[Nescafe]

I know SN2s go much better in aprotic solvents but what about nonpolar (THF) vs polar (DMF)?

Nescafe.

[discodermolide]

I know SN2s go much better in aprotic solvents but what about nonpolar (THF) vs polar (DMF)?

Nescafe.

I would imagine that they would proceed better in DMF.

[Nescafe]

I know SN2s go much better in aprotic solvents but what about nonpolar (THF) vs polar (DMF)?

Nescafe.

I would imagine that they would proceed better in DMF.

But why? Why does it go better In a polar solvent and not a non polar one. I understand that polar solvents solvate charges but on a sn2 don't we want the charges to "feel" eachother?

[discodermolide]

I know SN2s go much better in aprotic solvents but what about nonpolar (THF) vs polar (DMF)?

Nescafe.

I would imagine that they would proceed better in DMF.

But why? Why does it go better In a polar solvent and not a non polar one. I understand that polar solvents solvate charges but on a sn2 don't we want the charges to "feel" eachother?

Sorry I was wrong THF is better for the reasons you state. A quote from a Wiki page:

"The solvent affects the rate of reaction because solvents may or may not surround a nucleophile, thus hindering or not hindering its approach to the carbon atom. Polar aprotic solvents, like tetrahydrofuran, are better solvents for this reaction than polar protic solvents because polar protic solvents will be solvated by the solvent hydrogen bonding to the nucleophile and thus hindering it from attacking the carbon with the leaving group. A polar aprotic solvent with low dielectric constant or a hindered dipole end will favour SN2 manner of nucleophillic substitution reaction. Examples-DMSO,DMF,acetone etc."

My apologies.

[Dan]

That the solvent is aproic is far more important for S_N2 reactions than polarity. Polarity in S_N1 reactions shows a relatively large effect on rate because there are charged intermediates to solvate. If you consider the transition state [TS] for the rate determining step of S_N1:

R-X  [R^δ+---X^δ-]  R⁺ + X^-

The rate of the reaction is dependent on the difference in energy between the starting material and the TS (activation energy). Since there is a much greater degree of charge separation in the TS compared to the starting material, a more polar solvent stabilises the TS more than it stabilises the starting material and the activation energy drops.

For S_N2 we can play the same game:

Nu^-  + R-X  [Nu^δ----R---X^δ-]  Nu-R + X^-

Now if we compare the starting materials, with -ve charge localised on the nucleophile, to the TS, which has the charge spread  over the nucleophile and the leaving group, we see a slight decrease in charge separation in the TS compared to the starting materials. As a result, S_N2 reactions with charged nucleophiles slightly favour solvents of lower polarity (which is not what most people will tell you). There is a catch though, there is a reason people do not run these reactions with hexane as the solvent. The solvent must be polar enough to dissolve the nucleophile, so the least polar solvent you can normally get away with in this respect is THF or acetone, but commonly DMF is needed to dissolve the nucleophile salt.

For S_N2 with neutral nucleophiles (e.g. R₃P, R₂S etc.), you can do the same analysis to show that there is an increase in charge separation in the TS compared to the starting materials, and that a more polar solvent will increase reaction rate.

For a nice explanation of these concepts, I recommend: Peter Sykes - A Guidebook to Mechanism in Organic Chemistry

[orgopete]

The reaction rate of SN2 reactions depends on the nucleophile concentrations. If you were to dissolve an M⁺X^- reagent in THF and DMF, the concentration would be lower in THF. This will affect the reaction rate.

[PhDoc]

It appears to me that the explanations provided thus far are hitting on multiple aspects of a multifaceted answer.

One of the most important feature of using diplar aprotic solvents in SN2 reactions is that they solvate metal cations well, however not anions. Hence, if NaBr is your nucleophile, the Na+ will be very well solvated by DMF (for example), however the Br- will be free to react as if it were in the gas phase. This is the reason why halogen nucleophilicity parallels that observed in the gas phase,

Gas Phase        F- > Cl- > Br- > I-
Dipolar Aprotic  F- > Cl- > Br- > I-

Keep in mind that Wikipedia is not written by professors... it is written (and edited) by anyone who wishes to do so and who can provide references (that may or may not be verified).

The SN2 displacement of cyclopentyl chloride with NaBr will proceed faster with DMF as the solvent vs. THF. Keep in mind THF cannot solvate the metal cation, and hence you'll be left with an unreactive contact ion pair.

I understand the confusion on this issue from having surveyed over six undergraduate textbooks, and not finding high degrees of correlation amongst them with certain concepts. This is perhaps the most frustrating part of learning O-Chem because answers to problems will differ from one book to another.

[Nescafe]

It appears to me that the explanations provided thus far are hitting on multiple aspects of a multifaceted answer.

One of the most important feature of using diplar aprotic solvents in SN2 reactions is that they solvate metal cations well, however not anions. Hence, if NaBr is your nucleophile, the Na+ will be very well solvated by DMF (for example), however the Br- will be free to react as if it were in the gas phase. This is the reason why halogen nucleophilicity parallels that observed in the gas phase,

Gas Phase        F- > Cl- > Br- > I-
Dipolar Aprotic  F- > Cl- > Br- > I-

Keep in mind that Wikipedia is not written by professors... it is written (and edited) by anyone who wishes to do so and who can provide references (that may or may not be verified).

The SN2 displacement of cyclopentyl chloride with NaBr will proceed faster with DMF as the solvent vs. THF. Keep in mind THF cannot solvate the metal cation, and hence you'll be left with an unreactive contact ion pair.

I understand the confusion on this issue from having surveyed over six undergraduate textbooks, and not finding high degrees of correlation amongst them with certain concepts. This is perhaps the most frustrating part of learning O-Chem because answers to problems will differ from one book to another.

I understand from your explanation how a polar solvent is important in SN2s involving metal cations. But why is it that they solvate cations better than anions, I imagined that they solvated both very well. Also, what if we are only dealing with a neutral nucleophile such as R-NH2 and not M+ -X sort of specie?

Dan wrote " For SN2 with neutral nucleophiles (e.g. R3P, R2S etc.), you can do the same analysis to show that there is an increase in charge separation in the TS compared to the starting materials, and that a more polar solvent will increase reaction rate."  I can see what he is saying and I kinda dont. I thought if the nucleophile is neutral you can just use THF as you no longer deal with a charged nucleophile to begin with. There is charge separation in the T.S but not very significant amount and a nonpolar aprotic solvent (THF) will give the nucleophile a better chance in reacting with the electrophile.

Nescafe.

[Nescafe]

Can anyone answer the question in my last post.

I'm mainly confused about what Dan wrote about charge separation. :S
Nescafé