Ionic liquids are organic salts that are liquid at or near room temperature.
It has been found recently that such liquids can be useful solvents for organic
reactions. Often, the organic products can be removed from the ionic liquid by
extraction with, e.g., ether, without resorting to an aqueous workup. This can
be particularly useful when a precious metal catalyst is used in the reaction. PMID:24463635
The catalyst often remains in the ionic liquid, so that the catalyst solution
can be directly reused. 3-Bromo-5-fluoro-4-methylbenzoic acid Data Sheet

Roberta Bernini of the Univ. of Tuscia in Viterbo illustrated (Tetrahedron
Lett.
2003, 44, 8991.
DOI: 10.1016/j.tetlet.2003.10.002)
the power of this approach with the
MeReO3-catalyzed
oxidation of 1 to 2 in [bmim]BF4. 2-(2-Bromoethyl)-1,3-dioxolane Formula The product could
be extracted with ether and the catalyst-containing ionic liquid recharged with
substrate and H2O2 for four cycles before the conversion
and yield started to drop off. This drop off may be due to the accumulation of
water in the ionic liquid, which could be removed by distillation.

Alternative purification protocols are available. Zhaolin Sun of Lanzhou
University reports (Tetrahedron Lett. 2004, 45, 2681.
DOI: 10.1016/j.tetlet.2004.01.131)
that the ionic liquid TISC was specifically designed to promote
Beckmann rearrangement. TISC is not soluble in water, so the product caprolactam
was easily removed from the ionic liquid by extraction with water.

The counterion of the ionic liquid can be tuned to achieve one desired
reactivity or another. Martyn Earle of Queen’s University, Belfast has observed
(Org. Lett. 2004, 6, 707.
DOI: 10.1021/ol036310e)
that the reaction of toluene can be directed toward any of the three
products
6, 7, or 8, depending on the ionic liquid used.

Ionic liquids have been used for many other reactions. Examples include
Friedel-Crafts acylation (Tetrahedron Lett. 2002, 43, 5793.
DOI: 10.1016/S0040-4039(02)01185-1),
osmylation (Tetrahedron Lett. 2002, 43, 6849.DOI: 10.1016/S0040-4039(02)01461-2)
, Heck coupling (Tetrahedron Lett. 2003, 44, 8395.
DOI: 10.1016/j.tetlet.2003.09.120),
and Henry reaction (Tetrahedron Lett. 2004, 45, 2699.
DOI: 10.1016/j.tetlet.2004.01.129).