SN1 Reaction

*unimolecular substitution nucleophilic rxn*

• has carbocation intermediates
• results in a partially or completely racemic mixture
• tertiary carbon is favored
• nucleophile can attack on either side
• only [haloalkane] has an affect on the rate of rxn (rate-determining step)
• rate is governed by relative stabilities of intermediates
• possible for allyl and benzylic haloalkanes even if primary
• rarely occur on sp2 carbons and never on sp carbons
• polar protics are good solvents for SN1

SN2 Reaction
*bimolecular substitution nucleophilic rxn*

• no carbocation intermediates
• complete inversion of configuration
• backside attack
• primary carbons
• both [nucleophile] and [haloalkane] matter for rxn rate
• rate governed by steric hindrance (caused by bulky groups)
• never occur on sp2 or sp hybridized carbons
• polar aprotic solvents are good

Polar Aprotic Solvents

Cannot serve as hydrogen-bond donors. Do not interact strongly with ions and polar molecules. Accelerate SN2 reactions because they don't interact strongly with the nucleophile.

ex:
DMSO --> CH3SOCH3

Acetonitrile ---> CH3C=N

DMF --> HCON(CH3)2
Acetone --> CH3COCH3

Polar Protic Solvents

Hydrogen bond donors (contain -OH groups). Interact strongly with ions and polar molecules. Accelerate SN1 Reactions by stabilizing the charged carbocation intermediate.

Ex:
H2O (water)
HCOOH (formic acid)
CH3OH (methanol)
CH3CH2OH (ethanol)
CH3COOH (acetic acid)

- best leaving goups are most stable anions...(strongest conjugate acids)
Order of Leaving Groups (Best to Worst)
I-
Br-
Cl- ~ H2O
F-
CH3COO -
HO -
CH3O-
NH2-