The present invention provides an ionic liquid including a quaternary
phosphonium salt represented by general formula (1):
wherein R.sub.1, R.sub.2, R.sub.3, and R.sub.4 each represent a linear
alkyl group having 1 to 6 carbon atoms, a branched alkyl group having 3
to 6 carbon atoms, an alicyclic alkyl group having 3 to 6 carbon atoms,
or an alkoxyalkyl group represented by --(CH.sub.2).sub.nO--R.sub.5
(wherein n is an integer of 1 to 6; and R.sub.5 represents a methyl group
or an ethyl group); at least one of R.sub.1, R.sub.2, R.sub.3, and
R.sub.4 is an alkoxyalkyl group; R.sub.1, R.sub.2, R.sub.3, and R.sub.4
may be the same or different; R.sub.1 and R.sub.2 together may form a
ring; and X represents an anion. The present invention also provides a
reaction solvent including the ionic liquid.