Reactions
reactions are fast.
SN1 - Rate depends only on the concentration of the haloalkane. The carbocation forms
much slower than it reacts with other molecules. This makes SN1 reactions slow.
Role of solvent:
SN2 - Polar aprotic solvents favored. Examples: Acetone, THF (an ether), dimethyl
sulfoxide, n,n-dimethylformamide, hexamethylphosphoramide (HMPA).
Nonpolar solvents will also work, such as carbon tetrachloride (CCl 4 )
Protic solvents are the worst type for SN2 reactions because they ”cage,” or solvate, the
nucleophile, making it much less reactive.
SN1 - Polar protic solvents favored. Examples: H 2 O, Formic acid, methanol.
Aprotic solvents will work also, but protic solvents are better because they will stabilize
the leaving group, which is usually negatively charged, by solvating it. Nonpolar solvents
are the worst solvent for SN1 reactions because they do nothing to stabilize the carbocation
intermediate.
Role of nucleophile:
SN2 - Good nucleophiles favored
SN1 - Any nucleophile will work (since it has no effect on reaction rate)
Carbocation stability:
3
◦
carbon - most stable = SN1 favored
2
◦
carbon - less stable = either could be favored1 ◦carbon - seldom forms = SN2 favored
CH 3 +- never forms = SN2 favored
The reason why the tertiary carbocation is most favored is due to the inductive effect. In
the carbocation intermediate, there is a resulting formal charge of +1 on the carbon that
possessed the haloalkane. The positive charge will attract the electrons available. Because
this is tertiary, meaning that adjacent carbon atoms and substituents are available, it will
provide the most electron-density to stabilize this charge.