Ultrasound enhanced PTC conversion of benzamide to benzonitrile

Article abstract of DOI:10.1081/SCC-100105382

Enhancement of the reaction rate by combining the beneficial effects of PTC and ultrasound has been explored in the present study by taking the transformation of benzamide by dehydration to give benzonitrile as a model system. It was found that there is a

Full text of DOI:10.1081/SCC-100105382

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ULTRASOUND ENHANCED PTC CONVERSION OF
BENZAMIDE TO BENZONITRILE
Manickam Sivakumar a , Paramasivam Senthilkumar a & Aniruddha B. Pandit b
a Chemical Engineering Division, University Department of Chemical Technology (UDCT) ,
University of Mumbai , Matunga, Mumbai, 400 019, India
b Chemical Engineering Division, University Department of Chemical Technology (UDCT) ,
University of Mumbai , Matunga, Mumbai, 400 019, India
Published online: 09 Nov 2006.
To cite this article: Manickam Sivakumar , Paramasivam Senthilkumar & Aniruddha B. Pandit (2001) ULTRASOUND
ENHANCED PTC CONVERSION OF BENZAMIDE TO BENZONITRILE, Synthetic Communications: An International Journal for
Rapid Communication of Synthetic Organic Chemistry, 31:17, 2583-2587, DOI: 10.1081/SCC-100105382
To link to this article: http://dx.doi.org/10.1081/SCC-100105382
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SYNTHETIC COMMUNICATIONS, 31(17), 2583–2587 (2001)
ULTRASOUND ENHANCED PTC
CONVERSION OF BENZAMIDE
TO BENZONITRILE
Manickam Sivakumar, Paramasivam Senthilkumar,
and Aniruddha B. Pandit*
Chemical Engineering Division, University Department of
Chemical Technology (UDCT), University of Mumbai,
Matunga, Mumbai 400 019, India
ABSTRACT
Enhancement of the reaction rate by combining the beneficial
effects of PTC and ultrasound has been explored in the pres-
ent study by taking the transformation of benzamide by dehy-
dration to give benzonitrile as a model system. It was found
that there is a substantial reduction in the reaction time when
ultrasound was coupled with PTC.
INTRODUCTION
Liquid-liquid heterogeneous systems form an important place in the
domain of chemical reactions. The major limitation is the generation of
sufficient interfacial area especially for mass transfer controlled reactions.
*Corresponding author. E-mail: abp@udct.ernet.in
2583
Copyright & 2001by Marcel Dekker, Inc.
www.dekker.com

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SIVAKUMAR, SENTHILKUMAR, AND PANDIT
The role of PTC in enhancing the rate of the reaction under these conditions
is a well known phenomenon.1 Application of ultrasound for substantial
improvement of rate and product yields of variety of reactions is also an
area studied widely.2 Most of the reactions initiated by ultrasound proceeds
via free radical mechanism (true and direct chemical effect). In case of
liquid-liquid immiscible systems it causes homogenisation or generates
extremely fine emulsions/dispersions resulting in higher interfacial area,
which then leads to the desired chemical effect (indirect chemical effect).
This is an unique property of ultrasound since conventional methods of
mixing cannot produce such a high interfacial area, especially when one
of the phase used is highly viscous.3–5 Thus, ultrasound can play a dual
role of creating higher interfacial area as well as facilitating the process of
interfacial transport. Such useful effects which led to the use of ultrasound
to enhance the efficacy or even to replace PTC’s (solid/liquid) in some of the
chemical reactions.
SELECTION OF REACTION
The two-phase reaction, which has been selected under the study, is
the dehydration of benzamide to give benzonitrile. This reaction is of prac-
tical importance, since many of the cyano compounds are useful as drug
as well as agrochemical intermediates. Generally this reaction is carried out
by means of dehydration with the application of a number of acid catalysts
such as aluminium chloride, phosphorous pentoxide, phosphorous
oxy chloride, thionyl chloride and trimethylsilylpolyphosphate.6 The use
of these catalysts presents many difficulties in practice. Most of these are
required in large excess and many get destroyed during recovery of the
product. Also, the catalysts often render the medium corrosive which, can
result into a product contamination. Mildly acidic catalysts have also
been used in the reaction. Temperatures in the range of 280 to 380ꢀC
have to be employed for these acidic catalysts.7 Although high yields
of nitriles have been reported by vapour phase dehydration over zeolite
catalysts,8–9 this method also requires a high reaction temperature, say
400ꢀC and again it is restricted to volatile amides. PTC’s or quaternary
ammonium salts like Benzyltriethyl ammonium chloride (TEBA) have
also been employed in this conversion, by carrying out the reaction in a
two-phase mode.10 The main advantage of this reaction is that it proceeds
at room temperature with good yields in 2 hrs of time. In order to enhance
the reaction rates further, ultrasound was coupled with PTC in the above
transformation. The possible reaction scheme under the application of

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2585
ultrasound and PTC can be shown as,
EXPERIMENTAL
All Ultrasonically mediated dehydrations were carried out using
a variable amplitude/intensity type ultrasonic probe (Ace Glass Inc.,
New York, USA) having a driving frequency of 22.5 kHz, with a rated
power of 600 W. This horn has the provision for tuning, which is used to
overcome the drop in performance due to the continuous erosion at the horn
tip. In the present study, the horn was tuned prior to the study, in order to
obtain maximum performance. In all the studies, it was operated at 20%
amplitude i.e., at an electrical output of 120 Watts. All the chemicals used
were of analytical grade (S. D. Finechem, India) and were used without
further purification.
General Procedure: Dehydration of Benzamide by
Ultrasound and PTC
Mixture of 0.5042 g benzamide (4.17 mmol), 0.023 g TEBA (0.1 mmol),
10.1 ml chloroform and 3.3 g of NaOH (83.25 mmol, as 50% aqueous solu-
tion) were taken in a 25 ml reactor and irradiated by keeping the horn at the
interface for a given period of time at 25ꢀC with different conditions like
stirring with ultrasound and with only ultrasound irradiation. The reaction
mixture, including the white precipitate was then extracted with chloroform
(2 ꢁ10 ml), then washed with 10 ml water. Finally the obtained organic layer
was dried over Na2SO4 and the product was then distilled under reduced
pressure to afford oily residue, benzonitrile (b.p. 78–79ꢀC/20 mmHg).
Isolated benzonitrile was identified by direct comparison with an authentic

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SIVAKUMAR, SENTHILKUMAR, AND PANDIT
sample. A standard reaction with PTC was also carried out for 2 hrs by
taking the same quantities of the reactants as those taken for the corre-
sponding sonochemical reactions, as explained before.
RESULTS AND DISCUSSION
It can be seen from the above mechanism that the transfer of OH–
(from NaOH) from the aqueous phase to the organic phase (CHCl3) is the
rate controlling step. This transfer is mediated with the help of PTC and the
application of ultrasound along with PTC enhances the transfer rate sig-
nificantly. The experimental results obtained at different conditions are as
shown in the table and are discussed below.
From the table, it can be seen that in the absence of PTC, there was
no reaction even if sonication is carried out for 1hr. But, once a combina-
tion of sonication and PTC was applied to the system, it resulted in a better
overall effect. This clearly confirms that larger interfacial area generated by
the ultrasound is not the controlling factor in the reaction. Thus, the ultra-
sonic acceleration was ascribed to efficient mixing, enabling better mass
transfer between the two phases, providing the transfer of PTC through
this interface.
When irradiation was applied to the reaction without stirring, the
results obtained were similar as it was obtained with the combination of
stirring and sonication. This confirms that stirring doesn’t have any effect in
Table. Experimental Results of Ultrasonic Dehydration of Benzamide to Give
Benzonitrile Under Different Conditions
Yield of
Benzonitrile (%)
Conditions
Time (min)
(A) With PTC
Only Stirring
120
5
84.76
29.43
45.9
52.85
80.12
45.22
84.61
Stirring þ Ultrasound
Stirring þ Ultrasound
Stirring þ Ultrasound
Stirring þ Ultrasound
Only Ultrasound
10
15
20
10
20
Only Ultrasound
(B) Without PTC
Stirring þ Ultrasound
60
0

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CONVERSION OF BENZAMIDE
2587
the presence of sonication. In other way, again this reflects that only the
interfacial mass transfer controls the overall reaction rate, which is substan-
tially (6 fold), enhanced by the use of ultrasound. Thus, there is a tremen-
dous potential for combining PTC and ultrasound to get the desired
synergistic effects.
ACKNOWLEDGMENTS
MS gratefully acknowledges the Senior Research Fellowship (SRF),
by the University Grants Commission, Government of India. The authors
would also like to acknowledge the Department of Science and Technology
(DST), Govt. of India, for the funding of this project.
REFERENCES
1. Starks, C.M.; Liotta, C.L.; Halpern, M. Phase Transfer Catalysis,
Fundamentals, Applications and Industrial perspectives, Chapmann
and Hall Inc., New York, 1994, pp. 277–278.
2. Mason, T.J.; Lorimer, J.P. Sonochemistry, Theory, Applications and
Uses of Ultrasound in Chemistry, Ellis Horwood, Chichester, 1988,
pp. 88 and 92.
3. Majumdar, S.; Senthilkumar, P.; Pandit, A.B. Ultrasonics
Sonochemistry 1998, 5, 113.
4. Senthil Kumar, P., M. Chem. Engg. Thesis, Studies in cavitation, 1997,
University of Mumbai, India.
5. Majumdar, S.; Senthilkumar, P.; Pandit, A.B. Ind. Jnl. Chem. Tech.
1997, 4, 277.
6. Masataka, Y.; Sayaka, Y.; Tsuneo, I. Synthesis 1982, 7, 591.
7. Billenstein, S.; Blaschke, G. J. Am. Oil. Chemist. Soc. 1984, 61, 353.
8. Ramarao, A.V.; Rao, N.M.; Garyali, K.; Kumar, P. Chem. Ind.
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10. Saraie, T.; Ishigure, T.; Kawashima, K.; Morita, K. Tet. Lett. 1973, 2121.
Received in the UK May 15, 2000

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