Reactivity of sodium arenesulfinates in the substitution reaction to γ-functionalized allyl bromides

Article abstract of DOI:10.1002/poc.1629

The kinetics of nucleophilic bimolecular substitution reactions of γ-functionalized allyl bromides with non-substituted and p-substituted sodium arenesulfinates has been studied. Both the structure of allyl bromides and nucleophilicity of arenesulfinate ions exerted a significant effect on the values of the kinetic parameters such as the second-order rate constants k, activation energy E_A, and changes in the entropy ΔS≠, enthalpy ΔH≠, and free energy ΔG≠ of the formation of the activated complex from reactants. Based on the evaluation of kinetic parameters, the reactants could be arranged, according to their decreasing reactivity in the S_N2-reactions as follows: p-toluenesulfinate ion > benzenesulfinate ion> p-chlorobenzenesulfinate ion and 4-bromo-2- butenenitrile> 1,3-dibromopropene, respectively. Comparison was also made between the kinetic data obtained and some delocalization reactivity indexes for both the substrates and nucleophiles. The enthalpy-entropy compensation effect was observed for the reactions of sodium arenesulfinates with γ-functionalized allyl bromides. Copyright

Full text of DOI:10.1002/poc.1629

Research Article
Received: 19 March 2009,
Revised: 19 August 2009,
Accepted: 19 August 2009,
Published online in Wiley InterScience: 24 November 2009
(www.interscience.wiley.com) DOI 10.1002/poc.1629
Reactivity of sodium arenesulfinates in the
substitution reaction to g-functionalized
allyl bromides
a
a
a
*
Galina Khamis , Stoyanka Stoeva and Dimitar Aleksiev
The kinetics of nucleophilic bimolecular substitution reactions of g-functionalized allyl bromides with non-
substituted and p-substituted sodium arenesulfinates has been studied. Both the structure of allyl bromides and
nucleophilicity of arenesulfinate ions exerted a significant effect on the values of the kinetic parameters such as the
¼
¼
second-order rate constants k, activation energy EA, and changes in the entropy DS , enthalpy DH , and free energy
¼
DG of the formation of the activated complex from reactants. Based on the evaluation of kinetic parameters, the
reactants could be arranged, according to their decreasing reactivity in the SN2-reactions as follows:
p-toluenesulfinate ion > benzenesulfinate ion > p-chlorobenzenesulfinate ion and 4-bromo-2-butenenitrile > 1,3-
dibromopropene, respectively. Comparison was also made between the kinetic data obtained and some delocaliza-
tion reactivity indexes for both the substrates and nucleophiles. The enthalpy–entropy compensation effect was
observed for the reactions of sodium arenesulfinates with g-functionalized allyl bromides. Copyright ß 2009 John
Wiley & Sons, Ltd.
Keywords: kinetic resolution; nucleophilic substitution; substituent effects; sulfinate S-allylation
took place at a higher rate.[24] On the other hand, the kinetic
INTRODUCTION
studies conducted by Jarvis and Tong[25] showed that the
reactivity in the reactions of sodium benzenesulfinate with
a-halo-p-nitrobenzyl phenyl sulfones decreased in the order
The reactions of sulfinic acids or their salts with some electro-
philes, resulting in the formation of new C—S bonds constitute
one of the most important methods for the synthesis of
sulfones.[1] As reported in some publications,[2–8] the chemical
properties of sulfones make them very useful reagents in the
organic synthesis, since these products serve as synthetic
intermediates in the preparation of various chemically and
biologically significant compounds. The syntheses of sulfones
have been extensively described by Schank,[9] in his review, and
new, independent reports concerning the modification and
optimization of these types of important reactions have also
appeared.[10–19]
Relatively fewer studies, associated with the quantitative
evaluation of the reactivity of arenesulfinic acids toward saturated
and unsaturated electrophilic reactants, containing carbon–carbon
double bond have been published so far. Ogata et al. studied the
kinetics of nucleophilic addition of substituted arenesulfinic acids
to acrylonitrile[20] as well as p-benzoquinone.[21,22] These reactions
were found to follow second-order kinetics. The authors also found
that, at pH 2–4, the rate-determining step was the addition of
arenesulfinate ion to p-benzoquinone, whereas, at pH 4–6, addition
as well as deprotonation reactions took place with commensurable
rates. The basic kinetic parameters of the nucleophilic additions
of p-substituted arenesulfinic acids to 2-haloacrylonitriles were
also determined.[23] Lindberg[24] studied the kinetics of the
nucleophilic substitution reactions of arenesulfinate ions with
bromoacetamide and bromoacetate. The nucleophilicity of
sulfinate ions was found to decrease with the introduction of
electron-withdrawing groups such as chlorine atom and nitro
group in the aromatic nucleus. In all cases, the interaction
between bromoacetamide and p-substituted arenesulfinates
k
Br > kI > kCl, depending on the type of halogen.
A number of authors studied the interactions of arenesulfinic
acids with g-substituted allyl halides, which resulted in the
formation of structurally diverse allyl phenyl sulfones.[10,15,17,26]
However, information about the quantitative evaluation of these
kinds of reactions was not found. The aim of the present work is
associated with the kinetic studies of the reactions between
some non-substituted and p-substituted sodium arenesulfinates
with g-functionalized allyl bromides. The effect of substituents in
the reactants on the changes of basic kinetic parameters of these
reactions was discussed. Moreover, comparison of the results
concerning the delocalization reactivity indexes of arenesulfinate
ions and g-substituted allyl bromides was made.
EXPERIMENTAL
General remarks
All substituted and non-substituted sodium benzenesulfinates
were prepared by a known procedure, employing reduction of
the corresponding benzenesulfonylchlorides with 50% excess of
*
Correspondence to: S. Stoeva, Department of Organic Chemistry, Bourgas
Assen Zlatarov University, 8010 Bourgas, Bulgaria.
E-mail: sstoeva2001@yahoo.com
a
G. Khamis, S. Stoeva, D. Aleksiev
Department of Organic Chemistry, Bourgas Assen Zlatarov University, 8010
Bourgas, Bulgaria
J. Phys. Org. Chem. 2010, 23 461–467
Copyright ß 2009 John Wiley & Sons, Ltd.

G. KHAMIS, S. STOEVA AND D. ALEKSIEV
sodium sulfite.[17,27] 4-Bromo-2-butenenitrile was prepared
according to the procedure described by Masuyama et al.[26]
1,3-Dibromopropene was obtained by dehydrobromination of
1,2,3-tribromopropane in the presence of sodium hydroxide.[28]
The compounds thus synthesized were characterized by
elemental analysis and IR spectroscopy. Elemental microanalyses
were performed using a Carlo Erba 1104 instrument (Italy).
Infrared spectra were recorded with a Specord 75 IR instrument
(Germany). The samples were either prepared as KBr pellets or
pressed between NaCl plates as neat films. The absorption of
adducts, formed due to the interaction of sodium benzenesulfi-
nates with p-benzoquinone was determined at room tempera-
ture in aqueous-ethanol medium by using a UV-Vis spectropho-
tometer (Germany). Absorption maxima at 322–324 nm were
registered for all the adducts.
The experimentally obtained results, concerning the changes
in the concentrations versus the time of their interaction with
4-bromo-2-butenenitrile and 1,3-dibromopropene, respectively,
were processed by employing the integral method for the kinetic
evaluation of homogeneous liquid-phase reactions in a batch
reactor at several constant temperatures.[30,31] These results were
found to be in good agreement with a second-order kinetics
model. The correlation coefficient values exceeded 0.988, which
proved with a good approximation the linear dependences.
Verification of the adequacy of the kinetic equation with respect
to the experimental data was conducted by applying the Fisher
criterion (Fexp), according to the equation:[30,31]
2
^
sC2
sC
Fexp
¼
(1)
where s2C is the dispersion of reproducibility, measured in
mol2 Lꢁ2; and sC is the dispersion of adequacy in mol2 Lꢁ2
.
2
^
Rate measurements
The values of the dispersion of reproducibility for the
corresponding experimental series are presented in Table 1.
The Fisher criterion was calculated for each of the reactions at
temperatures corresponding to the middle of the interval in
which these reactions were conducted, and at a level of signi-
ficance a ¼ 0.05. Observing this value of a has been recom-
mended for studying and evaluating organic reactions.[30,31] The
experimental values of Fisher criterion (Fexp) were found to be
lower than the reference (tabulated) value (Ftable), which, in this
case was set at 8.941.[31]
The reaction mixture, containing an aqueous solution of sodium
arenesulfinate (8 ml, 1 mmol), g-functionalized allylic bromide
(1 mmol), dissolved in ethanol (8 ml) as well as glacial acetic acid
(0.06 ml, 1.05 mmol) was placed into a glass tube. Sodium
arenesulfinates were prepared by the interaction of equimolar
amounts of arenesulfinic acids with sodium hydroxide, dissolved
in double-distilled water. The glass reactor was placed into a
thermostat to maintain the temperature between 308 and 333 K,
(ꢀ0.1 K accuracy). Aliquots of 0.1 ml were periodically taken for
each of the eight experimental points. They were transferred into
50 ml flasks, cooled in ice water. Glacial acetic acid (0.1 ml) and
water–ethanol solution (2 ml, 1:1 volume ratio) of
p-benzoquinone (1 mmol) were also added. The flask was then
thermostated at 293 K for 20 min, and a constant volume of the
solution (50 ml) was maintained therein.
Due to the rapid interaction of unreacted arenesulfinic acids
with p-benzoquinone, a change of the light-yellow color of the
solution in the flask was observed. The corresponding adducts
such as 2,5-dihydroxy-40-substituted diphenyl sulfones were thus
obtained. The reaction took place at a high velocity at room
temperature (293–298 K) in a water–ethanol medium and at pH
2–4.[21,22] In this particular case, pH was maintained to be within
3.17–3.63 by using an acetate buffer. Following the absorption measure-
ments for the adducts by employing a UV-spectrophotometer with
absorbance maxima registered within 322–324 nm, the concentrations
of the unreacted arenesulfinic acids were determined by means
of a standardized linear dependence method as reference.[29]
Four parallel runs were conducted for each experimental point.
The accuracy of UV-analysis when determining the current
concentrations of arenesulfinic acids for the experimental series
studied changed within the 0.025–0.029% range.
Kinetic processing of the experimental results
Integral processing of experimental data was conducted, applying
the well-known second-order kinetics equation r ¼ kCACY, with the
first order toward each of reactants (sodium arenesulfinates and
4-bromo-2-butenenitrile or 1,3-dibromopropene). At CAo ¼ CYo and
stoichiometric coefficients equaling unity, this equation can be
expressed as follows:[30,31]
1
1
¼
þ kt
(2)
CAo CAi
where CAo is the initial concentration of the corresponding
arenesulfinic acid, in mol Lꢁ1; CAi is the current concentration of
the reacted arenesulfinic acid, in mol Lꢁ1; k is the second-order
rate constant, in L molꢁ1 minꢁ1; and t is the reaction time, in minutes.
To determine the activation parameters of the studied
[32–34]
reactions, Arrhenius and Eyring equations were used.
Calculation of electronic descriptors
Molecular geometries of the corresponding sodium benzene-
sulfinates, benzenesulfinic acids, 4-bromo-2-butenenitrile, and
Table 1. Values of the dispersion of reproducibility (s2C) and Fisher criterion (Fexp) calculated at Tav for the reaction of sodium
arenesulfinates with 4-bromo-2-butenenitrile and 1,3-dibromopropene
—
—
—
Br—CH —CH CH—C
—
—
—
Substrates
N
Br—CH —CH CH—Br
—
2
2
Nucleophiles
T (K)
sC2
323
4.71 ꢂ 10ꢁ12
1.993
320.5
2.54 ꢂ 10ꢁ13
3.025
323
9.33 ꢂ 10ꢁ13
3.657
323
2.47 ꢂ 10ꢁ13
2.965
320.5
1.67 ꢂ 10ꢁ11
3.072
323
1.34 ꢂ 10ꢁ12
3.266
Fexp
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Copyright ß 2009 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2010, 23 461–467

REACTIVITY OF SODIUM ARENESULFINATES
1,3-dibromopropene were optimized. Then the quantum-
chemical descriptors were calculated using the AM1 Hamiltonian
method of the MOPAC 93 program, a semi-empirical molecular
modeling routine.[35,36] The following local electronic descriptors
were determined: donor superdelocalizibility of a sulfur atom
(SE(S)) of arenesulfinate ions (Y-C6H4-SOꢁ2 , where Y ¼ H, CH3, or
Cl); acceptor superdelocalizability of an allylic carbon atom
(SN(Callyl)) in 4-bromo-2-butenenitrile and 1,3-dibromopropene,
respectively; charges of a sulfur atom (q(S)) and those of an allylic
carbon atom (q(Callyl)) for the same reactants; the partial electron
density of Callyl on the lowest unoccupied molecular orbital
(LUMO) in substrates (fN(Callyl)). Higher values of SE and SN
corresponded to stronger nucleophilicity and electrophilicity of
the atomic reaction centers, respectively.[37]
hydroxyl groups such as a mixture of ethanol and water, and the
negatively charged oxygen atoms in the sulfinate ions left the
sulfur atom as the only nucleophilic center, which was deter-
mined by its unshared electron pair.[33,42] Both the low electro-
negativity and facile polarizability of the sulfur atom caused its
effective bonding to the electrophilic reaction site (Callyl), according
to the principles of interaction between ‘soft’ bases and acids.[39] As
a result, the final products were the corresponding sulfones, formed
in accordance with Scheme 1. Their composition and structure
were confirmed previously by elemental microanalysis, IR, 1H
NMR, and 13C NMR.[17]
The kinetic studies, accompanying the preparation of allyl
phenyl sulfones described above, were conducted at the tem-
perature interval of 308–333 K. The changes in the concentration
of arenesulfinic acids versus reaction time during their interaction
with 4-bromo-2-butenenitrile and 1,3-dibromopropene at 323 K
are shown in Figs 1 and 2, respectively. Similar kinetic curves were
also obtained for each of the reaction temperatures at 70–94%
conversion of arenesulfinic acids. The experimental data
processing was performed, by employing the well-known
second-order kinetics equation, i.e., the first order with respect
to each of the reactants. The assumption of such a kinetic model
was based on the suggested synchronic reaction mechanism,
with the participation of both the reactants in the formation of an
activated complex. The validation of the kinetic equation for this
bimolecular process was conducted by determination of the
experimental values of the Fisher criterion (Fexp) for each of the
reactions (Table 1) and its comparison with the corresponding
tabular value, which, in this case, was Ftable ¼ 8.941. This value was
2.5–3 times as high as the experimental one, which corresponded
well to the Fexp ꢃ Ftable requirement.[30,31] Therefore, the
RESULTS AND DISCUSSION
Interaction of sodium benzenesulfinates with g-functionalized
allyl bromides gives rise to the formation of substituted allyl
phenyl sulfones, according to Scheme 1.
The reaction was conducted in a slightly acidic water–alcohol
medium,[17] in order to prevent the possible isomerization of allyl
phenyl- to vinyl phenyl sulfones.[38] It may be assumed that the
substitution of bromine at the allylic (Callyl) carbon atom by the
corresponding arenesulfinate ion occurs according to SN2-mechanism.
The following considerations supporting this assumption are put
forward: (i) primary allylic halides participate in the reaction with
a reaction center (Callyl), which is structurally similar to that of
primary haloalkanes, and no steric hindrances are involved;[39,40]
(ii) the presence of electron-withdrawing substituents such as CN
and Br in the molecular structure of substrates could hamper the
formation of stable allyl cations in accordance with SN1-type
mechanism but, on the other hand, would significantly increase
the electrophilicity of Callyl in the SN2-type reaction, particularly, in
the presence of a cyano group; and (iii) in the experimentally
prepared sulfones, no rearrangement products were detected;
the presence of only products of direct allylic substitution was
actually proved.[17] What is more, for the interaction of sodium
benzenesulfinates with highly functionalized primary (Z)-ketoallyl
halides, Kotti et al.[41] have also described that, in similar cases,
SN2 nucleophilic substitution reactions take place.
The implementation of this reaction in polar medium was a
prerequisite for the full dissociation of sodium arenesulfinates.
The strong acidity of arenesulfinic acids is well known. They are
completely dissociated in water, except at high acidities.[42] For
example, pKa values of benzenesulfinic, p-toluenesulfinic, and
p-chlorobenzenesulfinic acids determined in water at 298 K are
1.45, 1.55, and 1.15, respectively.[22] Moreover, the effective
formation of hydrogen bonds between solvents containing
Figure 1. Time dependence of the concentrations of benzenesulfinic
acid (1), p-toluenesulfinic acid (2), and p-chlorobenzenesulfinic acid (3) for
their interaction with 4-bromo-2-butenenitrile at 323 K
Scheme 1. Synthesis of substituted allyl phenyl sulfones
J. Phys. Org. Chem. 2010, 23 461–467
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G. KHAMIS, S. STOEVA AND D. ALEKSIEV
Figure 2. Time dependence of the concentrations of benzenesulfinic
acid (1), p-toluenesulfinic acid (2), and p-chlorobenzenesulfinic acid (3) for
their interaction with 1,3-dibromopropene at 323 K
Figure 3. Arrhenius plots for the interaction of benzenesulfinic acid (1),
p-toluenesulfinic acid (2), and p-chlorobenzenesulfinic (3) with
4-bromo-2-butenenitrile
suggested second-order kinetic equation adequately described
the experimental results obtained.
The values of the rate constants k, as a function of temperature
are presented in Table 2. The rate constants had relatively high
values, which is usually observed for SN2 reactions in allylic
systems.[39] Moreover, the temperature elevation of 108 within
the 308–333 K intervals resulted in a two- to threefold increase in
the corresponding k-values. This trend was more clearly
pronounced for those experimental series, in which nucleophiles
of lower reactivity such as p-chlorobenzenesulfinate ion had
participated. Such a trend was in agreement with the basic
mathematical formalism of the Arrhenius equation.[32]
Based on the Arrhenius plot of rate constants as shown in Figs 3
and 4 for the two experimental series, the values of the activation
energy EA and the pre-exponential factor (A) were determined
(Table 3). The corresponding values of the changes in entropy,
enthalpy, and free energy for the formation of the activated
complex from the reactants, participating in the SN2-reactions are
also listed in Table 3. The lowest values of these parameters were
observed for the reaction of the p-toluenesulfinate anion with
both the 4-bromo-2-butenenitrile and 1,3-dibromopropene.
Apparently, this resulted from the hyperconjugation effect,
involving both the methyl group and benzene nucleus, which led
to a more effective conjugation between the p-electrons of the
Figure 4. Arrhenius plots for the interaction of benzenesulfinic acid (1),
p-toluenesulfinic acid (2), and p-chlorobenzenesulfinic (3) with
1,3-dibromopropene
Table 2. Values of the rate constants k (L molꢁ1 minꢁ1) at different temperatures for the reaction of non-substituted and
p-substituted sodium arenesulfinates with 4-bromo-2-butenenitrile and 1,3-dibromopropene
Rate constants, k (L molꢁ1 minꢁ1
)
—
—
—
Br—CH —CH CH—C
—
—
—
Substrates
N
Br—CH —CH CH—Br
—
2
2
Nucleophiles
308 K
313 K
318 K
323 K
328 K
333 K
—
232.2 ꢀ 10.6
301.9 ꢀ 5.9
—
—
193.3 ꢀ 1.4
265.9 ꢀ 3.6
397.4 ꢀ 15.8
609.1 ꢀ 15.9
772.2 ꢀ 21.2
965.6 ꢀ 38.6
—
56.3 ꢀ 2.4
91.8 ꢀ 2.8
130.5 ꢀ 4.5
178.6 ꢀ 7.7
248.4 ꢀ 10.5
22.6 ꢀ 1.1
36.9 ꢀ 1.7
52.8 ꢀ 2.6
72.4 ꢀ 2.7
124.6 ꢀ 6.1
25.4 ꢀ 1.2
49.9 ꢀ 1.7
78.4 ꢀ 2.4
132.9 ꢀ 3.3
163.3 ꢀ 7.7
11.4 ꢀ 0.1
18.3 ꢀ 0.7
31.9 ꢀ 1.5
47.9 ꢀ 2.6
78.1 ꢀ 1.6
434.2 ꢀ 10.9
638.3 ꢀ 25.6
851.2 ꢀ 30.1
1084.1 ꢀ 28.1
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Copyright ß 2009 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2010, 23 461–467

REACTIVITY OF SODIUM ARENESULFINATES
Table 3. Basic kinetic parameters of the reaction of non-substituted and p-substituted sodium arenesulfinates with
4-bromo-2-butenenitrile and 1,3-dibromopropene
—
—
—
Br—CH —CH CH—C
—
—
Br—CH —CH CH—Br
—
Substrates
N
—
2
2
Nucleophiles
EA (kJ molꢁ1
)
63.09 ꢀ 0.02
2.01 ꢂ 1012
54.04 ꢀ 0.04
3.32 ꢂ 1011
70.77 ꢀ 0.02
81.45 ꢀ 0.06
1.11 ꢂ 1015
0.096 ꢀ 0.002
78.77 ꢀ 0.06
78.74 ꢀ 0.08
1.01
56.59 ꢀ 0.12
7.75 ꢂ 1011
84.94 ꢀ 0.01
1.65 ꢂ 1015
3.40 ꢀ 0.03
82.26 ꢀ 0.01
81.16 ꢀ 0.02
1.51
A (minꢁ1
)
1.48 ꢂ 1013
ꢁ35.81 ꢀ 0.14
68.09 ꢀ 0.02
79.66 ꢀ 0.12
1.35 ꢂ 10ꢁ2
¼
¼
¼
DS (J molꢁ1 Kꢁ1
)
ꢁ52.40 ꢀ 0.11
60.41 ꢀ 0.02
77.33 ꢀ 0.01
1.83 ꢂ 10ꢁ3
ꢁ67.32 ꢀ 0.10
51.40 ꢀ 0.04
72.80 ꢀ 0.17
3.05 ꢂ 10ꢁ4
ꢁ60.26 ꢀ 0.16
53.95 ꢀ 0.12
73.12 ꢀ 0.05
7.12 ꢂ 10ꢁ4
DH (kJ molꢁ1
DG (kJ molꢁ1
)
)
p
ring and sulfinate ions, and, also, enhanced their nucleophilicity.
The higher nucleophilicity of the sulfur atom in p-toluenesulfinate
anion was also confirmed by its higher donor superdelocaliz-
ability (SE(S) ¼ 0.2496 (a.u.)2/eV), as compared to that of benzene-
sulfinate and p-chlorobenzenesulfinate ions (Table 4). Among the
sulfinate ions explored, the weakest nucleophilic center was
established at the sulfur atom in sodium p-chlorobenzenesulfinate,
which was associated with the electron-withdrawing inductive
effect of chlorine atom, making the benzene ring ‘more positive’,[40]
and hampering the conjugation to the sulfonyl group.
Substituents such as the cyano group and bromine atom,
located at g-position toward the reaction center in the substrates
markedly influenced the reactivity of Callyl therein. As seen from
Table 2, the rate constant values for the interaction of both the
non-substituted and p-substituted arenesulfinates with 4-bromo-2-
butenenitrile were higher than those for the corresponding
interaction with 1,3-dibromopropene. This is apparently associ-
ated with the enhanced Callyl-type electrophilicity of 4-bromo-2-
butenenitrile under the influence of both the negative inductive
and resonance effects of the cyano group. Such an influence of
bromine, attached to vinyl group in 1,3-dibromopropene, was not
observed, since its negative inductive effect was, at least partially,
compensated by the positive resonance one, which led to a lower
electrophilicity of the reaction center (Callyl). The difference in the
substrate reactivity was particularly well pronounced in their
interaction with benzenesulfinate and p-chlorobenzenesulfinate
anions. In this case, the k-values for 4-bromo-2-butenenitrile were
1.50–2.24 times as high as those determined for 1,3-dibromopropene
as substrate.
The delocalization reactivity indexes of substrates (4-bromo-
2-butenenitrile and 1,3-dibromo-propene) are presented in
Table 5. Regardless of the fact that higher values of the acceptor
superdelocalizability of Callyl in 1,3-dibromopropene were
determined (SN(Callyl) varied from 0.2475 to 0.2612 (a.u.)2/eV),
compared to 4-bromo-2-butenenitrile (SN(Callyl) varied from
0.2374 to 0.2468 (a.u.)2/eV), the reactivity of the former chemical
was lower. Moreover, the atomic charges at the electrophilic site
(q(Callyl)) in these substrates differed slightly and could not
explain and define the differences in their reactivity. However, the
lower minimum and maximum values of the partial electron
density of Callyl at the lowest unoccupied molecular orbital
(LUMO) in 4-bromo-2-butenenitrile (fN(Callyl)), compared to those
in 1,3-dibromopropene (Table 5), obviously determined the
easier formation of its bond with sulfur in the corresponding
arenesulfinate ion. Therefore it could be assumed that the
interaction between sodium arenesulfinates and g-substituted
allyl bromides was more likely to be determined by orbital factors,
rather than by factors associated with the electronic charge. This
originated from the fact, that, in the activated complex, a bond
between the ‘soft’ nucleophilic center (S-atom in the reagents)
and the ‘soft’ electrophilic center (Callyl in substrates) was formed.
In such cases, the LUMO of the electrophilic center should
possess a relatively low energy.[33] On the other hand, the lower
Table 4. Reactivity indices of arenesulfinate ions
Arenesulfinate ions
Y-C6H4-SO2H
SE(S) (a.u.)2/eV)
q(S) (a.u.)
H
p-CH3
p-Cl
0.2444
0.2496
0.2419
1.4824
1.4768
1.4808
Table 5. Reactivity indices of 4-bromo-2-butenenitrile (1) and 1,3-dibromopropene (2)
SN(Callyl) ((a.u.)2/eV)
q(Callyl) (a.u.)
fN(Callyl) (eV)
No.
Compound
min.
max.
min.
max.
min.
max.
—
Br—CH —CH CH—CN
1
2
0.2374
0.2475
0.2468
0.2612
ꢁ0.2026
ꢁ0.1946
ꢁ0.2132
ꢁ0.2042
ꢁ0.0273
ꢁ0.0422
ꢁ0.3263
ꢁ0.4335
—
—
2
Br—CH —CH CH—Br
—
2
J. Phys. Org. Chem. 2010, 23 461–467
Copyright ß 2009 John Wiley & Sons, Ltd.
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G. KHAMIS, S. STOEVA AND D. ALEKSIEV
Scheme 2. Types of activated complexes formed in the interaction of arenesulfinate ions with 4-bromo-2-butenenitrile (A) and 1,3-dibromopropene (B)
enthalpies of activation for the interaction of arenesulfinate ions
¼
The steric factor p values correlated with the entropy changes
during the formation of the activated complex (transition state).
The lower negative DS -values, and, the lower p-values suggest
with 4-bromo-2-butenenitrile (DH ¼ 51.40–68.09 kJ molꢁ1), compared
¼
¼
to those with 1,3-dibromopropene (DH ¼ 53.95–82.26 kJ molꢁ1
)
(Table 3) indicated the higher stability of the activated complexes of
the SN2-reaction with the participation of the former substrate
(Scheme 2).
the rapid formation of the activated complex between reactants.
This is particularly well pronounced for the interaction of the
p-toluenesulfinate anion with both the 4-bromo-2-butenenitrile
and 1,3-dibromopropene. Based on the p-values shown in
Table 3, it could be assumed that, with the interactions of any of
the reagents with the corresponding substrates, the electro-
philicity of the reaction center (Callyl) in the latter plays a
determining role.
In this particular case, the stability of the complex A was not
only determined by the possibility of conjugation of the allylic
p-electron system with the p-orbital of the reaction center
(Callyl),[39,43] but, also, by its conjugation with the p-electrons of
the cyano group.
¼
¼
In the course of interaction of benzenesulfinate and
p-chlorobenzenesulfinate ions with 1,3-dibromopropene as a
substrate of reduced electrophilicity at the reaction center (Callyl),
an increase in the entropy of activation (slightly positive values)
was observed (Table 3). A decrease in the electrophilicity of Callyl
in 1,3-dibromopropene favors the formation of a late transition
state with these neutral S-nucleophiles, associated with a long
S—C bond. The change in the entropy of activation in that case
could be related to the change in the transition state of the
SN2-reactions studied,[44] more particularly, from the early
formation of the transition state with entropy-controlled reacti-
vity to the late formation of transition state with enthalpy-
controlled reactivity.
Plots of DH versus DS for the two experimental series gave
straight lines (Fig. 5). Hence, the reactions of arenesulfinates with
4-bromo-2-butenenitrile (line I) and 1,3-dibromopropene (line II)
were characterized by the isokinetic relationship or kinetic
compensation effect.[44] The values of the corresponding
isokinetic temperatures b(I) ¼ 528 K and b(II) ¼ 430 K were higher
than the experimental temperature range (308–333 K).
CONCLUSIONS
The kinetics of the nucleophilic substitution reaction of sodium
arenesulfinates with g-functionalized bromides was studied. The
principal kinetic and thermodynamic parameters associated with
¼
¼
¼
these reactions such as k, EA, A, p, DH , DS
DG were
determined. On the basis of their values, it can be assumed that
sodium p-toluenesulfinate had the highest reactivity for the
interaction with any of the substituted allyl bromides. On the
other hand, bromine atom bound to Callyl in
4-bromo-2-butenenitrile was replaced by the corresponding
arenesulfinate anion at a reaction rate, about twice as high as that
associated with the similarly bound bromine atom in
1,3-dibromopropene. Juxtaposition of the data from the kinetic
studies, obtained with the involvement of some local electron
reactivity indexes of sodium arenesulfinates and g-substituted
allylbromides was also made.
Acknowledgements
¼
¼
Figure 5. Compensation plots (DS vs. DH ) for the interaction of
p-toluenesulfinic acid, benzenesulfinic acid, and p-chlorobenzenesulfinic
acid with 4-bromo-2-butenenitrile (line I, points 1, 2, 3) and
1,3-dibromopropene (line II, points 10, 20, 30)
The authors thank Professor O. Mekenyan (Head of the Labora-
tory of Mathematical Chemistry, Bourgas Assen Zlatarov Univer-
sity) for providing the opportunity to apply the MOPAC 93
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J. Phys. Org. Chem. 2010, 23 461–467

REACTIVITY OF SODIUM ARENESULFINATES
software for evaluating the reactivity of the compounds under
study.
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