10-21-03 23:07
No 466005
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?
(Chief Bee)
10-22-03 03:04
No 466045
SN1 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/
(Hive Bee)
10-22-03 03:37
No 466051
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?
(Newbee)
10-23-03 06:52
No 466236
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.
(Hive Bee)
10-24-03 01:24
No 466422
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.