Solvent effects in nucleophilic substitution reactions of alkyl halides

Solvent Effects on Nucleophilic Substitution Reactions
SN1

• Polar solvent stabilizes transition state and carbocation intermediate.
  
• Polar protic solvent makes nucleophile less nucleophilic and stabilizes anionic leaving
  group. SN2
  
• Need polar solvent to dissolve nucleophile.
  
• Protic solvent slows rate by solvating nucleophile
  
• Aprotic solvent increases rate by binding cation and thus freeing nucleophile.
  

Example of effect of solvent on rate of reaction:

CH3CH2CH2CH2CH2X

+ NaCN SN2
CH3OH 20 h
Δ
DMSO 20 min

CH3CH2CH2CH2CH2CN 71 %
91 %

+ NaX

reaction 1
X = Br
reaction 2 X = Cl

Summary of SN1 versus SN2

SN1 SN2

Bond-breaking first (in rate-determining step)
Need stable carbocation (3° R-X)
First order reaction rate
If chiral alkyl halide → racemic product Carbocation rearrangements possible

Simultaneous bond breaking and bond forming in rate-determining step
Need minimal steric hinderance (methyl or 1° R-X)
Second order reaction rate
If chiral → inversion of chiral center

Polar, protic solvent Poor nucleophile

If 2° R-X
Polar, aprotic solvent Strong nucleophile