Genera overview of SN1 and SN2 mechanisms.

Can someone tell me what and how SN1 and SN2 mechanism work? How do they determine the rate of reaction etc? Which mechanisms do Primary (1), Secondary (2) and Tertiary (3) follow?

If you're at uni (the thread tag suggests otherwise though)... go get the textbook out...

SN1 - simple explanation - the haloalkane (I common reagent in SN1 & SN2, though it of course applies to many other compounds), heterolytically fissions to give the halide and carbocation. This is then attacked by the nucleophile to form a C-Nu bond.

Rate limiting step is the first. This requires groups which can stabilise the resultant carbocation. This is why tertiary haloalkanes undergo this faster.

SN2 - Nucleophile approaches 180 degrees to the C-X bond, and in one concerted step the C-X bond is broken and a new C-Nu bond is formed.

Rate is limited by the nucleophilicity of the nucleophile and the steric hindrance around the central carbon. There is more steric bulk in a tertiary haloalkane than a primary so SN2 is strongly disfavored for tertiary compounds.

Thank you. I am at an short vacation, so can't get textbooks here in the Lake District :P.
If I remember on top of my head (thanks to you);

SN2 (Substitution Nucleophilic Bimolecular Process) is favoured by Primary Haloalkanes and the SN1 mechanism is favoured by tertiary structure Haloalkanes. In the SN2 Mechanism, the C in surrounded by 5 groups (at least 2 very small H's). Suppose we have an HO (an Nu : ) or an Cl (an Halide) that has bonds that are in an state of forming or breaking with the Central C, and three R (alkyl) groups. They forms an 5 coordinate transition state:
-Very short life time
-HO bonds is forming at the same time as C-CL bonds breaks

The 3 Bulky R groups cannot fit around the central carbon along with the -OH and -Cl
so the Halide (Cl) is dropped from the central carbon atom because an atom/molecule cannot from from more than 4 bonds. So we have Haloalkane + Cl^-
Tiertary haloalkanes do not favour this mechanism (Steric effect?).


SN1 Mechanism are favoured by Tertiary haloalkanes

The alkyl group stabilize a +Ve carbon. The more alkyl group that are present, the more stable is the carbocation that forms, and the more likely it is to form.

Carbon cation

Primary (Most stable) > Secondary> Primary (least stable)


Did I get something wrong? Did I made made an mistake?
Just trying to refresh my memory on this topic.