Ionic liquids as catalytic green solvents for nucleophilic displacement
reactions
Christy Wheeler,a Kevin N. West,a Charles L. Liottab and Charles A. Eckert*a
a School of Chemical Engineering, Georgia Institute of Technology, Atlanta, GA, 30332-0100, USA.
E-mail: cae@che.gatech.edu
b School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332-0400, USA
Received (in Corvallis, OR, USA) 31st January 2001, Accepted 16th March 2001
First published as an Advance Article on the web 20th April 2001
We demonstrate the use of room-temperature ionic liquids
as a catalytic, environmentally benign solvent for the
cyanide displacement on benzyl chloride, replacing phase-
transfer catalyzed biphasic systems and thus eliminating the
need for a volatile organic solvent and hazardous catalyst
disposal.
solvent was motivated by its being one of the most widely used,
and therefore the most widely available, ionic liquids.
These reactions were carried out in a 25 ml volumetric flask
set in a recirculating heated bath and stirred with a magnetic stir
bar. The concentration of benzyl chloride in the ionic liquid was
1 M, and the amount of potassium cyanide was three times the
stoichiometric amount of benzyl chloride. While all of the
benzyl chloride was visually observed to be soluble, solid
potassium cyanide was present in the systems at all times.
Before introduction of benzyl chloride, the salt was stirred
overnight in the liquid so that the uniform particle sizes would
form and the salt would reach an equilibrium concentration.6
After the introduction of benzyl chloride, samples were drawn
and dissolved in cold acetonitrile before being analyzed by
HPLC equipped with a UV-Vis detector.
Reactions were carried out at 40, 60, and 80 °C, and
conversion is plotted in Fig. 1. Reaction rates were high, with
the reaction at 80 °C reaching complete conversion in less than
half an hour. Although the reactions at 60 and 80 °C show the
expected pseudo-first-order kinetic behavior (since the amount
of cyanide available for reaction should be constant), the
reaction at 40 °C appears to be zero order. This behavior
indicates that mass transfer of potassium cyanide into the
solvent is probably the rate-limiting step at the lower tem-
perature, which is consistent with the fact that the viscosity of
the ionic liquid is observed to decrease steeply with an increase
in temperature. Although solid salt is always present in the
reacting systems, the transfer into the liquid at higher tem-
peratures is apparently faster than the reaction step itself, such
that an equilibrium concentration of cyanide is achieved and
pseudo-first-order behavior is exhibited.
Ionic liquids have recently gained recognition as possible
environmentally benign alternative chemical process solvents.
Because of their vanishingly low vapor pressures, ionic species
do not contribute to VOC emissions as do most organic
solvents. Examples of their application in both reactions1,2 and
separations3,4 have been demonstrated. Although many types of
reactions have been investigated in ionic liquids, examples of
nucleophilic substitution reactions are absent from the lit-
erature.
Nucleophilic displacement reactions are often carried out
using phase-transfer catalysis (PTC) to facilitate the reaction
between the organic reactants and the inorganic ionic salts that
provide the nucleophiles.5 The phase-transfer catalyst, often a
tetraalkylammonium salt, acts as a shuttle for the reactant anion
between a polar phase that contains the salt reactant and a non-
polar phase that contains the organic reactant. This technique
not only overcomes the problem of contacting the reactants, but
also provides activation of the nucleophilic anion, since it is
much less tightly bound to a tetraalkylammonium cation than it
would be to a metal cation. In conventional PTC typical organic
solvents are environmentally undesirable species such as
methylene chloride or o-dichlorobenzene, and catalyst separa-
tion and recovery are significant challenges. With ionic liquids
as both the solvent and the catalyst, there are zero VOCs, and it
has been shown that product recovery is facile by CO2
stripping.3
Because ionic liquids are comprised of bulky organic cations,
they seem well suited for the types of reactions for which PTC
is effective. There even exists the possibility that the solvent
itself can act as a catalyst to activate the anion for reaction. The
ionic liquid cation might not be as effective a catalyst as most
PTCs; however, as a solvent, its high concentration should
overcome this limitation, yielding a high reaction rate.
We chose the cyanide displacement on benzyl chloride to
yield phenylacetonitrile, depicted in Scheme 1, as a model
reaction. For the solvent, we selected the ionic liquid 1-n-butyl-
3-methylimidazolium hexafluorophosphate (1), often called
The rate constants for each temperature are listed in Table 1.
An effective activation energy of 19 kcal mol21 is calculated
using the data at 60 and 80 °C. Because the nature of the solvent
changes with temperature, this activation energy includes
several factors other than the temperature dependence of the
intrinsic reaction rate, most especially the solubility of
potassium cyanide.
[bmim][PF6]. The reaction was chosen because it has been well
characterized in a variety of other systems. The choice of
Fig. 1 Conversion of benzyl chloride to benzyl cyanide (5 80 °C, - 60 °C,
: 40 °C).
Scheme 1 Cyanide displacement on benzyl chloride.
DOI: 10.1039/b101202a
Chem. Commun., 2001, 887–888
This journal is © The Royal Society of Chemistry 2001
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