CHEM01200604012 Dibakar Goswami - Homi Bhabha National ...

[pyridinium][Sn 2 Cl 5 ] etc. can be prepared by the reaction of a halide with a Lewis acid
(step IIa). In 1992, metathesis (step IIb) was first used to synthesize [emim][BF 4 ]. The
preparation of [emim][PF 6 ] shortly followed; and since then, thiocyanate, trifluoroacetate
and heptafluorobutonate salts have all been prepared by metathesis reactions.
NR 3
Step I
+ R'X
[R'R 3 N] + X
Step IIb
1. + metal salt M + [A] ; - MX (precipitation)
Step IIa
+ Lewis acid MX y
or 2. + Bronsted acid H + [A] ; - HX (evaporation)
or 3. Ion exchange resin
[R'R 3 N] + [MX y+1 ]
[R'R 3 N] + A
Scheme II.3.1
The physical and chemical properties of ionic liquids can be specifically varied
over a wide range by the selection of suitable cations and anions. 71 In general, 58e,72 the
ionic liquids can be considered to be polar phases with the solvent properties largely
determined by the ability of the salt to act a hydrogen-bond donor and/or acceptor and the
degree of localization of the charge on the anions. Furthermore the increasing chain length
of alkyl substituents on both the cations and anions leads to greater lipophilicity of ionic
liquids. Such ionic liquids which show a miscibility gap with water have also been used as
a substitute of volatile organic solvents in extractive separation processes.
More recently, ionic liquids have been referred as promising solvents for “clean
processes” and “green chemistry”. These reflect the current efforts to reduce drastically the
amounts of side products and also the solvent and catalyst consumption in chemical
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