Handbook of Solvents - George Wypych - ChemTech - Ventech!

21.2 Ionic liquids 1473
these alloys is the substantial vapor pressure of Al2Cl6 associated with the acidic region of
these melts.
This is of course not the case when working with room temperature ionic liquid
systems. Electrochemical and spectroscopic studies of cobalt, 138 copper, 139 and nickel, 140
have been carried out in the AlCl3-butylpyridinium chloride molten salt system. The direct
current and pulsed current electrodeposition of Ni-Al alloys has also been shown in acidic
AlCl3-butylpyridinium chloride ionic liquids. This particular alloy has also been shown to
be successful in AlCl3-[C2-mim]Cl as have Co-Al and Cu-Al. 141 Electrochemical techniques
can also be used to calculate the diffusion coefficients of metal ions. Table 21.2.6 shows the
calculated diffusion coefficients and stokes-Einstein products of cobalt(II), 104 copper(I), 142
nickel(II) 143 and zinc(II) 144 in the 40-60 mol% [C2-mim]Cl-AlCl3 ionic liquid.
21.2.4.1 Electrosynthesis
Electrosynthesis provides an attractive alternative to conventional methods used for performing
synthetic chemistry. It can effect the clean, complete conversion of starting material
to product without using hazardous or toxic experimental conditions. Ionic liquids
possess many advantages over conventional solvents typically used in electrochemical experiments.
Their polar nature allows them to dissolve large concentrations of a wide variety
of organic and inorganic compounds. They possess no vapor pressure, are stable on heating
and because they are completely ionized the need for a supporting electrolyte is eliminated.
Such benefits are not associated with many of the non-aqueous systems employed in electrochemical
applications.
Electrochemical studies in ionic liquids are common; 145-148 however the application of
this technology to molecular synthesis remains largely unexplored. Some of the examples
to date deal with electro-polymerizations. Osteryoung et al. prepared polyaniline on platinum
and glassy carbon electrodes by anodic oxidation of the monomer in imidazolium
based chloroaluminate ionic liquids. 149 Elsewhere the formation of polyfluorene was
achieved by Janiszewska and Osteryoung. 150 Actual transformation of an organic moiety
was achieved electrochemically in 1-methyl-3-butylimidazolium chloroaluminate systems.
The reactions studied dealt with the reduction of aromatic ketones. 151 Reductions of
perylene and phenazine have also been achieved electrochemically in basic mixtures of aluminum
trichloride and 1-ethyl-3-methylimidazolium chloride. 152,153 The use of a neat ionic
liquid 1-ethyl-3-methylimidazolium hydrogen dichloride [C2-mim][HCl2] was also investigated
with regard to the reduction of phenazine. 152 Most of the work in electrosynthesis has
been carried out in air- and moisture-sensitive ionic liquids, those containing
chloroaluminates and [HCl2] - ; little investigation into the electrosynthetic possibilities presented
by air and moisture stable ionic liquids has been attempted. 154
21.2.5 PHYSICAL CHARACTERIZATION
21.2.5.1 Viscosity
Viscosity is probably the most important physical property for initially determining the
“processability” of a solvent. It is necessary for all calculations involving fluid flow, (pumping,
mixing, etc.) as well as in the estimation of heat transfer and diffusion coefficients. Information
on the change in viscosity as a function of temperature, solvent or reagent
addition is required for the efficient and design of any handling equipment. Ideally one
would like the viscosity of a fluid to be as low as possible allowing for the fluid to be