Thermal reactions of chloroarenes with hydrogen sulfide in the presence of methanol

Article abstract of DOI:10.1007/s11178-006-0009-9

Gas-phase reactions of chloroarenes (ClC₆H₄X, X = H, 4-CH₃, 4-OH, 4-Cl, 4-CF₃) with hydrogen sulfide or its precursors were investigated in the presence of methanol, which was a stronger H-donor than hydrogen sulfide. Introducing methanol increased the selectivity of arenethiols formation at X = H and 4-CH₃ and did not affect the reaction selectivity at acceptor X. The efficiency of methanol influence was considered from the viewpoint of free-radical reaction mechanism and the stability of the arenethiyl radicals. 2005 Pleiades Publishing, Inc.

Full text of DOI:10.1007/s11178-006-0009-9

Russian Journal of Organic Chemistry, Vol. 41, No. 11, 2005, pp. 1624-1630. Translated from Zhurnal Organicheskoi Khimii, Vol. 41, No. 11, 2005,
pp. 1658-1664.
Original Russian Text Copyright Ó 2005 by Deryagina, Sukhomasova, Levanova, Papernaya, Korchevin.
.
Thermal Reactions of Chloroarenes with Hydrogen Sulfide
in the Presence of Methanol
E.N. Deryagina, E.N. Sukhomasova, E.P. Levanova, L.K. Papernaya, and N.A. Korchevin
Faworsky Irkutsk Institute of Chemistry, Siberian Division, Russian Academy of Sciences, Irkutsk, 664033 Russia
e-mail: papern@irioch.irk.ru
Received November 30, 2004
Abstract—Gas-phase reactions of chloroarenes (ClC6H4X, X = H, 4-CH3, 4-OH, 4-Cl, 4-CF3) with hydrogen sulfide
or its precursors were investigated in the presence of methanol, which was a stronger H-donor than hydrogen
sulfide. Introducing methanol increased the selectivity of arenethiols formation at X = H and 4-CH3 and did not
affect the reaction selectivity at acceptor X. The efficiency of methanol influence was considered from the
viewpoint of free-radical reaction mechanism and the stability of the arenethiyl radicals.
hydrogen sulfide significantly raised the selectivity of the
2-thiophenethiol formation.
High-temperature (500–700°C) gas-phase reactions
of haloarenes with hydrogen sulfide provide a simple one-
stage procedure for preparation of the corresponding
arenethiols and diaryl sulfides [1]. The target products in
these processes are thiols, and the corresponding diaryl
sulfides, products of haloarenes reduction (benzene and
its derivatives), and fusion products (dibenzothiophene)
are regarded as side products. Variation of temperature
and the contact time of reagents in the reaction zone,
and also the use instead of the hydrogen sulfide of its
simple organic derivatives (providing the hydrogen sulfide
through thermal decomposition) frequently make it
possible to improve the selectivity of the process of thiol
formation. The study of the kinetic characteristics of the
reactions of formation for thiol and sulfide revealed that
the factor governing the rates of these processes was
the stability of the corresponding thiyl radicals controlling
their concentration in the reaction zone [2, 3]. However
the change in the concentration of the arenethiyl radicals
can be achieved by introducing into the system additional
reagent (hydrogen donors). Methanol can play the part
of an efficient hydrogen donor transforming into a stable
formaldehyde molecule.
In extension of the studies on the effect of the proton
donors on the direction of reactions involving thiyl radicals
we investigated gas-phase reactions of various chloro-
arenes Ia–Ie with the hydrogen sulfide in the presence
of methanol additive. In reactions with diethyl disulfide
(precursor of the hydrogen sulfide) only chloroarenes Ia
and Ib were involved. The products of these reactions
were arenethiols IIa–IIe, diaryl sulfides IIIa–IIIe, and
the corresponding reduction products IVa–IVe. Their
formation occurred along the following scheme.
(1)
(2)
(3)
X = H (a), CH3 (b), OH (c), Cl (d), CF3 (e).
At X = H dibenzothiophene (V) also occurred in the
reaction products. Applying diethyl disulfide resulted in
formation of its pyrolysis products (mainly thiophene, and
also thienothiophene and carbon disulfide). We previously
considered their formation paths [5, 6], and their overall
yield amounted to 5–10% with respect to the initial diethyl
disulfide. Besides the condensation of chlorobenzene with
ethenethiol generated by the thermolysis of the diethyl
disulfide [7] led to formation of benzothiophene in this
.
RS is thiyl radical.
We showed formerly [4] that methanol introduction
into the gas-phase reaction of 2-chlorothiophene with the
1070-4280/05/4111-1624Ó2005 Pleiades Publishing, Inc.

THERMAL REACTIONS OF CHLOROARENES WITH HYDROGEN SULFIDE
1625
process with the yield not exceeding 5% with respect to
chlorobenzene (Ia) introduced into the reaction.
selectivity of thiol IIa at 600–650°C was high (Table 1).
The side products in this reaction are thiophene,
benzothiophene, and thienothiophene. Bringing methanol
into this system also increased the selectivity of thiol IIa
formation. The best results were obtained at the use of
15 (600–630°C) and 25 mol % of methanol (600°C).
Therewith the yield of the side products was reduced.
Applying 50–75 mol% of methanol raised the selectivity
of thiophenol (IIa) formation at 630–650°C, but here the
yield of the side reaction products grew.
Thermal thiylation of chloroarenes was studied in
a flow system. A mixture of chloroarene and methanol
(10, 15, 25, 40, 50, and 75 mol % of methanol with respect
to chloroarene) was brought into the reaction zone in
a flow of hydrogen sulfide. The mixture of chloroarene
with methanol and diethyl disulfide was brought into the
reactor in a nitrogen flow. The reactions in the presence
of methanol were compared with the processes without
it. The reaction conditions and the yields of the products
in reactions with compounds Ia and Ib are presented in
Tables 1 and 2.
4-Methylchlorobenzene (Ib) reacted with the hydrogen
sulfide at 620–640°C providing 4-methylthiophenol (IIb)
and bis(4-methylphenyl) sulfide (IIIb). Therewith the
yield of thiol IIb was less or equal to that of sulfide IIIb
[17.6/29.6 (620°C) and 18.1/15.3% (640°C)] (Table 2).
Thus the reaction was not selective with respect to thiol.
Introduction of methanol favors the increase in selectivity
of thiol IIb formation: at the methanol concentration
20 mol % IIb/IIIb = 47.3/18.0 (630°C), at 25 mol% 48.5/
31.6 (620°C), and at 50 mol% 45.7/22.9% (640°C).
Simultaneously the yield of toluene (IVb) decreased, and
the other side products did not form at all.
The reactions of chloroarenes with hydrogen sulfide
(or diethyl disulfide) in the presence of methanol start at
higher temperature. This phenomenon is caused by
capturing of radicals initiating thermal reactions by
methanol as efficient hydrogen donor.
R is any radical formed at the thermal degradation of
molecules.
Thiol IIb formed selectively in reaction of 4-methyl-
chlorobenzene (Ib) with diethyl disulfide. The yield of
thiol IIb at 600–640°C is approximately two times larger
that the yield of sulfide IIIb (50 and 30% respectively)
(Table 2). Therewith the yield of toluene (IVb) did not
exceed 19%, but the other side products appeared (thio-
phene and thienothiophene). The reaction of 4-methyl-
chlorobenzene (Ib) with diethyl disulfide in the presence
of methanol in amount of 15, 25, or 50 mol% occurred
with greater selectivity with respect to thiol IIb. The best
results were obtained at the use of 50 mol% of methanol.
At 620°C the yields of compounds IIb and IIIb equal to
68.3 and 14.6%, and at 630°C to 64.0 and 17.7%
respectively. Simultaneously the yield of side products
toluene (IVb) and thiophene decreased, and the others
did not form.
However when the concentration of the initiating
radicals reaches a sufficient level begin thermal reactions
of chloroarenes thiylation. Therewith the methanol
addition in some cases significantly affects the selectivity
of thiols IIa and IIb formation.
In the absence of methanol chlorobenzene (Ia) the
most efficiently reacted with the hydrogen sulfide at 620
and 650°C.At 620°C thiophenol (IIa) and diphenyl sulfide
(IIIa) formed in equal amount [30.8% calculated on the
reacted chlorobenzene (Ia)], and at 650°C the yield of
thiol IIa exceeded that of sulfide IIIa (27.1 and 7.4%
respectively), but a considerable quantity of side products
arose under these conditions: benzene (IVa) (yield up to
39%) and dibenzothiophene (V) (3–4%) resulting in a
high conversion of chlorobenzene (Ia) (Table 1). Bringing
into the system methanol in amount of 15 mol % resulted
in a sharp increase in the selectivity of thiol IIa formation
at 650°C : the yield of thiol IIa was 55.4%, and sulfide
IIIa 19.2% with respect to the reacted chlorobenzene
(Ia). Therewith the yield of benzene (IVa) fell to 5%,
and dibenzothiophene (V) did not form at all. At 620°C
the selectivity of thiol IIa formation is also high, but the
conversion of chlorobenzene remained low. The other
methanol concentrations are less efficient.
4-Hydroxythiophenol (IIc) is an interesting monomer
and semiproduct of the organic synthesis. The direct gas-
phase reaction of 4-chlorophenol (Ic) with the hydrogen
sulfide at 540–560°C did not yield thiol IIc selectively
for it involved considerable reduction of the initial
compound into phenol (IVc). The yield of phenol (IVc)
was always than that of thiol IIc (58/21 at 540°C and 49/
43% at 560°C). Therewith the conversion of initial
compound Ic was relatively low (up to 32%). The
introduction of methanol into this reaction in amount of
25 mol% provided a slight effect at 580°C. Simultaneous-
The reaction of chlorobenzene (Ia) with the diethyl
disulfide also afforded thiol IIa and sulfide IIIa, and the
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DERYAGINAet al.
1626
Table 1. Gas-phase reaction of chlorobenzene (Ia) with hydrogen sulfide (runs nos. 1–20) and diethyl disulfide Et2S2 (runs nos.
21–40) in the presence of methanol. Molar ratio (Ia) : H2S = 1:2.5; reagents contact time 35–40 s. Molar ratio (Ia): Et2S2 = 1:2;
reagents contact time 50–55 s
RUSSIAN JOURNALOF ORGANIC CHEMISTRY Vol. 41 No. 11 2005

THERMAL REACTIONS OF CHLOROARENES WITH HYDROGEN SULFIDE
1627
Table 2. Gas-phase reaction of 4-methylchlorobenzene (Ib) with hydrogen sulfide (runs nos. 1–17) and diethyl disulfide Et2S2
(runs nos. 18–40) in the presence of methanol. Molar ratio (Ib): H2S = 1:2.5; reagents contact time 40–45 s. Molar ratio (Ib):
Et2S2 = 1:2; reagents contact time 45–50 s
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DERYAGINAet al.
1628
ly the conversion of 4-chlorophenol (Ic) grew (37%),
the yield of thiol IIc increased (58.3%), and the yield of
phenol (IVc) decreased (32%). The corresponding bis(4-
hydroxyphenyl) sulfide (IIIc) formed in neither case. The
effect from methanol addition in amount of 15 and
50 mol% is even less pronounced.
sulfide IIIe was 14%. Bringing the methanol into the
system in amount of 25 mol% only slightly improved the
selectivity of thiol IIe formation at 640°C (yield of
compound IIe 31.4, that of IIIe 15.1%] and reduced the
yield of the reduction product IVeto 19.4%.
The results obtained are in agreement with the stability
of arenethiyl radicals whose stability we have
quantitatively estimated before [2]. The formation of thiols
II and diaryl sulfides III by reactions (1) and (2) occurs
involving thiyl radicals HS· and RC6H4S· in substitution of
the chlorine in the aromatic ring [1].
The reaction of 1,4-dichlorobenzene (Id) with
hydrogen sulfide occurred at 560–580°C to a low conver-
sion (25–30%). Yields of 4-chlorothiophenol (IId) are
somewhat lower or equal to the yields of bis(4-
chlorophenyl) sulfide (IIId) (32–42.3/45–37.5%). The
addition of methanol at any concentration practically did
not favor increasing the conversion of initial compound
Id and selectivity with respect to thiol IId. The best result
was obtained at 590°C using 25 mol% of methanol (IId/
IIId = 45.3/38.3%, conversion of compound Id 34%).
The rate of reaction (7) and consequently the selectivity
of thiols II formation depend on the concentration of the
radicals RC6H4S·. When the only H-donor in the system
is the hydrogen sulfide the less stable arenethiyl radicals
generated from the chloro derivative and the hydrogen
sulfide further react with it faster than with the initial
chloro derivative, and with the more stable radicals the
opposite is valid. In the presence of stronger H-donors
than the hydrogen sulfide the thiyl radicals are faster
converted into the hydrogen sulfide and the corresponding
thiol.
For the sake of comparison we studied the gas-phase
reaction of 2,5-dichlorothiophene with hydrogen sulfide.
This reaction proceeded to a low conversion with strong
tarring at 490–510°C giving as the main product of sul-
furization bis(5-chlorothienyl) sulfide, and the expected
5-chloro-2-thiophenethiol did not form at all. 2,5-Dichloro-
thiophene to a large extent was reduced into 2-chloro-
thiophene that was converted into 2-thiophenethiol and
bis(2-thienyl)sulfide. The introduction of methanol into
this reaction did not favor formation of 5-chloro-2-
thiophenethiol but accelerated the process as a whole.
Therewith grew the yields of all reaction products, both
the main one and the side compounds.
The high concentration of the hydrogen sulfide in the
reaction zone (the ratio chloroarene: H2S = 1:2–2.5)
ensures sufficiently high concentration of HS· radicals
notwithstanding their consumption by reaction (8).
However at the high methanol amount reaction (8) leads
to decrease in the concentration of HS· radicals, and the
selectivity of thiol formation diminishes. Therefore an
optimum range of methanol addition exists where the
selectivity of thiol formation increases (15–25 mol% with
respect to chloroarene).
4-Trifluoromethylchlorobenzene in the absence of
methanol efficiently reacted with hydrogen sulfide
(conversion up to 70%) in a narrow temperature range
(610–620°C). The yield of 4-trifluoromethylthiophenol
(IIe) is greater than that of bis(4-trifluoromethylphenyl)
sulfide (IIIe) [25.8/14.3 (610°C) and 17.6/10.1%
(620°C)]. However the process gave rise to a consider-
able amount of the reduction product, trifluoro-
methylbenzene (IVe) (29.6–45.2%). The decrease in the
reaction temperature by 10°C resulted in a significant
reduction in conversion (to 20%). Interestingly, at this
temperature no thiol IIe was obtained, and the yield of
(8)
(9)
The improvement of the process selectivity for thiols
IIa and IIb originates apparently from the fact that the
methanol, being more efficient H-donor than the hydrogen
sulfide, considerably reduces the concentration of radicals
RC6H4S· by reaction (9) and decelerates reaction (7).
Arenethiyl radicals destabilized by the acceptor
substituents (X = OH, Cl, CF3) react with chloroarene
along scheme (7) at a high rate [2], and therefore here
the methanol addition virtually does not affect the
selectivity of the process.
(4)
(5)
(6)
(7)
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THERMAL REACTIONS OF CHLOROARENES WITH HYDROGEN SULFIDE
1629
It was established that methanol addition did not
considerably increase the rate of reduction of initial
chloroarenes into arenes IV. This fact suggests that
reaction (3) apparently occurs due to the halophilic attack
of thiyl radicals on the chlorine atom.
Reaction of chlorobenzenea (Ia) with diethyl
disulfide in the presence of methanol (Table 1, run
no. 26). Into a reactor at 630°C was charged a mixture
of 0.97 g (8.5 mmol) of chlorobenzene (Ia), 2.11 g
(17 mmol) of diethyl disulfide, and 0.041 g (1.3 mmol) of
methanol at a rate of 2 ml/h in a flow of nitrogen
(1.2 l/h). Contact time 52 s. In 90 min we obtained at the
output of the reactor 0.9 g of liquid condensate containing
according to GLC 0.28 g of thiophenol (IIa), 0.078 g of
diphenyl sulfide (IIIa), 0.06 g of benzene (IVa), 0.018 g
of benzothiophene, 0.001 g of carbon disulfide, 0.02 g of
thiophene, 0.013 g of thienothiophene, and 0.43 g of
unreacted initial chlorobenzene (Ia).
(10)
(11)
Inasmuch as the presence of methanol decreases the
thiyl radicals concentration, reaction (10) should not
significantly accelerate.
The results obtained permit making an improvement
in the preparative procedure of thiophenol and 4-methyl-
thiophenol synthesis from the corresponding chloroarenes
and hydrogen sulfide.
Reaction of 4-methylchlorobenzene (Ib) with
hydrogen sulfide in the presence of methanol
(Table 2, run no. 4). Into a reactor at 630°C was charged
a mixture of 1.12 g (8.9 mmol) of 4-methylchlorobenzene
(Ib) and 0.057 g (1.8 mmol) of methanol at a rate of
3 ml/h in a flow of hydrogen sulfide (1.4 l/h). Contact
time 42 s. In 21 min we obtained at the output of the
reactor 0.99 g of liquid condensate containing according
to GLC 0.31 g of 4-methylthiophenol (IIb), 0.11 g of bis-
(4-methylphenyl) sulfide (IIIb), 0.14 g of toluene (IVb),
and 0.43 g of unreacted 4-methyl-chlorobenzene (Ib).
EXPERIMENTAL
Initial reagents and condensates formed in the course
of reaction were analyzed by GLC on a chromato-graph
LKhM 80-MD-2, column 2000´3 mm, liquid phase XE
60 (5%), oven temperature linearly programmed at a
rate 12 deg/min, carrier gas helium.
Thermal reactions were carried out in a flow system
at atmospheric pressure in a hollow quartz pipe of 19 mm
diameter. The length of the reaction zone was 250 mm.
The quartz pipe was placed into a pipe electric heater
where the temperature was automatically controlled. The
initial liquid and crystalline reagents were charged into
the reaction zone with an automatic syringe batcher;
therewith the crystalline reagents were preliminary melted
in the batcher heated with a nichrome coil. The velocity
of gas (nitrogen or hydrogen sulfide) flow was manually
controlled with the help of rheometers. Reagents contact
time was estimated from the volume of the reaction zone
and supply rate of all reagents in gas state at the reaction
temperature.
Reaction of 4-methylchlorobenzene (Ib) with
diethyl disulfide in the presence of methanol
(Table 2, run no. 37). Into a reactor at 630°C was charg-
ed a mixture of 0.37 g (2.9 mmol) of 4-methylchloro-
benzene (Ib), 0.7 g (5.7 mmol) of diethyl disulfide, and
0.046 g (1.4 mmol) of methanola at a rate of 3 ml/h in
a flow of nitrogen (1.2 l/h). Contact time 48 s. In 21 min
we obtained at the output of the reactor 0.36 g of liquid
condensate containing according to GLC 0.09 g of
4-methylthiophenol (IIb), 0.022 g of bis(4-methylphenyl)
sulfide (IIIb), 0.013 g of toluene (IVb), 0.013 g of carbon
disulfide, 0.022 g of thiophene, and 0.2 g of unreacted
4-methylchlorobenzene (Ib).
Reaction of chlorobenzene (Ia) with hydrogen
sulfide in the presence of methanol (Table 1, run
no. 8). Into a reactor preliminary flushed with nitrogen
was charged at 650°C a mixture of 1.60 g (14 mmol) of
chlorobenzene (Ia) and 0.067 g (2 mmol) of methanol at
a rate of 3 ml/h in a flow of hydrogen sulfide (1.6 l/h).
Contact time 35 s. In 29 min we obtained at the output of
the reactor 1.37 g of liquid condensate containing
according to GLC 0.51 g of thiophenol (IIa), 0.15 g of
diphenyl sulfide (IIIa), 0.05 g of benzene (IVa), and
0.66 g of unreacted initial chlorobenzene (Ia).
Gas-phase reactions of chloroarenes Ic–Ie in the
absence and in the presence of methanol were carried
out in a similar way.
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