Sonication effect on the reaction of 4-bromo-1-methylbenzene with sodium sulfide in liquid-liquid multi-site phase-transfer catalysis condition - Kinetic study

Article abstract of DOI:10.1016/j.ultsonch.2014.08.006

The synthesis of di-p-tolylsulfane from the reaction of 4-bromo-1-methylbenzene (BMB) with sodium sulfide was carried out using a multi-site phase-transfer catalyst (MPTC) viz., 1,4-dihexyl-1,4-diazoniabicyclo[2.2.2]octanium dibromide and ultrasonic irradiation in a liquid-liquid reaction condition. The overall reaction rate is greatly enhanced when catalyzed by multi-site phase-transfer catalyst (MPTC) combined with sonication (40 kHz, 300 W) in a batch reactor than catalyzed by MPTC without sonication. Effects on the reaction due to various operating conditions, such as agitation speed, different ultrasound frequencies, different phase-transfer catalysts, different organic solvents, the amount of MPTC, temperature, amount of sodium sulfide, effect of sodium hydroxide, volume of n-hexane and the concentration of 4-bromo-1-methylbenzene. The reaction obeys a pseudo first-order rate law and a suitable mechanism was proposed based on the experimental observation.

Full text of DOI:10.1016/j.ultsonch.2014.08.006

Accepted Manuscript
Sonication effect on the reaction of 4-bromo-1-methylbenzene with sodium sul-
fide in liquid-liquid multi-site phase-transfer catalysis condition - kinetic study
Pachaiyappan Abimannan, Varathan Selvaraj, Venugopal Rajendran
PII:
S1350-4177(14)00265-X
DOI:
http://dx.doi.org/10.1016/j.ultsonch.2014.08.006
ULTSON 2675
Reference:
To appear in:
Ultrasonics Sonochemistry
Received Date:
Revised Date:
Accepted Date:
25 June 2014
10 August 2014
12 August 2014
Please cite this article as: P. Abimannan, V. Selvaraj, V. Rajendran, Sonication effect on the reaction of 4-bromo-1-
methylbenzene with sodium sulfide in liquid-liquid multi-site phase-transfer catalysis condition - kinetic study,
Ultrasonics Sonochemistry (2014), doi: http://dx.doi.org/10.1016/j.ultsonch.2014.08.006
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Manuscript
Sonication effect on the reaction of 4-bromo-1-methylbenzene with sodium sulfide in
liquid-liquid multi-site phase-transfer catalysis condition - kinetic study
By
a
Pachaiyappan Abimannan & Varathan Selvaraj
a
&
,a
Venugopal Rajendran*
,a
*
Head of the Department of Chemistry,
Pachaiyappa’s College for Men,
Kanchipuram, Tamil Nadu,
India-631 501.
E-mail: rajendranv1967@gmail.com; venugopal.rajendran@ymail.com;
abimannan01@gmail.com; selvarajptc@gmail.com
Fax No: +91-44-27268824
1

Abstract
The synthesis of di-p-tolylsulfane from the reaction of 4-bromo-1-methylbenzene
BMB) with sodium sulfide was carried out using a multi-site phase-transfer catalyst
MPTC) viz., 1,4-dihexyl-1,4-diazoniabicyclo[2.2.2]octanium dibromide and ultrasonic
(
(
irradiation in a liquid-liquid reaction condition. The overall reaction rate is greatly
enhanced when catalyzed by multi-site phase-transfer catalyst (MPTC) combined with
sonication (40 kHz, 300W) in a batch reactor than catalyzed by MPTC without
sonication. Effects on the reaction due to various operating conditions, such as agitation
speed, different ultrasound frequencies, different phase-transfer catalysts, different
organic solvents, the amount of MPTC, temperature, amount of sodium sulfide, effect of
sodium hydroxide, volume of n-hexane and the concentration of 4-bromo-1-
methylbenzene. The reaction obeys a pseudo first-order rate law and a suitable
mechanism was proposed based on the experimental observation.
Keywords
Ultrasound irradiation; multi-site phase-transfer catalysis; sodium sulfide; 4-
bromo-1-methylbenzene.
1
. Introduction
The reaction between two reactants in two different phases are usually very
difficult to carry out. When applying a protic solvent in a severe condition for such a
system, the conversion can be enhanced but generally accompanied with some
unfavorable side-reactions. Such drawbacks can be overcome by using phase-transfer
catalysis. The advantages of applying phase-transfer catalysts are to enhance the reaction
rates and higher product selectivity under very mild reaction conditions. Nowadays,
2

phase-transfer catalysis is an inevitable potential tool for enhancing the efficiency,
improving safety and reducing environmental impact. The advances of the same in the
recent years have made a remarkable impact in the organic synthesis and are being
enormously employed to a multitude of organic transformations such as substitution,
displacement, condensation, epoxidation and polymerization [1]. Numerous reactions
have been investigated to explore the mechanisms and kinetics of phase-transfer catalysis
especially for liquid-liquid system [1-3]. Nowadays, phase-transfer catalysis is widely
applied in manufacturing pharmaceuticals, agricultural chemicals, perfumes, flavors, dyes
and specialty polymers, and is extended to the pollution and environmental control
processes.
Thioethers, including di-p-tolyl sulfide, which have been widely employed as
perfume additives [4] and inverse phase-transfer catalysts [5-13]. Organosulfur
compounds are used in pharmaceuticals, dyestuffs and agrochemicals. Many drugs
contain sulfur, early examples being antibacterial sulfonamides known as sulfa drugs.
Sulfur is a part of many bacterial defense molecules. Most β-lactam antibiotics, including
the penicillins, cephalosporins and monolactams contain sulfur [14]. Organosulfur
compounds are responsible for some of the unpleasant odors of decaying organic matter.
The main features of phase-transfer catalysis are that a highly active intermediate
would be form and transported between the phases to conduct the intrinsic reaction [15-
1
7]. Recently, in phase-transfer catalysis, multi-site phase transfer catalyst shows
significant enhancement in the reaction rate [15-22]. The main advantage of multi-site
phase-transfer catalyst is, it has more than one active site and has the ability to transfer
+
more number of anionic species (M Y ) from aqueous phase to organic phase per cycle
−
3

+
−
in contrast to single-site PTC which transfer only one anionic species (M Y ) per cycle.
In our laboratory too, several multi-site phase transfer catalysts are synthesized and used
for various reactions [16-22].
Ultrasound-assisted organic synthesis as a green synthetic approach is a powerful
technique that is being more and more accelerate organic reactions [23-25]. The
conventional methods suffer from some drawbacks, such as prolonged reaction times,
high temperatures and decomposition of phase-transfer catalyst due to instability at
higher temperature. The use of ultrasound minimize those drawbacks i.e. shorten the
reaction time and avoid use of high temperatures [26]. Thus, the development of a
simple, efficient and green method for the synthesis of di-p-tolylsulfane is our destination
in this work.
Ultrasonic irradiation in bi-phase reaction can increase interfacial area coupled
with local hot-spot generation, and has been demonstrated to promote high reaction rate
in organic synthesis [27-31]. Ultrasound irradiation combined with liquid–liquid phase-
transfer catalyst (LL-PTC) has revealed significant improvement in the reaction rate. But
the application of ultrasound in LL-PTC especially catalyzed by multi-site phase-transfer
catalyst was rarely reported.
Our objective was to develop the process and explore the role of multi-site phase-
transfer catalyst (MPTC) in the di-p-tolylsulfane synthesis using a green non-polar
solvent like n-hexane which is easier to remove under vacuum evaporation or water bath
distillation. The enhancement of apparent reaction rate or conversion under the ultrasonic
irradiation combined with multi-site phase-transfer catalyst (MPTC) was also explored.
We have also studied the kinetic aspects such as, the effect of the agitation speed, the
4

different kind of ultrasound frequencies, the volume of organic solvent, the amount of
multi-site phase-transfer catalyst, kinds of phase-transfer catalyst, kinds of organic
solvent, the amount of sodium sulfide, the volume of n-hexane, the amount of sodium
hydroxide and temperature. A rational mechanism was proposed based on the
experimental data. A kinetic model was built, in which a pseudo-first order rate law is
applied to find the rate constant and the activation energies of the reaction.
In the past, the application of ultrasound in heterogeneous liquid-liquid phase-
transfer catalysis especially multi-site phase-transfer catalyzed reactions was rarely
studied. To the best of our knowledge, there is no literature report available on the
kinetics of synthesis of di-p-tolylsulfane catalyzed by a new multi-site phase-transfer
catalyst under ultrasonic irradiation.
2
2
. Experimental
.1. Chemicals, catalysts and solvents
All the reagents, including sodium sulfide, 4-bromo-1-methylbenzene, DABCO
(
1,4-diazabicyclo[2.2.2]octane), n-hexyl bromide,
bromide (TBAB), tetrabutylammonium chloride (TBAC), tetrabutylammonium iodide
TBAI), tetrahexylammonium bromide (THAB), tetraoctylammonium bromide (TOAB),
biphenyl, tetrabutylammonium
(
n-hexane, toluene, chlorobenzene, anisole and other reagents for synthesis were
guaranteed grade (GR) chemicals and were used without further purification.
5

2
.2. Instrumentation
1
13
H NMR and C spectra were recorded on a Bruker 300 MHz and 75 MHz
respective using TMS as an internal standard. Gas chromatography was carried out using
a GC-Varian 3700 model. Ultrasonic water bath, Equitron, Media Instrument
Manufacturing Company, Chennai, India-600 004. The ultrasonic generator was a
thermostatic bath equipped with dual frequencies (28/40 kHz) and electric power 300 W
with 0.0126 W/mL of power density.
2
.3. Ultrasonic process equipment
Ultrasonic energy is transmitted to the process vessel through the liquid medium,
usually water in the tank. For safety purpose, the sonochemical reactor consisted of two
layers stainless steel body. The sonochemical reactor configuration used in the present
work is basically an ultrasonic bath. The internal dimension of the ultrasonic cleaner tank
is 48 cm x 28 cm x 20 cm with liquid holding capacity of 5 liters. Two types of
frequencies of ultrasound were used in these experiments, which are 28 kHz and 40 kHz
with each output as 300 W. Both ultrasounds separately produces through a flat
transducer mounted at the bottom of the sonicator. The reactor was a 250 mL three-
necked Pyrex round-bottom flask. This reaction vessel was supported at the centre of the
ultrasonic cleaning bath 2 cm above from the position of the transducer to get the
maximum ultrasound energy. All the experimental parameters were done at 40 kHz with
output power of 300 W.
3
.1 Synthesis of a new dual-site phase-transfer catalyst (MPTC)
A mixture of 1,4-diazabicyclo[2.2.2]octane (DABCO, 5g, 44.58 mmol), n-hexyl
bromide (18g, 15.3 mL, 109.04 mmol) and 80 mL of ethanol was placed in a 250 mL
6

three necked round bottomed Pyrex flask. The reaction was carried out at 60°C for 24
hours and was gently refluxed in the nitrogen atmosphere. The solvent was then
completely removed under vacuum and onium salt, i.e., 1,4-dihexyl-1,4-
diazoniabicyclo[2.2.2]octanium dibromide, (MPTC; Scheme 1) was washed with n-
hexane (4 x 25 mL). The Colorless jelly liquid was stored in CaCl
2
desiccators. Yield:
1
2%; H NMR (300 MHZ,
9
1
3
D
2
O); δ. 0.818-0.926 (t, 3H -CH
3
in hexyl group), 1.090-
+
in hexyl group), 1.708-1.811 (m, 2H, N -CH
.110(d, 6H -CH
2
2
-CH
2
in hexyl group),
+
.388-3.444(t, 2H, N -CH
+
13
2
hexyl group) 3.966 (s, 6H, the cyclic N -CH
), 18.94 (-CH –CH
- CH in hexyl group carbon) 31.06(-CH
2
), C NMR (75
MH
hexyl group carbon), 26.22(N - CH
CH - in hexyl group carbon), 51.16 (N - CH
Z
, D
2
O): δ 12.90(-CH
3
2
3
in hexyl carbon), 23.52(-CH
2
-CH
2
–CH
3
in
+
2
2
2
2 2
-CH -CH –
+
+
in hexyl group carbon), 65.13 ( N -CH
3
2
2
cyclic Carbon); Elemental analysis: Calculated: C, 48.91%; H, 8.68%; Br, 36.15%; N,
6
.35%; and Found: C, 48.88%; H, 8.66%; Br, 36.13%; N, 6.33%.
3
.2 Synthesis of di-p-tolylsulfane
2
To the well powdered Na S (5g), 6 mL water was added under overhead stirring
for few minutes to generate the sulfur anion. Then the 4-bromo-1-methylbenzene (2g,
1
1.69 mmol) and the synthesized MPTC (0.2 g) in n-hexane (30 mL) were added slowly.
o
The reaction mixture was heated at 60 C for 2 hours with vigorous stirring. The progress
of the reaction and end of the reaction was evidenced by thin layer chromatography
(
TLC). The product was isolated by simple extraction with n-hexane (3 x 25 mL). The
organic layer was collected and the solvent was evaporated under reduced pressure. The
product was obtained as a pure derivative (di-p-tolylsulfane; Scheme 2) in the form of
brownish oily liquid having unpleasant smell of organosulfur after subjected to the
7

column chromatography (SiO
2
) employing hexane: ethyl acetate (9.8:0.2) as an eluent
Scheme-2). The selectivity of the di-p-tolylsulfane is 100% and it is evident from the GC
gas chromotography) measurement.
(
(
4
. Reaction mechanism and kinetic model
In this work, the reaction between 4-bromo-1-methylbenzene (BMB) and sodium
sulfide (Na
2
S) were employed in the presence of multi-site phase-transfer catalyst namely
)and ultrasonic
1
,4-dihexyl-1,4-diazoniabicyclo[2.2.2]octanium dibromide (Q(Br)
2
frequency 40 kHz (300 W). The reaction mechanism is represented in Scheme 3, which is
similar to the mechanism proposed by Yadav and Bisht [32] and Hsiao et al. [33].
The overall reaction can be represented as:
when the organic and aqueous reactants are immiscible in the aqueous and organic
phases, the substitution reaction (Eq. (2)) and ion-exchange (Eq. (1)), mainly take place
in the phase-transfer catalyst. In addition, the lipophilicity of the catalyst is a crucial
factor for influencing the rates of Eq. (1) and (2).
4
.1. Definition
The conversion (X) of 4-bromo-1-methylbenzene (BMB) is defines as follows:
X=1-{[BMB]
where [BMB]
time (t) t=0 and t>0, respectively.
0
/[BMB].o,i
}
1
o
and [BMB]o,i represent the concentration of 4-bromo-1-methylbenzene at
4
.2 Rate expression
The rate expression for this reaction may be expressed as:
8

-
r
BMB = kapp [BMB]
o
2
Where kapp is the apparent reaction rate constant.
This reaction is carried out in a batch reactor, so the diminution rate of BMB with time (t)
can we expressed as
-
d[BMB]
on integrating the Eq. (3) yields:
0
ln{[BMB] /[BMB].o,i} = -ln(1-X) = kapp t
0
/ dt = -rBMB = kapp [BMB]
o
3
-
4
Using Eq. (4), we can get the kapp value experimentally by plotting –ln (1-X) against time,
t).
(
5
. Results and discussion
The reaction was conducted on a 250 mL three-necked Pyrex round-bottom flask
this permits agitating the solution, inserting the water condenser to recover organic
reactant and taking samples and feeding the reactants. This reaction vessel was supported
at the centre of the sonicator. Known quantities of n-hexane (30 mL), well powdered
sodium sulfide (5g) in 6 mL water and 0.2g biphenyl (IS-internal standard) were
introduced into the reactor. Then, 2 g of 4-bromo-1-methylbenzene (11.69 mmol), 0.2g of
the synthesized MPTC were introduced to the reactor to start the reaction. The reaction
mixture was stirred at 600 rpm. The phase separation was almost immediate on arresting
the stirring process. Samples were collected from the organic layer at regular time
2
intervals and each run consists of six samples. A pinch of anhydrous CaCl was placed in
the sample vials to absorb any moisture present in the organic layer and 0.5 mL of n-
hexane was added to vials to dilute the solution. The kinetics was followed by estimating
the amount of 4-bromo-1-methylbenzene that disappeared and measured by a gas
9

Chromatography (GC-Varian 3700 model). The analyzing conditions were as follows;
Column, 30 m x 0.525 mm i.d. capillary column containing 100% poly(dimethyl
0
siloxanen); injection temperature, 250 C; FID detector (300 C). Yields were
0
determined from standard curve using biphenyl as an internal standard.
5
.1 Effects of stirring speed
To determine the influences of mass-transfer, the stirring speed was varied from 0
to 800 rpm along with ultrasound irradiation (40 kHz, 300W) using 1,4-dihexyl-1,4-
diazoniabicyclo [2.2.2]octanium dibromide (MPTC). In principle, the homogeneous
reaction is independent of the agitation speed but heterogeneous reaction is dependent to
the agitation speed. It can be seen from the Table-1 that the increase in the reaction rate is
produced with a raise in the stirring speed from 0 to 400 rpm. The agitation speed
exceeds 400 rpm; the conversion of 4-bromo-1-methylbenzene is not dependent to the
agitation speed. This result is different from that of other reaction systems that require a
high agitation speed to attain larger interfacial area to increase the mass transfer rate [32-
3
4]. However, the active intermediate of the catalyst (QS), which is produced by ion
exchange between sodium sulfide and MPTC in the inter-phase and is hydrophobic, likes
to stay in the organic phase, i.e., it is easy to transfer the active intermediate of the
catalyst from the aqueous phase to the organic phase, in which the interfacial area is not
so important. Therefore, the reaction kinetics is controlled by the chemical reaction for
stirring speed exceeds 400 rpm. Hence, the stirring speed was set at 600 rpm for
subsequent experiments. On comparing the rate constants for the mechanical stirring and
the ultrasonic irradiation, the mechanical stirring lonely has influenced the reaction more
1
0

effectively than the ultrasound irradiation alone but the combination of both reveals more
effect on the reaction rate i.e., the results are as follows,
(
i)In the absence of stirring speed and in the presence of the effect of ultrasonic
−
condition at 40 kHz (300 W) the observed rate constant is 6.76 × 10 , min .
3
−1
(
ii) In the presence of stirring speed (600 rpm) and in the absence of the effect of
−
3
−1
ultrasonic condition at 40 kHz (300 W) the observed the kapp is 8.62×10 , min
conventional method).
iii) In the presence of both condition, i.e., at 600 rpm combined with the
(
(
−
3
−1
ultrasonic wave frequency 40 kHz (300 W) the kapp value is 35.26×10 , min .
From this observation the ultrasound irradiation enhances the reaction rate almost
six times with respect to the conventional method. It may be due to in the presence of
ultrasonic energy increase the collision rate between the organic and aqueous phase and
decrease the surface area between the two layers [35]. This implies that the synergetic
effect of both ultrasound and stirring enhanced the reaction rate.
5
.2 Effect of Ultrasonic Power
Ultrasonic irradiation is defined as acoustic waves which are produced from low
frequency high power ultrasound (20 kHz -100 MHz) [36, 37]. They create cavities
which are the formation, growth and implosive collapse irradiated with sound [37]. In
case of volatile molecules cavities are believed to act as a micro reactor i.e., as the
volatile molecule enter the microbubbles and the high temperature and pressure
produced during cavitational break their chemical bonds thus reacting with other
specious [38-40]. It has been reported that a combination of PTC and ultrasound is often
better than either of the two techniques alone [41]. In recently, few works only are
1
1

reported that multi-site phase-transfer catalyst (MPTC) combined with ultrasound shows
the highest potential to increase the rate of the reaction or conversion [16-22, 42].
To study the effect of ultrasound frequency, two different ultrasonic frequencies
(
28 kHz, 40 kHz) is used, having the same output power of 300 W. The reaction is also
carried out without ultrasound (conventional method). The mass-transfer resistant plays
an important role in interfacial reaction, although the use of ultrasound increases the
mixing of the two phases and enhances the mass-transfer [36]. Possible reason is in the
liquid–liquid bi-phase system, the presence of ultrasonic wave energy used to disrupt the
interface by cavitational collapse near the liquid–liquid interface and impels jets of one
liquid into other, forming fine droplets and it leads to a dramatic enhancement in the
interfacial contact area through which transfer of species can take place. The kinetic
profile of the reaction is obtained by plotting –ln (1-X) against time. In our experimental
-
3
-1
condition, the presence of ultrasonic condition the kapp values are 21.51 x 10 , min and
-
3
-1
5.26 x 10 , min for 28 kHz (300W) and 40 kHz (300W), respectively (Table-2).
3
Hence, the overall kapp value was increased by increasing the ultrasonic frequency in the
order of 0 kHz (silent condition) <28 kHz (300W) <40 kHz (300W) for our system
(
Table 2) .Similar trend was observed by Entezari coworkers [43, 44].
5
.3 Effect of the amount of MPTC
The influence of the amount of MPTC (viz., 1,4-dihexyl-1,4
diazoniabicyclo[2.2.2]octanium dibromide) on the arylation of sodium sulfide has been
studied by varying amount of MPTC concentration from 0.1g to 0.3g with respect to 4-
bromo-1-methylbenzene (BMB) under ultrasound irradiation (40 kHz, 300W) keeping
other experimental parameters constant. The pseudo-first order rate constant were
1
2

evaluated from -ln (1-X) versus time. As shown in Fig.1, the conversion is very low when
the reaction was carried out in the absence of MPTC. However, the reaction rate or
conversion is dramatically enhanced when MPTC was added to the reaction solution
when 0.2g of MPTC added to reaction, the conversion is merely 73% in 120 minutes and
at higher concentration of MPTC the conversion quickly reach 76% in 40 minute of
reaction and after 40 min, the conversion marginally differed due to mass transfer
resistance is take place for higher catalyst concentration. Therefore all the further
experiments were done at 0.2g of catalyst concentration. The increase in the conversion is
due to the synergic effect of ultrasound [45, 46].
5
.4 Effect of the concentration of 4-bromo-1-methylbenzene
To investigate the influence of 4-bromo-1-methylbenzene (BMB) on the kinetics of
synthesis of di-p-tolylsulfane under ultrasonic irradiation condition (40 kHz, 300W) and
MPTC, the amount of 4-bromo-1-methylbenzene (BMB) was varied from 5.84 mmol to
1
7.54 mmol. The results are shown in Table-3, the data clearly indicates that the kapp
value increases with increasing the amount of 4-bromo-1-methylbenzene (BMB). This
observation due to presence of more number of active sites in the MPTC and higher
concentration of substrate (BMB) had co-operatively influence the reaction and thus
enhance the more number of contacts between catalyst and substrate (BMB), and hence it
is reflected in enhanced kapp values. In addition with that, the ultrasound irradiation
enhance the formation of active catalysts and it reflected in the enhanced rate of the
reaction, it may be due to reduces the surface area between the two phases, and hence
more reactants collide to each other simultaneously we get higher kapp value [29,33, 47,
4
8].
1
3

5
.5 Effect of temperature
To study the temperature effect, the temperature is varied from 40 to 80 C
Figure.2) for the reaction of sodium sulfide with 4-bromo-1-methylbenzene keeping
o
(
other operating conditions are constant. The kinetic profile of the reaction is obtained by
plotting -ln (1-X) versus time. The conversion is increased when the reaction carried out
increasing temperature along with ultrasound (Figure 2) [49, 50]. The reason is that the
number of reactant molecules which possess higher activated energy at a higher
temperature and thus the ultrasonic wave easily passes through the reactor [51]. The other
point is that the collision of the reactants at higher temperature is also increased. Hence,
the apparent rate constant or conversion is higher at the higher temperature. The energy
-1
of activation (Ea) is calculated from the Arrhenius plot (Fig. 5) which is 56.45 kJ.mol
-
1
for ultrasonic condition and for silent condition it is 71.47 kJ.mol . This higher activation
energy demonstrates that this reaction was kinetically controlled and the mass-transfer
resistance between phases was unimportant. In addition ultrasound is used to overcome
the mass-transfer resistant and avoid the use of higher temperature for the reaction
system. It is evident from higher activation energy and higher stirring speed that the
present reaction system follows the interfacial mechanism [52-55].
5
.6 Effect of the amount of sodium sulfide
2
The effect of the amount of sodium sulfide (Na S) on the conversion of 4-bromo-
1
-methylbenzene is shown in Figure.3. To study this effect, only the sodium sulfide
concentration is increased keeping water as constant (6 mL). In principle, the
concentration of sodium sulfide affects the distribution of the catalyst between two
phases. The active catalyst in the aqueous phase is solvated in the lower concentration of
1
4

sodium sulfide in 6 mL water. The hydration is decreased by increasing the concentration
of sodium sulfide. Hence, the reaction rate is increased by increasing the amount of
sodium sulfide. However, when the concentration of sodium sulfide exceed a certain
value (approximately 5 g per 6 mL water) the reaction rate decreased, it may be due to
the catalyst concentration that decreased at sodium sulfide phase (salting out effect). The
k
6
app value increases by increasing the sodium sulfide amount up to 5 g sodium sulfide per
ml water and does not increase apparently beyond this concentration (Figure 3).
Therefore, the conversions are obviously affected by the amount of sodium sulfide, so we
choose 5 g sodium sulfide per 6 mL water for the further study.
5
.7 Effect of organic solvents
The influence of various organic solvents on the rate of arylation of sodium
sulfide was followed under otherwise standard reaction conditions. The distribution of
phase-transfer catalyst between aqueous and organic medium depends on the polarity of
the solvent used. We test the five solvents are toluene, n-hexane, chlorobenzene,
cyclohexane and chloroform for this study. The polarity of organic solvent affects the
reaction rate along with the effect of ultrasound. From the plot of –ln (1-X) against time,
the kapp values are obtained. It is clear from the Table-4; chlorobenzene possesses a
higher kapp value among the five organic solvents used.
5
.8 Effect of volume of n-hexane
In a homogeneous reaction, the reaction follows the intrinsic kinetic law. The
conversion or the reaction rate is directly proportional to the concentration of the
reactants. A dilute concentration of the reactant is obtained using a large amount of
organic solvent. The conversion of 4-bromo-1-methylbenzene is decreased with the
1
5

increase in the volume of n-hexane, even though the interfacial contact area is increased
with increase in the amount of organic solvent. Another possible reason is effective
mixing of two layers and fruitful collision between the reactants occurred when the
organic solvent volume is less by the combined effect of stirring and ultrasonic waves.
Nevertheless, a sufficient amount of organic solvent would be required to dissolve a
certain amount of organic reactant, which is 4-bromo-1-methylbenzene. From Table.5,
the kapp value is inversely proportional to the volume of hexane, as expected.
5
.9 Effect of MPTC and single-site phase-transfer catalysts
Quaternary ammonium salts are generally used as phase-transfer catalysts to
promote the reaction rate. Several phase-transfer catalysts were employed in addition
with MPTC to evaluate their efficacy in the reaction of sodium sulfide with 4-bromo-1-
0
methylbenzene at 60 C and 600 rpm under ultrasonic condition (40 kHz, 300W) with 3
mol% of PTCs with respect to 4-methyl bromobenzene. Along with MPTC, The other
five different phase-transfer catalysts were tetrabutylammonium bromide (TBAB),
tetrabutylammonium chloride (TBAC), tetrabutylammonium iodide (TBAI),
tetrahexylammonium bromide (THAB) and tetraoctylammonium bromide (TOAB).
Table-6 depicts the apparent rate constants for these six catalysts. Among these, MPTC
shows higher reactivity due to it contains two active sites therefore; the conversion is as
fast as compared with other single site PTCs [29]. The greater efficiency of the MPTC is
+
attributed to the higher lipophilic character of the cation (Q ) supplied by the MPTC
2
+ 2-.
catalyst, leading to the formation of intermediate with the sulfide anion as Q S The
ion-pairs are able to enter the organic media in which the displacement reaction takes
place. In general, a more oleophilic cation is more effective to transfer anions into the
1
6

organic phase. Therefore, the order of catalytic reactivity are MPTC > TOAB > THAB >
TBAI > TBAB > TBAC. In addition, the role of counter anion is also important.
Comparing the results for TBAB, TBAC and TBAI, the order of the reactivity of these
−
−
−
anions is I > Br > Cl . Since softer and more oleophilic anions are more effective than
harder and less oleophilic anions, the order of catalytic reactivity is TBAI > TBAB>
TBAC [47]. In general, the iodide of a quaternary cation is a co-catalyst, forming 4- iodo-
1
-methylbenzene in situ and rapidly which is more reactive than the 4-bromo-1-
methylbenzene [53, 55].
5
.10 Effect of volume of water
The reaction of sodium sulfide with 4-bromo-1-methylbenzene under ultrasound
condition (40 kHz, 300 W) was examined by varying the amount of water from 3 to 12
mL, under standard reaction conditions. Apparent rate constants were obtained from the
plot of -ln(1-X) against time. Generally, the volume of water directly affects the
concentration of sodium sulfide in the aqueous phase. Therefore, the conversion will be
affected by the volume of water (Figure.4). This is due to the hydration of the active
catalyst (QS) as the volume of water increased.
5
.11 Effect of the amount of sodium hydroxide
The rate of arylation of sodium sulfide depends on the strength of the sodium
hydroxide. Only sodium hydroxide concentration is varied keeping other standard
reaction conditions. Table-7, shows the effect of sodium hydroxide amount in the
aqueous phase on the di-p-tolylsulfane production. The kapp value increased from 0.5 g to
2
g of sodium hydroxide (NaOH). It may be due to increase the basicity of aqueous phase
[
56]. From 2 g to 5 g of sodium hydroxide, the kapp value tremendously decreased.
1
7

According to Hsiao and Weng [57] adding salt (NaOH or NaBr) to the aqueous solution
would inhibit the ionization of sodium sulfide, thus the rate of formation of the product is
decreased and the second reason is the amount of phase-transfer catalyst salted out from
the aqueous phase and thereby the reaction rate is decreased.
5
.12 Effect of alkyl and aryl halides
Table 8 shows that the reaction follows pseudo-first order law in the presence of
MPTC and excess amount of sodium sulfide. It was found aryl halide carrying electron-
donating groups could be smoothly converted to the desired products. The coupling of
aryl iodides containing electron-withdrawing groups also thioether in good conversion.
Aryl halides carrying an ortho substituent were also found to readily participate in the
reaction. Further, the most reactive organic reactant is allyl bromide. The reaction is 100
%
completed within 10 min for allyl bromide. It may be due to its smaller molecular size
and the conjugation of pi bond. Among the alkyl bromides sec-propyl bromide is the least
reactive one because of steric hindrance in its reaction. From 1-propyl bromide to 1-octyl
bromide the kapp value decreases due to increasing the carbon chain of the molecules [58-
6
2].
6
. Conclusion
Based on the results of this study, the following conclusions may be drawn:
(
(
1) The multi-site phase-transfer catalyst has been successfully synthesized and tested
for the synthesis of the di-p-tolylsulfane from sodium sulfide and 4-bromo-1-
methylbenzene (BMB) under ultrasonic irradiation condition.
2) The efficiency of the MPTC compared with the known phase-transfer catalysts, in
which the MPTC shows higher activity.
1
8

(
3) From the kinetic study, the rate of the reaction is found to be directly dependent
on each kinetic variables, viz., [MPTC], [substrate], ultrasonic frequency, stirring
speed and temperature.
(
(
4) The rate of the reaction decreases with increase in the concentration of sodium
sulfide and volume of n-hexane.
5) The rate of the reaction initially increases and then decreases with increase in the
concentration of sodium hydroxide and increases in the volume of water.
6) Based on the experimental result, the interfacial mechanism was proposed.
7) The combination of ultrasound and MPTC resulted in better efficacy as compared
to the individual operation.
(
(
Acknowledgments
The authors would like to thank The University Grants Commission, New Delhi,
India, for financial support for this research work. We also thank The Pachaiyappa’s
Trust, Chennai, Tamil Nadu, India-600 030, for their grant permission to do this research
work.
1
9

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9

Caption of Figures
Figure 1
2 2
Effect of the amount of MPTC on the conversion: 5g of Na S, 6 mL H O, 0.2g of
biphenyl (internal standard), 11.69 mmol of 4-bromo-1-methylbenzene, 30 mL of
0
n- hexane, 600 rpm, 60 C; ultrasound condition (40 kHz, 300 W).
Figure 2
2 2
Effect of temperature on the conversion: 5g of Na S, 6 mL H O, 0.2g of biphenyl
(
internal standard), 11.69 mmol of 4-bromo-1-methylbenzene, 0.2g of MPTC, 30 mL of
n-hexane, 600 rpm, and ultrasound condition (40 kHz, 300 W).
Figure 3
Effect of sodium sulfide concentration: 11.69 mmol of 4-bromo-1-methylbenzene, 0.2g
0
of biphenyl (internal standard), 0.2g of MPTC, 60 C, 600 rpm, ultrasound condition (40
kHz, 300 W).
Figure 4
Effect of volume of water on the conversion: 5g of Na
2
S, 0.2g of biphenyl (internal
0
standard), 11.69 mmol of 4-bromo-1-methylbenzene, 0.2g of MPTC, 60 C, 600 rpm,
3
0 mL of n-hexane, ultrasound condition (40 kHz, 300 W).
2
0