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The Hive => Chemistry Discourse => Topic started by: stratosphere on October 21, 2003, 11:07:00 PM

Title: best solvent for sn1
Post by: stratosphere on October 21, 2003, 11:07:00 PM

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?

Title: Solvents for SN1
Post by: Rhodium on October 22, 2003, 03:04:00 AM

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.

(https://www.thevespiary.org/rhodium/Rhodium/hive/hiveboard/picproxie_docs/000466005-file_4nmo.gif)

Ref:

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

Title: thats one of the reasons i contimplated dmso,...
Post by: stratosphere on October 22, 2003, 03:37:00 AM

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?

Title: How bout a mix
Post by: Disciple on October 23, 2003, 06:52:00 AM

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.

Title: i also didn't realize dmso had such a high...
Post by: stratosphere on October 24, 2003, 01:24:00 AM

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.