Mechanisms

S_N1 Substitution Nucleophilic - Unimolecular

S_N1 mechanism has two steps 1) rate determining dissociation of leaving group to form carbocation and then 2) attack of nucleophile on carbocation to form the substituted product.

• Rate=-k[R-LG]
• Nucleophile not important (it's not in RDS)
• 3^o > 2^o > 1^o > methyl
• no stereochemistry
• Favors polar solvents

S_N2 Substitution Nucleophilic - Bimolecular

S_N2 mechanism occurs in one step in which the nucleophile attacks alkyl halide from the back side of the C-LG bond.

• Rate=-k[Nu][R-LG]
• Nucleophile sterics and electronics important.
• methyl > 1^o > 2^o > 3^o   (Sterics are important here, see also β-effect)
• Stereochemical inversion (Walden Inversion) resulting from backside attack
• Favors polar aprotic solvents

Why backside attack?

1. LG is in the way for frontside attack
2. Backside has the required empty antibonding orbital to accept electrons from Nu (Nucleophile)

How to determine which mechanism is occuring?

Its easy for methyl, primary (1^o) and tertiary (3^o) halides and tosylates.  Methyl and 1^o halides and tosylates will undoubtedly follow the S_N2 pathway since they can not form stable carbocations required for SN1 and they have least steric hindrance (favors S_N2).  Tertiary (3^o) will follow S_N1 pathway since they can form stable cations.   Tertiary (3^o) have the most steric congestion which limits nucleophilic attack in S_N2 like reactions.  Secondary halides and tosylates are difficult to predict (in practice they are even more difficult to predict).  Bare in mind that elimination reactions (E1/E2) can compete with substitution and often times elimination products form as well, especially in the presence of nucleophiles that are very basic.