best solvent for sn1

[stratosphere]

i was trying to decide what the best solvent for a sn1 substitution would be, that would not react with the resultant carbocation.

in particular for a relativly hindered secondary iodo alkane (4-iodo-3-hydro-arcoline), with the iodine to be replaced with benzoate ion, id assume this would only have a good chance to proceed via a sn1.

i was thinking maybe dmso?

[Rhodium]

S_N1 reaction rates are enhanced by protic, polar solvents (which has a high dielectric constant) which make the rate
determining step faster because they facilitate the production of the ionic intermediates by stabilizing the separated charges.



Ref:

http://falcon.sbuniv.edu/~ggray/CHE3304/oc11outl.html

[stratosphere]

thats one of the reasons i contimplated dmso, is its high dielectric constant.
but i understand it does not solvate anions well, so the rate at which iodine leaves might be slowed.

the problem with most protic solvents that i can think of is that they are apt to participate in side reactions with the carbocation and create for example a ether or alcohol rather then the desired benzoate-ester.

so what would be a good compromise of "proticity", polarity, and low reactivity?

[Disciple]

DMSO although it has a high dielectric polarization is Aprotic, you need a Protic solvent with a high dielectric polarization.
A mix of 80% water and 20% ethanol is reported to give a rate increase of 14,000 times that of the reaction in ethanol, this mix might give you enough solubility and a good rate of reaction. But in the end it depends on your compounds solubility and reactivity as to which solvent you choose.

[stratosphere]

i also didn't realize dmso had such a high b.p., that would make workup rather difficult.
 
since im trying to make a bezoate ester, and don't want the ether or alchol to form from reaction with the solvent, perhaps a good strategy would be to start with methanol(which reactes 10^5 times more slowly then water with tertbutylchloride) saturated with sodium benzoate, if it proceeds too slowly then increase the water:methanol ratio.