
(2H, m, H-4, and 5), 8.8 (1H, d, H-6). The same pat-
tern of nmr spectra were observed for the hydrogen
atom of the pyridine ring of other esters. Found: C,
44.76; H, 4.79; N, 7.38; S, 17.32. C7H9NSO3 requires:
C, 44.92; H, 4.81; N, 7.48; S, 17.12%).
Scheme III
mechanism for pyrolytic eliminations involving 1,5-
hydrogen shifts (Scheme IV) a spectrum of transition-
state structures exists. For some compounds, breaking
of the
bond is the most important rate-deter-
C9D
Isopropyl-2-Pyridine Sulfonate. ␦(CDCl3), 1.3 (6H,
d, CH(CH3)2), 5.0 (1H, m, CH(CH3)2). Found: C,
47.79; H, 5.43; N, 7.13; S, 15.88. C8H11NO3S requires:
C, 47.76; H, 5.47; N, 6.96; S, 15.92%).
mining step, while for others it is nucleophilic, basic,
or protophilic attack upon the ␤-hydrogen. Changes in
structure within a given compound type have been
shown to change the nature of the transition state
within these limits (which correspond to either a more
El-like, or a more Ei-like, transition state).
2-Phenylethyl-2-Pyridine Sulfonate. ␦(CDCl3), 3.0
(2H, t, CH2C6H5), 4.6 (2H, t, OCH2), 7.2 (5H, s, C6H5).
Found: C, 59.13; H, 5.12; N, 5.46; S, 11.99.
C13H13NO3S requires: C, 59.07; H, 4.94; N, 5.32; S,
12.16%).
2 - p - Methoxyphenylethyl - 2 - Pyridine Sulfonate. ␦
(CDCl3), 3.0 (2H, t, CH2Ar), 3.8 (3H, S, OCH3), 4.6
(2H, t, SO3CH2), 6.8 (2H, d, 2 and 6 Ar), 7.1 (2H, d,
3 and 5 Ar). Found: C, 57.24; H, 5.15; N, 4.85; S,
10.83. C14H15NO4S requires: C, 57.33; H, 5.12; N,
4.77; S, 10.92%).
Scheme IV
In order to learn more about the pyrolysis reaction
of other sulfonate derivatives, we felt it would be valu-
able to have rate data for a number of pyridine and
quinoline sulfonate esters. So in this article we have
examined the gas-phase elimination reaction of 2-pyr-
idine (A) and 8-quinoline (B) sulfonate esters and
measured their rate of pyrolysis.
Ethyl-8-Quinoline Sulfonate. ␦(CDCl3), 1.3 (3H, t,
CH2CH3), 4.4 (2H, q, CH2CH3); 7.6–7.9 (3H, m,
H-3, 5 and 6-quinoline); 8.3–8.5 (2H, m, H-4 and
7-quinoline); 9.1 (1H, m, H-2-quinoline). The same

GAS-PHASE PYROLYTIC REACTIONS OF ESTERS OF SULFONIC ACID
773
Table I Rate Coefficient and Arrhenius Parameters for Pyrolysis of 2-Pyridine Sulfonate
Esters
SO3R
N
Ϫ1
104k(s
Ϫ1
R
T(K)
)
log A(s
)
Ea/kJ mol
C2H5
420.1
426.8
434.4
442.7
450.2
458.3
463.1
466.3
473.2
0.55
1.08
2.23
4.78
9.28
18.58
27.47
35.98
62.37
13.99 Ϯ 0.012
146.67 Ϯ 0.106
CH(CH3)2
382.9
392.5
401.4
409.1
414.8
419.7
424.6
1.88
4.81
9.61
18.53
26.71
40.08
56.09
11.19 Ϯ 0.021
109.17 Ϯ 1.67
CH2CH2Ph
438.7
442.2
449.9
457.5
464.5
469.6
474.5
487.2
1.13
1.51
2.91
5.36
9.27
13.67
19.71
49.01
12.53 Ϯ 0.038
138.41 Ϯ 0.335
CH2CH2
OMe
398.7
407.4
415.5
421.8
432.5
445.2
451.3
0.53
1.15
2.22
3.87
9.08
11.52 Ϯ 0.003
120.57 Ϯ 0.027
23.51
36.52
2-p-Methoxyphenylethyl-8-Quinoline Sulfonate. ␦
pattern of nmr spectra were observed for the hydrogen
atom of the quinoline ring of other esters. Found: C,
55.63; H, 4.67; N, 5.87; S, 13.63. C11H11NO3S re-
quires: C, 55.69; H, 4.64; N, 5.91; S, 13.50%).
(CDCl3), 2.8 (2H, t, CH2Ar), 3.7 (3H, s, OCH3), 4.5
(2H, t, SO3CH2) 6.7 (2H, d, 2 and 6 Ar), 7.0 (2H, d,
3 and 5 Ar). Found: C, 62.81; H, 5.14; N, 4.18; S,
9.32. C18H17NO4S requires: C, 62.97; H, 4.94; N, 4.08;
S, 9.32%).
Isopropyl-8-Quinoline Sulfonate. ␦ (CDCl3), 1.2
(6H, d, CH(CH3)2); 5.1 (1H, m, CH(CH3)2). Found: C,
57.59; H, 5.13; N, 5.47; S, 12.81. C12H13NSO3 re-
quires: C, 57.7; H, 5.18; N, 5.57; S, 12.75%).
Analysis of Reaction Products
Solutions of the esters in chlorobenzene were passed
down a reactor column packed with helices. The col-
umn was heated to temperatures comparable to those
used in the kinetic investigation [5]. The products of
pyrolysis were swept out using a stream of nitrogen
gas, and the effluents were collected in cold traps.
2-Phenylethyl-8-Quinoline Sulfonate. ␦ (CDCl3), 2.9
(2H, t, CH2Ar), 4.5 (2H, t, OCH2); 7.2 (5H, s, C6H5).
Found: C, 65.20; H, 4.83; N. 4.44; S, 10.27.
C17H15NO3S requires: C, 65.18; H, 4.79; N, 4.47; S,
10.22%).

774
AL-AWADI ET AL.
Table II Rate Coefficients and Arrhenius Parameters for Pyrolysis of 8-Quinoline Sulfonate
Esters
N
SO3R
Ϫ1
104k(s
Ϫ1
R
T(K)
)
log A(s
)
Ea/kJ mol
C2H5
432.4
444.4
455.0
467.6
472.5
477.5
482.7
0.81
2.32
5.09
12.05
16.70
23.80
34.07
11.28
Ϯ0.002
127.05
Ϯ0.019
CH(CH3)2
CH2CH2Ph
CH2CH2
381.3
389.0
402.0
413.0
422.0
432.0
0.88
1.73
5.36
12.10
22.40
52.50
10.87
Ϯ0.028
108.97
Ϯ2.23
432.8
447.4
460.0
470.0
476.5
482.7
0.63
2.20
6.64
13.60
25.30
41.10
13.31
Ϯ0.026
145.23
Ϯ2.27
413.8
417.5
425.3
433.1
441.1
448.8
456.7
464.9
0.85
1.19
2.36
4.62
8.98
16.62
30.62
56.45
12.55
Ϯ0.005
131.66
Ϯ0.043
OCH3
The product obtained from the pyrolysis was con-
firmed by nmr and GC-MS spectroscopy.
Kinetic Runs and Data Analysis. Aliquot parts
(0.2 ml) of very dilute solutions (ppm) of neat sub-
strates in acetonitrile as an internal standard were pi-
petted into the reaction tube, which was then sealed
under vacuum and placed inside the pyrolyzer for
600 s at a temperature where 10–20% pyrolysis was
deemed to occur. The content of the tube were then
analyzed using the HPLC probe.
At least three kinetic runs were repeated for each
5–10ЊC rise in temperature of the pyrolyzer and
for the same time interval until 90–95% pyroly-
sis was achieved. The rates were followed over a
temperature range of ca. 50 K and the rate coeffi-
cients were calculated using the expression for a
first-order reaction: kt ϭ ln a0/a. The Arrhenius
parameters were obtained from a plot of log k vs.
1/(T/K).
Kinetics
Reaction Set-up. Preliminary kinetic results were ob-
tained on a flow-system featuring a Eurotherm 093
pyrolysis unit coupled to a Perkin Elmer Sigma 115
gas chromatograph. The kinetic data reported are from
a reactor set-up including: HPLC (Bio-rad Model
2700) fitted with a UV/VIS detector (Bio-rad Model
1740); HPLC: Column LC-8, 25 cm, 4.6 mm, 5 ␮m
(Suppel. Co.); and a CDS custom-made pyrolysis unit
for the thermolysis reactions. The pyrolysis tube is
jacketed by an insulating aluminum block fitted with
a platinum resistance thermometer and a thermocouple
connected to a Comark microprocessor thermometer.

GAS-PHASE PYROLYTIC REACTIONS OF ESTERS OF SULFONIC ACID
775
Table III Rate Data at 500 K for Pyrolytic Reaction of 2-Pyridine and 8-Quinoline Sulfonate
Esters
Ϫ1
102k/s
SO3R
N
N
SO3R
R
C2H5
4.58
1.02
CH(CH3)2
CH2CH2Ph
60.74
2.6
30.75
1.37
8.35
6.19
CH2CH2
OMe
RESULTS AND DISCUSSION
over the phenyl group could be attributed to the
ϩM effect of the OMe group on the stability of
the carbocation center; on the other hand, it sug-
gests the unimportance of p-C9H bond cleav-
age.
The compounds gave excellent and reproducible first-
order kinetic results with linearity up to Ͼ95% reac-
tion. The data is summarized in Tables I and II; the
rates of pyrolysis at 500 K are given in Table III.
The main features of the results are as follows:
We therefore conclude (based on the rate data) that
the mechanism of the gas-phase elimination reaction
of 2-pyridine and 8-quinoline sulfonate esters pre-
sented in this study operate via carbocation interme-
diate, and the work of Corey et al. further confirms
this finding, as shown in Scheme I.
1. Each of the pyridine ester is consistently more
reactive than the quinoline ester. This follows
from the fact that C92 of the pyridine will re-
ceive a greater electron-withdrawing effect from
the nitrogen atom (which will make it the most
electron-deficient carbon in the ring), and this
indirect electron-withdrawal effect of the nitro-
gen atom in the 2-pyridine esters should facili-
tate C9O bond cleavage, while for the 8-quin-
oline esters, this is not the case. This finding
suggests that pyridine sulfonates in (A) are bet-
ter leaving groups than quinoline sulfonates
in B.
The support of the University of Kuwait through research
grant SC 091 and the facilities of Analab and SAF are grate-
fully acknowledged.
BIBLIOGRAPHY
2. The isopropyl ester in both series are more re-
active than their ethyl counterparts. This effect
is consistent with greater stabilization of the
transition state with the developing secondary
carbocation character compared to the primary
ethyl one.
3. The effect on the reaction rate of replacing the
␤-hydrogen atom by phenyl group in both se-
ries, unlike carboxylic acid ester pyrolysis, does
not sterically accelerate the reaction.
1. Corey, E. J.; Posner, H.; Atkinson, R. F.; Wingard,
A. K.; Halloran, D. J.; Radzik, D. M.; Nash J. J. J Org
Chem 1989, 54, 389.
2. Kaufmann, D.; DeMeijene, A.; Luk, K.; Overton, K.;
Stothers, J. B. Tetrahedron 1982, 38, 977.
3. Al-Awadi, N. A.; Elnagdi, M. H.; Kaul, K.; Illingovan,
S.; El-Dusouqui, O. M. Tetrahedron 1988, 54, 4633.
4. Al-Awadi, N. A.; Elnagdi, M. H.; Kaul, K.; Illingovan,
S.; El-Dusouqui, O. M. J Phys Org Chem 1999, 12,
654.
4. The activation effect of the p-methoxyphenyl
5. DePuy, C. H.; King, E. R. Chem Rev 1960, 60, 431.