Kinetics and mechanism of pyrolysis of sulphonyl hydrazones and oximes. Part 2 - Structural effects and molecular reactivity

Article abstract of DOI:10.1002/(SICI)1099-1395(199908)12:8<654::AID-POC168>3.0.CO;2-L

The kinetics and products of the pyrolytic reaction of six tosyl arenecarboxaldoximes were studied over the temperature range ca 334 - 401 K to yield the following Arrhenius log A/s^-1 and E_a / KJ mol^-1, respectively: 10.92 and 98.25 for tosyl benzaldoxime and 10.83 and 100.4 for m-nitro-, 10.66 and 97.53 for p-chloro-, 11.80 and 107.8 for m-chloro-, 11.60 and 101.5 for p-methyl- and 10.84 and 97.75 for p-methoxybenzaldehyde O-[(4-methylphenyl)sulpnonyl]oxime. At 500 K, the oxime compounds were found to be 9.4 × 10³ - 2.7 × 10⁴-fold more reactive than their hydrazone analogues. Copyright

Full text of DOI:10.1002/(SICI)1099-1395(199908)12:8<654::AID-POC168>3.0.CO;2-L

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 12, 654–658 (1999)
Kinetics and mechanism of pyrolysis of sulphonyl
hydrazones and oximes. Part 2ÐStructural effects and
molecular reactivity
Nouria A. Al-Awadi,* Mohamed H. Elnagdi, Kamini Kaul, Swaminathan Ilingovan and
Osman M. E. El-Dusouqui
Department of Chemistry, University of Kuwait, P.O. Box 5969, Safat 13060, Kuwait
Received 25 October 1998; revised 23 1999; accepted 8 March 1999
ABSTRACT: The kinetics and products of the pyrolytic reaction of six tosyl arenecarboxaldoximes were studied over
1
the temperature range ca 334 – 401 K to yield the following Arrhenius log A/s and Ea / kJ mol 1, respectively:
10.92 and 98.25 for tosyl benzaldoxime and 10.83 and 100.4 for m-nitro-, 10.66 and 97.53 for p-chloro-, 11.80 and
107.8 for m-chloro-, 11.60 and 101.5 for p-methyl- and 10.84 and 97.75 for p-methoxybenzaldehyde O-[(4-
methylphenyl)sulphonyl]oxime. At 500 K, the oxime compounds were found to be 9.4 Â 103 – 2.7 Â 104-fold more
reactive than their hydrazone analogues. Copyright 1999 John Wiley & Sons, Ltd.
KEYWORDS: sulphonyl hydrazones; sulphonyl oximes; pyrolysis; kinetics; mechanism; structural effects;
molecular reactivity
INTRODUCTION
In Part 1,1 we reported the first kinetic and mechanistic
study on the non-catalysed thermal elimination reaction
of a number of tosyl arenecarboxaldehyde hydrazones.
The reactions were found to be unimolecular and to
involve a cyclic six-membered transition state. The study
also featured Hammett correlation of the effects of
substituents on the H-bond donor acidity of the
hydrazone hydrogen [Scheme 1 (i)]. This analysis
parallels an earlier finding from the pyrolysis of a
heterocyclic hydrazone system where the development of
aromatic character of an incipient pyridyl ring was
considered to be an important factor in directing the
elimination pathway [Scheme 1 (ii)].2
Scheme 2. Oxime elimination fragments
The mechanism of the present pyrolytic reaction of the
arenecarboxaldoximes follows an identical pathway. The
Arrhenius parameters and relative rates suggest that the
polarity of the oxime N—O bond (a) and the concomitant
charge development are the major contributors to the
relatively high reactivity observed for these compounds,
rather than the protophilicity of bond (b) or the H-bond
donor acidity of (c) (see Scheme 2). It has been noted
previously1 that kinetic and mechanistic investigations on
the pyrolysis of sulphonyl hydrazones are available but
only for reactions in solution.3–5 This is also the case for
the oximes.6
RESULTS AND DISCUSSION
Scheme 1. Six-membered transition state
First-order rate constants were calculated individually for
tosyl benzaldoxime (1) and m-nitro- (2), p-chloro- (3), m-
chloro- (4), p-methyl (5) and p-methoxybenzaldehyde O-
[(4-methylphenyl)sulphonyl]oxime (6) over a range of
temperature of ꢀ46 K. Each rate coefficient is an average
*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: Kuwait University; Contract/grant number:
SC 088.
Copyright 1999 John Wiley & Sons, Ltd.
CCC 0894–3230/99/080654–05 $17.50

PYROLYSIS OF SULPHONYL HYDRAZONES AND OXIMES
655
Table 1. Kinetic data, rate coef®cients (103 k/s 1) at 380 K and Arrhenius parameters for pyrolysis of p-CH3C6H4SO3N
oximes
=
CHAr
1
1
1
Compound
Ar
T/K
104 k/s
Log A/s
Ea/kJ mol
103 k/s 1 (380 K)
1
C6H5
335.0
343.3
354.0
361.6
367.0
375.0
385.0
351.3
367.6
373.9
378.0
384.7
397.2
338.9
345.9
367.6
374.8
378.5
385.0
353.1
369.0
384.4
392.1
401.0
334.2
346.0
355.8
360.5
371.1
376.9
384.6
345.0
354.6
363.6
365.1
372.3
375.4
384.0
389.1
395.2
0.40
1.51
2.67
5.38
8.12
16.5
40.6
0.83
3.54
6.58
8.74
15.5
45.1
0.40
2.08
6.76
12.2
15.4
24.8
0.68
3.49
14.3
24.5
57.0
0.53
1.83
5.24
8.10
21.0
31.6
67.2
1.10
2.70
6.49
8.40
12.6
16.6
34.5
54.0
83.2
10.92
Æ0.19
98.25
Æ1.32
2.60
2
3
m-NO2C6H4
p-ClC6H4
10.83
Æ0.01
100.4
Æ0.05
1.07
1.80
10.66
Æ0.26
97.53
Æ1.8
4
5
m-ClC6H4
11.80
Æ0.24
107.8
Æ1.7
0.93
4.44
p-CH3C6H4
11.60
Æ0.25
101.5
Æ1.78
6
p-CH3OC6H4
10.84
Æ0.28
97.75
Æ1.20
2.53
of at least three kinetic runs with deviation ꢁ Æ 2%. The
rate constants and the temperatures at which the
measurements were made are given in Table 1. Arrhenius
plots of the kinetic data showed strict linearity for ꢀ95%
reaction. A representative plot was included in Part 1.1
The log A/s and Ea/kJ mol data are also given in
Table 1; the values are correct to within Æ0.15 and Æ1.0,
respectively.
The present thermal elimination reactions were
ascertained to be homogeneous, polar and unimolecular.
It is noteworthy that the reactions are free from radical
and catalytic pathways.7 The reactions were tested for
their homogeneous first-order nature either by introdu-
cing a change of up to sixfold in substrate concentration
or a change of up to 50% in reaction volume. The strict
linearity of the Arrhenius plots over the reaction range
(ꢀ95%) and temperature span (46–50 K) further
substantiates the homogeneous first-order kinetics of
the reactions.
The reactions are deemed free of reactor-surface
effects since an increase in the ratio of reactor surface-
to-reactor volume of approximately ninefold did not
produce any significant change in rate constants, within
experimental error. The reaction surface was varied by
using empty reaction tubes and tubes packed with glass
1
1
1
helices. The Arrhenius log A/s (11.3 Æ 0.5) and Ea/kJ
1
mol (103 Æ 5) values are what might be expected for
polar pyrolytic reactions. The polar nature of the
reactions is further confirmed by the results of the
Hammett correlation.1 The first-order kinetics and
product analysis of the reaction coupled with the
expected thermodynamic stability of the incipient frag-
Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 654–658 (1999)

656
N. A. AL-AWADI ET AL.
Table 2. Rate coef®cients (k/s 1) at 500 K and relative rates (krel) of oxime and hydrazone analogues: p-CH3C6H4SO2XN
=
CHAr
Parameter
Ar
C6H5
m-NO2C6H4
p-ClC6H4
p-CH3OC6H4
1
k/s (oxime)
4.53
1.68
2.20
2.33
2.96
1.95
4.25
1.99
104k/s 1 (hydrazone)
krel
26 964
9 442
15 179
21 794
ments of pyrolysis point to a reaction which is unimol-
ecular in nature. The quantitative estimation of the
composition of the reaction mixture was carried out in
presence of a thermally stable inert internal standard and
using high-performance liquid chromatography (HPLC)
with UV–visible detection. Further, parallel reactions
were carried out in the presence and absence of free-
radical scavengers. The rates were not changed by the
addition of radical traps to the reaction mixture.
compounds is ꢀ = 0.125 Æ 0.025. This observation might
be the result of an appreciable contribution to overall
molecular reactivity from bond (c).
The effect of structure on reactivity in gas-phase and
vapour-phase pyrolytic reactions has been reviewed.13
Molecular reactivity in the oxime compounds is domi-
nated by the greater polarity of the N—O s-bond (a) and
the associated residual charge developed across the
bond.
The elimination fragments in the pyrolysate were
identified using on-line gas chromatography–mass spec-
trometry (GC–MS), Fourier transform (FT) IR pyroprobe
and 1H NMR spectroscopy. Each of the oximes 1–6 gave
p-methylbenzenesulphonic acid and the corresponding
cyanoarene compound (Scheme 2). All the reactions of
the substrates under study were free from secondary
decomposition processes.
To facilitate comparison of molecular reactivities,
rates of reaction were calculated at 500 K for both the
hydrazone and oxime series. The rate coefficients and
relative rates of oximes 1, 2, 3 and 6 and their hydrazone
counterparts are given in Table 2. The results in Tables 1
and 2 allow the following conclusions to be drawn.
The oximes are consistently more reactive than the
hydrazones, showing large relative rate differences of
9.44 Â 103 – 2.70 Â 104-fold.
In each analogous hydrazone–oxime pair where the
structural difference between the two systems is asso-
ciated with bond (a), the increase in reactivity of ca 104-
fold could be attributed primarily to the polarity of this
bond. Where a change in reactivity of this magnitude or
higher has been obtained in reactions of similar nature,
the reason could be traced to a switch in the bond
affecting the change9,11 and a non-linear Hammett
correlation has been reported.11 Failure to obtain a linear
Hammett correlation for the present oxime series parallel
to that reported for the hydrazone compounds might be
due to this structural modification, where the contribution
to reactivity from bond (a) is considered to dominate,
thus superseding any effective contribution from H-bond
donor acidity associated with bond (c). A review is
available14 in which the relationship between non-linear
Hammett structure–reactivity patterns and changes in
effective molecular structure and associated reactivity, or
change in the rate-controlling step of a reaction or change
in the mechanistic pathway of the reaction is outlined and
discussed.
The greater reactivity of the oximes over the
hydrazones is attributed to the differential in the polarity
of the oxime (N—O) bond compared with the hydrazone
(N—NH) bond. The relative contribution to the molecu-
lar reactivity of each of the three bonds involved in the
electron flow described by the curved arrows in the
elimination pathway (Scheme 2) has been documented in
earlier studies.8–11 It has been noted in these studies that
any one of bonds (a), (b) or (c) in Scheme 2 could be the
prime contributor to molecular reactivity, or that the
overall reactivity is a product of synergism of two or all
three bonds. Non-synchronous bond breaking would be
the reason for polarity of the concerted transition state,
and an effective larger relative contribution from a
particular bond might under favourable structural con-
straints lead to a linear Hammett correlation.11,12
EXPERIMENTAL
Synthesis
Benzaldehyde, p-chlorobenzaldehyde, m-chloro-
benzaldehyde, p-methylbenzaldehyde and p-meth-
oxybenzaldehyde O-[(4-methylphenyl)sulphonyl]-
oxime.6 To a rapidly stirred solution of 3 mmol of
benzaldoxime in 20 ml of dry diethyl ether, 3 mmol of
powdered sodium amide were added and the mixture was
stirred for about 5 h. The mixture was filtered and the
precipitate was washed several times with diethyl ether.
p-Toluenesulphonyl chloride (0.36 g, 2.5 mmol) in 5 ml
of diethyl ether was slowly added to a rapidly stirred
slurry of the sodium salt of the oxime in 5 ml of diethyl
The effects of aryl substituents on the H-bond donor
acidity of the aldehydic hydrogen is evident in the
hydrazone series, and has provided a suitable set for a
Hammett correlation analysis.1 The value of the Ham-
mett reaction constant for the pyrolysis of the hydrazone
Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 654–658 (1999)

PYROLYSIS OF SULPHONYL HYDRAZONES AND OXIMES
657
ether. The salt produced was removed by filtration and
Kinetic and product analysis
10 ml of hexane were added to the filtrate. On cooling,
the oxime of the arenesulphonate crystallized out and was
filtered. Recrystallization from 1:1 (v/v) mixture of
diethyl ether and hexane gave the pure product in 55–
60% yield.
Instrumentation. The pyrolyser was a CDS custom-
made unit. It consisted of an insulated aluminium block
to ensure a low temperature gradient and resistance to
high temperature, a Pyrex reaction vessel to fit in a
groove along the cylindrical axis of the block, a platinum
resistance thermometer and a thermocouple connected to
a microprocessor. The temperature of the block was
controlled by means of a Eurotherm 093 precision
temperature regulator. The temperature setting was
achieved with a digital switch which allows temperature
read-out accurate to Æ0.5°C. The actual temperature of
the reactor groove was measured by means of a Comark
microprocessor thermometer. Comparative quantitative
analyses of reaction mixtures were carried out using
HPLC with an LC-8 column (25 cm  4.6 mm. i.d.) and a
UV–visible detector (Bio-Rad, Shimadzu SPD-10AV).
Samples were injected using a Supelco 5 mm precision
syringe.
m-Nitrobenzaldehyde O-[(4-methylphenyl)sulpho-
nyl]oxime. The sodium salt of m-nitrobenzaldoxime
was prepared as described above. This salt is dissolved in
5 ml of 1:1 acetone–water mixture and then added slowly
with stirring to a cooled solution of p-toluenesulphonyl
chloride in 2 ml of acetone. From the mixture kept at
15°C, the oxime sulphonate ester separated out, was
filtered, washed with 50% acetone solution and recrys-
tallized from hexane–diethyl ether to give pure yield of
about 50%.
Characterization
Characterization of the substrates and the constituent
fragments of the pyrolysates included elemental analysis,
GC–MS, FT-IR and 1H NMR spectroscopic analysis. The
GC–MS Instrument was a Finigan MAT INCOS XL
quadrupole mass spectrometer. FT-IR spectra were
measured using a Perkin-Elmer model 2000 spectro-
meter, H NMR spectra were recorded on a Bruker AC-
80 spectrometer and the elemental analyser was a LECO
CHNS-932 unit.
All melting-points are uncorrected.6 1H NMR spectra and
results of the elemental analysis of the compounds under
study are as follows.
Benzaldehyde O-[(4-methylphenyl)sulphonyl]oxime
(1): m.p. 84–86°C (found: C, 60.66; H, 4.86; N, 4.65;
S, 11.65. C14H13NO3S requires C, 61.09; H, 4.72; N,
5.09; S, 11.63%); 1H NMR (80 MHz; CDCl3), ꢁ 2.44 (s,
1
3H, CH3), 7.26–7.98 (m, 9H, ArH), 8.23 (s, 1H, HC=N).
m-Nitrobenzaldehyde O-[(4-methylphenyl)sulphonyl]-
oxime (2): m.p. 112 –113°C (Found: C, 52.02; H, 4.05;
N, 8.47; S, 10.61. C14H12N2O5S requires C, 52.5; H,
3.75; N, 8.75; S, 10.0%); 1H NMR (80 MHz;
CD3COCD3), ꢁ 2.46 (s, 3H, CH3), 7.45–8.50 (m, 8H,
Kinetic runs and data analysis. Dilute (ppm) standard
solutions of the substrates in a suitable solvent (e.g.
acetonitrile) were prepared and an internal standard (e.g.
chlorobenzene) was added to each solution. Care was
taken to ensure that both the solvent and the internal
standard were 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 peak which was one third higher than
that of the internal standard in the analytical HPLC trace.
Preparation of a standard solution allows several kinetic
runs to be performed on the same reaction mixture.
A sample (0.2 ml) of the standard solution was placed
in a reaction tube (9 ml) and the contents were sealed
under vacuum. The tube was placed swiftly in the
pyrolyser, which had been preheated to a threshold
temperature that together with a convenient reaction time
would allow only 10–20% pyrolysis. The contents of the
reaction tube were then analysed using HPLC. For each
substrate, the rate coefficient represents an average from
triplicate runs with rate agreement to within Æ2%. Rates
were measured at regular intervals of 5–10°C over a 46–
50 K temperature range and up to ꢀ95% reaction.
ArH), 8.74 (s, 1H, HC=N). p-Chlorobenzaldehyde O-
[(4-methylphenyl)sulphonyl]oxime (3): m.p. 109–110°C
(Found: C, 54.25; H, 3.82; N, 4.03; S, 10.99.
C14H12NO3SCl requires C, 54.28; H, 3.87; N, 4.52; S,
1
10.34%); H NMR (80 MHz; CDCl3), ꢁ 2.44 (s, 3H,
CH3), 7.25–7.99 (m, 8H, ArH), 8.19 (s, 1H, HC
=N). m-
Chlorobenzaldehyde O-[(4-methylphenyl)sulphonyl]-
oxime (4): m.p. 90–92°C (Found: C, 54.08; H, 3.87; N,
4.15; S, 10.39. C14H12NO3SCl requires C, 54.28; H, 3.87;
N, 4.52; S, 10.34%); 1H NMR (80 MHz; CDCl3), ꢁ 2.44
(s, 3H, CH3), 7.25–7.86 (m, 8H, ArH), 8.25 (s, 1H,
HC=N). p-Methylbenzaldehyde O-[(4-methylphenyl)-
sulphonyl]oxime (5): m.p. 108–110°C (Found: C, 62.49;
H, 5.21; N, 4.52; S, 11.22. C15H15NO3S requires C,
62.28; H, 5.19; N, 4.84; S, 11.07%); 1H NMR (80 MHz;
CDCl3), ꢁ 2.35 (s, 3H, CH3), 2.42 (s, 3H, CH3), 7.18–7.99
(m, 8H, ArH), 8.18 (s, 1H, HC=N). p-Methoxybenzal-
dehyde O-[(methylsulphonyl)]oxime (6): m.p. 99–101°C
(Found: C, 59.26; H, 4.86; N, 4.21; S, 11.01.
C15H15NO4S requires C, 59.02; H, 4.91; N, 4.59; S,
Rates were reproduced using empty reaction tubes and
tubes packed with glass helices in order to increase the
reactor surface by up to about ninefold. This precaution
was to test for reactor surface effects. Further, rates were
1
10.49%); H NMR (80 MHz, CDCl3), ꢁ 2.43 (s, 3H,
CH3), 3.82 (s, 3H, H3CO), 6.93–7.99 (m, 8H, ArH), 8.16
(s, 1H, HC=N).
Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 654–658 (1999)

658
N. A. AL-AWADI ET AL.
measured in the presence and absence of free-radical
scavengers (e.g. cyclohexene) to confirm the non-
involvement of free-radical pathways.
The rate coefficients were calculated using the
expression kt = lna0/a for first-order reactions. The
Arrhenius parameters were obtained from a plot of log
k vs 1/T (K), an example of which was shown in Part 1.1
The elimination rate constant at T (K) is given by
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1
1
log k ˆ log Aꢂs
†
EaꢂkJ mol †=4:574 TꢂK†
Product analysis. The products analysed were for
pyrolyses conducted at temperatures commensurate with
those of the kinetic measurements. No secondary
decomposition was observed for any of the compounds
studied. In both the kinetic runs and product analysis, the
initial conversion of 10–20% and reaction extent of
ꢀ95% pyrolysis have been maintained.
Acknowledgements
The support of Kuwait University through research grant
SC 088 and the facilities of Analab and SAF are
gratefully acknowledged.
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Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 654–658 (1999)