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SN1 and SN2 Reactions

SN2 and SN1 Compared

\(S_N2\) \(S_N1\)
One step (concerted) mechanism Stepwise mechanism with a \(C+\) Intermediate
Rate determination based on two molecules Rate determination based on one molecule
Product changes chirality Products have inverted and retained configuration
Reactivity order of alkyl halides
Reactivity order of alkyl halides
Reaction rate improved by polar aprotic solvents Polar solvents help stabilise the carbocation
No carbocation rearrangements Carbocation rearrangements

Leaving Group (LG)

  • A good leaving group must be more electronegative than the C atom
    • This allows a partial dipole to exist
    • Also ensures that the LG will take an electron pair with it.
  • It must also be weaker than the nucleophile that is substituting it.
  • The best leaving groups are those that stabilise the negative charge in the transition state (polarizability, delocalisation of \(e^−\))
  • The greater the stability of the transition state, the lower the energy required to get through the reaction
  • Weak bases make good leaving groups, strong bases make poor ones. Alcohols, Ethers and Amines don’t make good leaving groups and should be converted to more appropriate ones,


The SN2 reaction is also known as a backside attack, because of the way that it forces the leaving group off from behind, as shown in the next example:

Steric Hinderance

Since the nucleophile needs to be able to access the \(\delta +\) C, hinderance from the other groups attached can cause the reaction to become less active.

Relative Reactivity \(<\)1 1 500 40,000 2,000,000

The Nucleophile

The stronger the nucleophile, the more quickly the reaction will happen. This can be achieved in multiple ways


Anions are much better nucleophiles than formally neutral species


Nucleophilicity usually increases going down a period of the periodic table \(\ce{I > Br > Cl}\)

Species \(\ce{OH−, NH2−, OR−}\) \(\ce{F-}\) \(\ce{Cl-}\) \(\ce{Br-}\) \(\ce{I-}\) \(\ce{TosO-}\)
Relative Reactivity <<1 1 200 10,000 30,000 60,000

The Solvent

  • The reaction is most effective in solvents that won’t protonate the nucleophile though will allow the polar species to exist in solution
  • An aprotic solvent with a low-moderate polarity is best
Solvent \(\ce{CH3OH}\) \(\ce{H2O}\) DMSO DMF \(\ce{CH3CN}\)
Relative Reactivity 1 7 1300 2800 5000

Summary of SN2

Nucleophile Product Class of Resulting Compound
\(\ce{OH-}\) \(\ce{CH3CH2OH}\) alcohol
\(\ce{OR-}\) \(\ce{CH3CH2OR}\) ether
\(\ce{X-}\) \(\ce{CH3CH2X}\) alkyl halide
\(\ce{CN-}\) \(\ce{CH3CH2CN}\) nitrile
\(\ce{NH3-}\) \(\ce{CH3CH2NH3+X-}\) alkyl ammonium salt
\(\ce{H2O}\) \(\ce{CH3CH2OH}\) alcohol


The General Reaction

Carbocation Stability

  • Since the reaction is dependent on a carbocation, its stability is paramount for the speed of the reaction
  • Carbocations on more branched structures are more stable and thus the reaction will occur more rapidly
Relative Reactivity \(<\)1 1 12 1,200,000
  • This is due to the slight dislocation of the electron density from around the carbocation

The Nucleophile

Since the nucleophile is not involved in the rate determining step, it has no effect on the rate of the reaction.

The Solvent

Since the carbocation is stabilised by a polar environments, polar solvents such as water and methanol are faster for this reaction than nonpolar solvents

Solvent Ethanol 40% Water
60% Ethanol
80% Water
20% Ethanol
Relative Reactivity 1 100 14,000 100,000