Solvent effect in ambident nucleophiles

[Titan97]

If nucleophile is ambident and solvent is polar aprotic, more electronegative atom is free to attack the substrate.
Since more electronegative atom is a hard base, it forms a stronger bond with hard acids like carbocation. This promotes formation of carbocation by substrate. Hence, S_N1 reaction should take place

But, polar aprotic solvent favours S_N2 reaction.

So which mechanism does the reaction follow?

 

[CrazyNinja]

Can you give the exact nucleophile, substrate, solvent and reaction conditions involved? If it is a technical question, give an example. It would help in making the debate more easier to follow.

 

[Titan97]

Example, if CN- was the nucleophile, N is more electronegative. If I used polar protic solvent, Hydrogen bonding will happen with N. So N is hindered and can't act as a good nucleophile.

If the solvent was aprotic, there is nothing to stop N.

Now, (in polar aprotic solvent) N is a hard base and it promotes formation of carbocation (hard base likes hard acids). Then N will attack the carbocation. If carbocation forms, the reaction is SN1. But polar aprotic solvents are generally used for sn2 right?

 

[CrazyNinja]

The thing with organic chemistry is this uncertainty. Here is what I know: Every possible reaction happens in every possible case.

What is necessary is which reaction gives the major product. In your case, SN1 may give the major product, but we cannot decide unless the experiment is already performed.

But polar aprotic solvents are generally used for sn2 right?

Yes they are. But that doesn't mean other reactions cannot occur. As I said, every possible reaction occurs, but the amount formed could be indeed very tiny. If you are familiar with chemical kinetics, you must know the collision theory of reactions. An organic reaction takes place because of collision between molecules. The molecules can be made to collide anyway possible. And because of the different factors involved (orientation, place of collision, KE of participants,etc), all possible products will form. The major product will be decided based on how majority of the collisions occur.