Many attempts have been made to correlate the nature of solvent with reaction rate or nucleophilicity. Huges & Ingold have proposed the following qualitative theory of solvent efects;

i)Ions & polar molecules, when dissolved in polar solvents, tend to become solvated.

ii) For a given solvent, solvation tends to increase with increasing magnitude of charge on the solute molecules or ions.

iii)For a given solute, solvation tends to increase with increasing dipole moment of the solvent.

iv)For a given magnitude of charge, solvation decreases as the charge is spread over a larger volume.

v)The decrease of solvation due to the dispersal of charge will be less than that due to its destruction.

Thus if the reaction of SN2 type is to be carried out with -SH a solvent which is less polar will be useful. In a polar solvent the SH ion may get solvated and thus its nucleophilicity may decrease.