Phase Transfer Catalyzed Reduction of Nitrotoluenes by Aqueous Ammonium Sulfide: Kinetic Study

Topics: Chemical reaction, Chemistry, Reaction rate Pages: 22 (3034 words) Published: March 3, 2014
Phase Transfer Catalyzed Reduction of Nitrotoluenes by Aqueous Ammonium Sulfide: Kinetic Study

Sunil K. Maitya, Narayan C. Pradhan*, b, Anand V. Patwardhanc a

Department of Chemical Engineering, IIT, Kharagpur, India, Email: sunil_maity@che.iitkgp.ernet.in b
Department of Chemical Engineering, IIT, Kharagpur, India, Email: ncp@che.iitkgp.ernet.in c
Department of Chemical Engineering, IIT, Kharagpur, India, Email: avp@che.iitkgp.ernet.in

* Corresponding author: Phone: +91-3222-283940; Fax: +91-3222-255303; E-mail: ncp@che.iitkgp.ernet.in Keywords: Liquid-liquid phase transfer catalysis; Ammonium sulfide; Nitrotoluene.

Abstract
The reduction of nitrotoluenes (o-, m- and p-) using aqueous ammonium sulfide as the reducing agent was carried out in an organic solvent, toluene, under liquid-liquid mode with phase transfer catalyst (PTC), tetrabutylammonium bromide (TBAB). The selectivity of toluidines was found to be 100%. The reaction rate of m-nitrotoluene was found to be highest among the three nitrotoluenes followed by p- and o-nitrotoluene. The effects of different parameters such as speed of agitation, catalyst concentration, sulfide concentration, concentration of nitrotoluene, and temperature on the reaction rate and conversion were studied. The rate of reaction of nitrotoluene was found to be proportional to the concentration of catalyst, to the square of the concentration sulfide, and to the cube of the concentration of nitrotoluene. The apparent activation energy for this kinetically controlled reaction was estimated as 19.43, 21.45 and 25.54 kcal/mol for ONT, PNT and MNT, respectively.

1. Introduction
Phase transfer catalysts (PTC) are widely used
to intensify otherwise slow heterogeneous reactions
involving an organic substrate and an ionic
reactant, either dissolved in water (liquid-liquid) or
present in solid state (liquid-solid). Phase transfer
catalysis is now an attractive technique for organic
synthesis because of its advantages of simplicity,
reduced consumption of organic solvent and raw
materials, mild operating conditions, and enhanced
reaction rates and selectivity. Among several
varieties of phase transfer catalysts, quaternary
ammonium salts are most preferred for their better
activity and ease of availability.
Two mechanisms, interfacial and bulk, are
generally used for liquid-liquid phase- transfer
catalysis based on the lipophilicity of the
quaternary cation.
The bulk mechanism, as
suggested by Starks [1] and Starks and Liotta [2], is
applicable to catalysts that are not highly lipophilic
or that can distribute themselves between the
organic and the aqueous phase, such as benzyl
triethyl ammonium, dodecyl trimethyl ammonium,
and tetrabutyl ammonium salts. In the interfacial
model, catalysts such as tetrahexyl ammonium and
trioctyl methyl ammonium salts remain entirely in
the organic phase because of their high lipophilicity
and exchange anions across the liquid-liquid
interface [3]. Tetrabutyl ammonium bromide

(TBAB) has been reported to be the most active
PTC among six different catalysts used to intensify
the reaction of benzyl chloride with solid sodium
sulfide [4]. Several researchers have also carried
out reduction of nitroarenes by sodium sulfide
using TBAB as PTC [5-8]. However, there is no
reported work in the literature on the preparation of
aryl amines using aqueous ammonium sulfide in
presence of phase transfer catalyst, TBAB.
The reduction of nitrotoluenes to the
corresponding amines is commercially very
important as the products toluidines have wide
commercial applications as intermediates for dyes,
agrochemicals and pharmaceutical products. The
use of ammonium sulfide as a reducing agent is of
considerable practical value due to some inherent
advantages of the method over other conventional
processes. For example, catalytic hydrogenation
requires more expensive equipment and hydrogen
handling facility; additional...

References: Techniques,
Academic: New York, 1978.
Weinheim, 1983.
Chem. Res. 29 (1990) 1103-1108.
Res. 31 (1992) 1606-1609.
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