Gas-phase pyrolysis of thiopheneacetic acids, thienylethanols, and related compounds - Protophilicity of ring π-electrons and relative acidities of hydrogen-bond donors of hydroxyl groups

Article abstract of DOI:10.1139/v02-067

Based on kinetic data of thermal gas-phase elimination reactions, the following Arrhenius log A (s^-1) and E_a (kJ mol^-1) values, respectively, are obtained: 10.76 and 153.5 for 3-thiopheneacetic acid (1), 10.08 and 149.4 for 2-thiopheneacetic acid (2), 12.04 and 207.1 for 2-(3-thienyl)ethanol (3), 11.55 and 203.3 for 2-(2-thienyl)ethanol (4), 10.91 and 123.4 for 2-thiopheneglyoxylic acid (5), 11.05 and 223.8 for 1-(2-thienyl)propan-1-one (6), and 10.33 and 149.8 for 3-thiophenemalonic acid (7). The products of these pyrolytic reactions were either carbon dioxide or formaldehyde in addition to methylthiophene or thiophenecarboxaldehyde. Both positional and molecular reactivities of the substrates and related compounds are compared, and the results are rationalized on the basis of a reaction pathway involving a concerted six-membered transition state.

Full text of DOI:10.1139/v02-067

499
Gas-phase pyrolysis of thiopheneacetic acids,
thienylethanols, and related compounds —
protophilicity of ring ꢀ-electrons and relative
acidities of hydrogen-bond donors of hydroxyl
groups
Ibtehal A. Al-Juwaiser, Nouria A. Al-Awadi, and Osman M.E. El-Dusouqui
Abstract: Based on kinetic data of thermal gas-phase elimination reactions, the following Arrhenius log A (s–1) and Ea
(kJ mol–1) values, respectively, are obtained: 10.76 and 153.5 for 3-thiopheneacetic acid (1), 10.08 and 149.4 for 2-
thiopheneacetic acid (2), 12.04 and 207.1 for 2-(3-thienyl)ethanol (3), 11.55 and 203.3 for 2-(2-thienyl)ethanol (4),
10.91 and 123.4 for 2-thiopheneglyoxylic acid (5), 11.05 and 223.8 for 1-(2-thienyl)propan-1-one (6), and 10.33 and
149.8 for 3-thiophenemalonic acid (7). The products of these pyrolytic reactions were either carbon dioxide or formal-
dehyde in addition to methylthiophene or thiophenecarboxaldehyde. Both positional and molecular reactivities of the
substrates and related compounds are compared, and the results are rationalized on the basis of a reaction pathway in-
volving a concerted six-membered transition state.
Key words: thiophenes, gas-phase, pyrolysis, kinetics, mechanism.
Résumé : Sur la base de données cinétiques de réactions d’élimination thermiques en phase gazeuse, on a déterminé
les valeurs suivantes d’Arrhenius suivantes log A (s–1) et Ea (kJ mol–1) pour respectivement l’acide 3-thiophèneacétique
(1, 10,76 et 153,5), l’acide 2-thiophèneacétique (2, 10,08 et 149,4), le 2-(thién-3-yl)éthanol (3, 12,04 et 207,1), le 2-
(thién-2-yl)éthanol (4, 11,55 et 203,3), l’acide 2-thiophèneglyoxylique (5, 10,91 et 123,4), la 1-(thién-2-yl)propan-1-one
(6, 11,05 et 223,8) et l’acide 3-thiophènemalonique (7, 10,33 et 149,8). Les produits de ces réactions de pyrolyse sont
le bioxyde de carbone ou le formaldéhyde ainsi que le méthylthiophène ou le thiophènecarboxaldéhyde. On a comparé
les réactivités positionnelles ainsi que moléculaires des substrats examinés ici ainsi que celles de composés apparentés
et on rationalise les résultats en se basant sur une voie réactionnelle impliquant un état de transition concerté à six
chaînons.
Mots clés : thiophènes, phase gazeuse, pyrolyse, cinétique, mécanisme.
[Traduit par la Rédaction] Al-Juwaiser et al. 503
Introduction
and gas-phase reactions suggest that the 2- and 3-thienyl
ring systems act as electron withdrawing and electron donat-
ing groups, respectively (6). An electron withdrawing effect
would be expected to augment the electrophilic nature of the
hydroxyl hydrogen of both the alcohol and carboxylic acid
moieties of substituted thiophenes. The present study in-
volves thiophenes substituted at the 2- and 3-positions with
groups in which terminal O-H moieties serve as H-bond do-
nor acids, and where a protonic hydrogen is available for
intramolecular electrophilic H-exchange processes. Reactions
of this nature, for which a concerted six-membered transi-
tion state (TS) is postulated, are expected to have favourable
conformations and effective intramolecular pre-organization.
A representative example is the thermal gas-phase elimina-
tion reaction of 2-thiopheneglyoxylic acid, the final product
of which is 2-thiophenecarboxaldehyde (Scheme 1).
Thiophene is classified as an electron-rich (ꢀ-excessive)
heteroaromatic compound, with its aromaticity between that
of furan and benzene, and reactivity towards electrophiles
ca. 103- to 105-fold higher relative to benzene (1, 2). The
structural effects associated with the 2,3-conjugative interac-
tion and the high 2:3-bond order superimposed upon an in-
trinsic 2 > 3 positional reactivity serve to explain the relative
rates and partial rate factors obtained in electrophilic aro-
matic substitution reactions of thiophene and its N, O, and
Se analogues and monosubstituted derivatives (3–5). Hammett
substituent-replacement constants reported for both solution
Received 16 September 2001. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on 30 April 2002.
Previous studies on comparable reactions have lead to the
conclusion that, during the development of the TS of the
elimination process, either bond a, b, or d (Scheme 1) could
be effectively displaced further away from the reacting site
compared with the other two bonds, or that a combined
I.A. Al-Juwaiser, N.A. Al-Awadi,1 and O.M.E. El-Dusouqui.
Department of Chemistry, University of Kuwait, P.O. Box
5969, Safat 13060, Kuwait.
1Corresponding author (e-mail: nouria@kuc01.kuniv.edu.kw).
Can. J. Chem. 80: 499–503 (2002)
DOI: 10.1139/V02-067
© 2002 NRC Canada

500
Can. J. Chem. Vol. 80, 2002
Scheme 1. Reaction pathway, concerted six-membered TS, and
final product of thermal gas-phase elimination.
Table 1. Kinetic data for pyrolysis of compounds 1–7.
Compd. T (K) k × 104 (s–1) Compd. T (K) k × 104 (s–1)
3.303
6.171
9.986
1.574
4.747
1
564.3
574.2
584.1
5
443.4
453.5
463.5 12.25
473.2 23.47
477.2 25.59
483.4 41.20
493.0 68.35
503.1 83.67
588.8 15.13
593.8 22.74
603.4 28.43
613.0 45.07
617.8 50.98
effect on the observed reactivity involves either two or all
three of the bonds (7–11). The effect of the double bond (a)
is associated with the lability (protophilicity) of its ꢀ-elec-
trons, the effect of b depends on bond polarity, and that of d
results from the relative acidity of the H-bond donor and the
magnitude of the positive charge developing on the hydro-
gen. In addition, an important factor in these elimination re-
actions is the relative thermodynamic stability of the
incipient moieties leading to the products (10a, c). It is note-
worthy that a six-membered TS has also been proposed for
decarboxylation reactions of heteroarylacetic acids in aque-
ous media (12).
2
3
563.2
573.3
583.8
593.4
603.4 13.31
613.2 20.07
623.4 31.63
1.795
2.627
5.749
8.984
6
7
774.9
785.1
795.2
805.5
813.4
821.9
0.894
1.748
2.506
3.853
4.755
4.932
684.8
695.1
705.1
715.3
720.8 11.78
725.2 16.64
735.3 20.75
745.6 32.85
1.574
3.229
5.149
7.390
563.3
573.8
583.7
594.6 15.00
599.0 18.82
604.4 25.24
613.9 37.30
2.069
5.866
8.515
Results and discussion
Kinetics
The kinetic data of the gas-phase elimination reactions of
the seven compounds (1–7) under study are given in Table 1.
Each rate constant represents an average from a set of three
values obtained in three separate kinetic runs; all three val-
ues were measured at the same temperature. The agreement
between the rate-constant values in each set is within ±2%.
Each elimination process was monitored until 90–95% of re-
action was completed. The temperature range (54 ± 7 K)
over which each compound was investigated is a prerequisite
for reliable kinetic studies (9–11). Arrhenius parameters were
obtained using the results from Table 1. The Arrhenius plots
were linear over the temperature range studied, with correla-
tion coefficients of the order of 0.990 ± 0.009. For example,
for 3-thiopheneacetic acid (1) at 53.5 K the correlation coef-
ficient is 0.9994. Arrhenius log A (s–1), DS# (J mol–1 K–1), the
energy of activation (Ea, kJ mol–1), and the first-order rate
constants (k, s–1) of the elimination reactions at 600 K are
recorded in Table 2. The values of log A (s–1) show a narrow
range for each set of compounds (10.5 ± 0.4 for the acids
and 11.8 ± 0.2 for the alcohols), but a wider range for the Ea
(kJ mol–1) values, an indication of dependence of the reac-
tion on Ea. This seems to be a common feature of polar ther-
mal gas-phase elimination processes (13). The values for
DS# (J mol–1 K–1) are negative and relatively large, which
might be the result of considerable pre-organization leading
to the formation of the six-membered TS postulated for such
gas-phase reaction pathways (7). It is to be noted that an ear-
lier but independent preliminary study by our group on the
present substrates gave almost identical reaction rate factors
at 450 K; however, only the rates for pyrolysis at 600 K are
reported here.
4
704.5
714.9
725.1
735.2
745.2 17.66
755.4 31.49
765.3 45.79
3.003
4.191
9.145
9.865
peratures comparable with those used in the kinetic investi-
gations. The reactions were allowed ample residence time to
ensure maximum pyrolysis. The constituents of the
pyrolysates were analyzed using GC–MS, FT-IR, and 1H
NMR techniques. Conversion of the acid and alcohol start-
ing materials into reaction products was monitored in part
by observing the absence of the characteristic FT-IR absorp-
tion peaks of the substrates. Pyrolysis of 2-thiopheneacetic
acid (2) and 2-(2-thienyl)ethanol (4) gave 2-methylthiophene
C+
(m/z: M = 98 and base peak at 97; the corresponding val-
ues for their precursor substrates are M = 142 and M
C+
C+
=
128, respectively). On the other hand, pyrolysis of 3-
thiopheneacetic acid (1) and 2-(3-thieny)ethanol (3) pro-
duced 3-methylthiophene with m/z values identical to those
of their 2-substituted analogues. Differentiation between 2-
methylthiophene and 3-methylthiophene was derived from
their respective 1H NMR spectra (2-methylthiophene
(CDCl3) d: 3.2–3.5 (s, 3H, CH3), 6.8–7.7 (m, 3H, HAr); 3-
methylthiophene (CDCl3) d: 2.1–2.3 (s, 3H, CH3), 6.9–7.5
(m, 3H, HAr)). Pyrolysis of 2-thiopheneglyoxylic acid (5)
Reaction products
Reaction products for complete pyrolysis were obtained
for all substrates except 1-(2-thienyl)-1-propanone, since its
complete pyrolysis required temperatures beyond the operat-
ing temperatures of the reactors. Pyrolyses were carried out
either in a flow reactor or in a sealed-tube pyrolyzer at tem-
C+
gave 2-thiophenecarboxaldehyde (m/z: M = 112, and frag-
ment ions at 111 and 83; ꢁmax: 1655 (C=O); and d (CDCl3):
7.3–8.3 (m, 3H, HAr), 10.0 (s, 1H, HCO)). It is noteworthy
© 2002 NRC Canada

Al-Juwaiser et al.
501
Table 2. Arrhenius parameters and rate coefficients (k, s–1) at 600 K and DS# values for pyrolysis of compounds 1–7.
T
(K)
Ea
log A
DS#
Corr.
Coeff.
k
Compd.
(kJ mol–1)
(s–1)
(J mol–1K–1)
(s–1)
1
2
3
4
5
6
7
53.5
60.2
60.8
60.8
59.7
47.0
50.6
153.5
149.4
207.1
203.3
123.4
223.8
149.8
10.76
10.08
12.04
11.55
10.91
11.05.
10.33
–44.8
–57.7
–20.3
–29.6
–41.8
–39.2
–53.0
0.9994
0.9866
0.9943
0.9906
0.9918
0.9814
0.9827
2.51 × 10–3
1.20 × 10–3
1.03 × 10–6
7.08 × 10–7
1.48
3.72 × 10–9
1.95 × 10–3
Scheme 2. Concerted four-membered TS (a) and six-membered
TS (b) for pyrolysis of 3-thiopheneacetic acid.
Scheme 3. Calculated reaction mechanism for thermal gas-phase
elimination of diacetamide.
attack. (iii) Theoretical ab initio calculations on the mecha-
nism of similar thermal gas-phase elimination reactions gave
results based on a six-membered TS that agree well with ex-
perimental values. In addition, these calculations have re-
placed the proposed four- and five-membered TS with five-
and six-membered structures, respectively (17, 18).
The theoretical studies used analytical gradients at the
MP2//RHF/6–31G* level of theory, optimization routines,
and microcanonical probability fluxes through TS to map
PES and to calculate activation barriers and rate constants
for the thermal reactions. The compounds whose elimination
reactions were assigned a concerted six-membered TS in-
clude diacetamide, b-hydroxyketones, acetic anhydride, and
methyl benzoylformate (17). Concerted five-membered TS
were calculated for the pyrolysis of pyruvic acid, benzoyl
that 3-thiophenemalonic acid (7) gave pyrolysis products
and a rate constant similar to those of 3-thiopheneacetic
acid, an indication that the latter seems to be the primary de-
composition product, which then pyrolyzes further into 2-
methylthiophene and CO2.
Reaction pathway and molecular reactivity
The present results for substrates 1–7 suggest a homoge-
neous unimolecular first-order elimination process. More-
over, the magnitudes of the Arrhenius-parameter values are
in the range expected for such reactions (7, 13–16). The ho-
mogeneous nature and the lack of reactor-surface effects
were proven by increasing the available surface area for re-
action using tubes packed with glass helices. An increase of
ca. 50% in surface area produced no noticeable change in
the reaction rates. Kinetic runs were also conducted in the
presence and absence of cyclohexene (free-radical scaven-
ger). The rates, however, remained the same within experi-
mental error. Additional analysis of the pyrolysates gave no
indication of a radical-based reaction product.
Product formation can be rationalized either (i) on the ba-
sis of a mechanism involving a concerted four-membered TS
or (ii) by a pathway with a concerted six-membered TS. The
two mechanisms are shown in Scheme 2 for the gas-phase
elimination reaction of 3-thiopheneacetic acid.
Although it is much simpler to explain product formation
using the four-membered TS mechanism (Scheme 2a), theo-
retical and experimental evidence favour the six-membered
TS (Scheme 2b). This evidence includes the following:
(i) the conformation of the cyclic six-membered structure is
known to be much more stable compared with the highly
strained four-membered counterpart. (ii) The development of
the six-membered TS requires a more facile intramolecular
transfer of the electrophilic hydrogen to the protophilic
thiophene ring positions, which are amenable to electrophilic
formic acid, 3-hydroxy-3-methyl-2-butanone ( -oxoacids
a
and -hydroxyketone), and a large number of -substituted
a
a
carboxylic acids (17, 18). The theoretical calculations fur-
ther confirm that the development of the TS involves
intramolecular hydrogen transfer to a protophilic site and
concomitant formation and breaking of the other bonds in
the concerted structure. Typically, the reaction pathway cal-
culated for the thermal gas-phase elimination reaction of
diacetamide shown in Scheme 3 reproduces the cyclic six-
membered TS, which accounts for the kinetic values and the
reaction products reported in the experimental study of this
substrate (17a, 19).
The six-membered TS mechanism (Scheme 2b), presently
adopted to account for the kinetic results and reaction prod-
ucts of the thiophene compounds (1–7), has also been pro-
posed for the thermal elimination reaction of 3-butenoic acid
(8), 3-buten-1-ol (9), and the ꢀ-deficient 2-(2-ethoxy)pyri-
dine systems (7, 15). The values of the Arrhenius parameters
reported for the latter systems are comparable to those ob-
tained in the present study.
The gas-phase molecular and positional reactivities of the
thiophene compounds (1–7) and related systems (8–10)
given in Scheme 4 allow for the following comparisons to be
made:
(i) the thiopheneacetic acids 1, 2 are more reactive than
their thienylethanol analogues (3, 4) by a factor ca. 2 × 103.
© 2002 NRC Canada

502
Can. J. Chem. Vol. 80, 2002
Scheme 4. Rate coefficients (k, s–1) at 600 K and selected rate
ratios for pyrolysis of present substrates (1–7) and related com-
pounds (8–10).
2.5 × 10–4 s–1 at 600 K, protophilic alkene ꢀ bond) is much
lower than that of 4-hydroxy-2-butanone (k = 1.5 × 10–2 s–1
at 600 K, protophilic carbonyl moiety) (15). The effect of
aromaticity on molecular reactivity is demonstrated when
thiopheneglyoxylic acid (5) is shown to be twofold more re-
active than benzoylformic acid (10); the latter being the
more aromatic in nature (20).
(iv) 3-Thiopheneacetic acid (1) is more reactive than its 2-
isomer (2) by a factor of ca. 2, whereas the corresponding
relative rate factor for the thienylethanols (3 and 4) is 1.5.
These rate factors, though moderate, are nevertheless consis-
tent with the accepted intrinsic 2 > 3 positional reactivities
of ꢀ-excessive five-membered aromatic heterocycles (4, 5).
In the present systems, the protonic H of the group at the 3-
position is reacting with the more protophilic ꢀ-bond at the
nuclear 2-position.
(v) 3-Thiophenemalonic acid (7) has two equivalent
carboxylic acid groups that are equally susceptible to
decarboxylation during pyrolysis, whereas 3-thiopheneacetic
acid (1) only has one such group. Statistically, 7 would be
expected to be twice as reactive as 1; however, the rate con-
stants of the elimination reaction of the two acids are com-
parable (2.0 × 10–3 s–1 and 2.5 × 10–3 s–1, respectively). The
reason that the higher reactivity expected for 7 is not ob-
served might be due to intramolecular H-bonding involving
the two acid groups, with an adverse effect on the molecular
reactivity of acid 7.
Experimental
Materials, techniques, and instrumentation
The acids, alcohols, and ketone (1–7) under investigation
are commercially available from Aldrich. The substrates and
the constituents of their pyrolysates were characterized using
1
GC–MS, FT-IR, and H NMR spectroscopy. Instrumentation
This large difference in molecular reactivity is ascribed to a
more acidic H-bond (bond d, Scheme 1) and to the magni-
tude of the positive charge developing on the incipient hy-
drogen of the OH group of substrates 1, 2. The difference in
reactivity becomes much larger (a factor of 4 × 108) when
the O–H moiety of 5 is compared with a C–H bond of re-
lated ketone 6.
included a Finnigan Mat INCOSXL for GC–MS studies, a
PerkinElmer 2000 for the FT-IR, and a Bruker AC80 for the
NMR analysis. Kinetic runs were conducted in a Chemical
Data System (CDS) custom-made pyrolyzer comprising an
insulated aluminium block fitted with a platinum-resistance
thermometer and a thermocoupler connected to a Comark
microprocessor thermometer. The temperature of the alu-
minium block was controlled by a digital Eurotherm 093
precision temperature regulator. It is to be noted that alu-
minium was chosen for its low temperature gradient and re-
sistance to elevated temperatures. HPLC analysis of kinetic
runs was carried out on a Bio-rad Model 2700 coupled with
a Bio-rad 1740 UV–vis detector.
(ii) 2-Thiopheneglyoxylic acid (5) is 1.2 × 103 more reac-
tive than 2-thiopheneacetic acid (2). This is a consequence
of the effect of the relative bond polarity (bond b, Scheme 1)
on molecular reactivity, and on the repulsion between the
positive charge on the carbon atoms of the adjacent carbonyl
groups of 2, which result in a more polar bond (b) in 2.
(iii) Aliphatic 3-butenoic acid (8) and 3-buten-1-ol (9) are
both more reactive than their heteroaromatic thienyl counter-
parts (3- and 2-thiopheneaceatic acid (1, 2) and 1-(3-
thienyl)- and 1-(2-thienyl)ethanol (3, 4), respectively). The
rate factors involved are, respectively, 18 and 37 for the for-
mer, and 244 and 355 for the latter pair of analogues. This
pattern of relative reactivities indicates that although the
thiophene ring is ꢀ-excessive in nature, its ꢀ bonds (bond a,
Scheme 1) are still less protophilic than the aliphatic ꢀ
bonds. Comparative protophilicity of bond a also explains
why the rate of the elimination reaction of 3-buten-1-ol (k =
Kinetic measurements and product analysis
Procedures for kinetic measurements and data treatment
and analysis of pyrolysates using flow-reactors have been
described in detail elsewhere (9a, 10b, 19). Reaction prod-
ucts were obtained in sealed-tube pyrolyzers. Acetonitrile
was used as the solvent for both kinetic and reaction product
studies, and either benzene or 1,2,3-trichlorobenzene as the
internal standard in the quantitative chromatography mea-
surement of the extent of the reaction.
© 2002 NRC Canada

Al-Juwaiser et al.
503
10. (a) N.A. Al-Awadi, K. Kaul, and O.M.E. El-Dusouqui. Can. J.
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Acknowledgments
We wish to thank Tommy Mathew for a preliminary ki-
netic study on the present compounds. The support of the
University of Kuwait received through research grant SC
088 and the facilities of ANALAB/SAF is gratefully ac-
knowledged.
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