2. SN2 Nucleophilic Substitution - Review
Recall: nucleophilic substitution of a bromoalkane
Reaction occurs when
bromoethane is added to a
dilute NaOH solution.
3. SN2 Reaction with Cyanide ions
Bromoethane + CN- ions (KCN, reflux in methanol) - mechanism
Overall
8. Effect of nucleophile on rate of
substitution
The electron density of the nucleophile affects the rate of
substitution.
- Anions are more reactive than neutral molecules.
Reactivity depends on how easily the nucleophile can
donate electrons
- N is less electronegative than O
Rate of Reaction
CN- > OH- > NH3 > H2O
9. Effect of R-X on rate of substitution
Rate of substitution is fastest when the leaving halide ion
is iodine and slowest when the leaving halide ion is
fluorine. WHY?
10. Effect of R-X on rate of substitution
Rate of substitution depends on whether the
halogenoalkane is primary, secondary or tertiary.
1o
Rate of SN2 Rate of SN1
Reaction 2o Reaction
Increases Increases
3o
11. Effect of nucleophile on rate of
substitution
The concentration of the nucleophile affects the rate of
SN2 substitution but not SN1.