[betterbesafehero]

Please elaborate it with a picture and mechanism. I studied wikipedia page but have some doubt.

Please also tell that why it is called Sni, and how it is different from SN1 and SN2 mechanism ?

Is it unimolecular or bimolecular ?

[Archer]

It is, effectively an S_N1 reaction. Normally in S_N1 of chiral molecules the carbocation intermediate is planar so there is no retention of configuration as the neucleophile can attack either face.

However in S_Ni the neucleophile can only attack one face as it is part of the reagent used in the reaction.

IUPAC designation is D_N + A_ND_e

The classic example is thionyl chloride reacting with a chiral alcohol. The neucleophilic Cl^- comes from the intermediate sulfurochloridoite ion which is stronly and closely ion-paired with the carbocation (which is planar). because of this coordination the Cl^- cannot attack from behind the planar carbocation so it can only attack the front.

The following website has a good mechanism

http://www.chemthes.com/rxn-type_dp.php?type=10

Another example is the decomposition of alkylchloroformates to aklyl chlorides and CO₂

[betterbesafehero]

That's pretty similar to what wikipedia page suggests. Anyway thanks a lot..!