Advices For Studying Organic Chemistry

6.14B THE EFFECT OF THE CONCENTRATION AND STRENGTH OF THE

NUCLEOPHILE

1. Neither the concentration nor the structure of the nucleophile affects the rates of
   SN1 reactions since the nucleophile does not participate in the rate-determining
   step.


2. The rates of SN2 reactions depend on both the concentration and the structure of
   the nucleophile.


3. Nucleophilicity: the ability for a species for a C atom in the SN2 reaction.
   
   
   1. It depends on the nature of the substrate and the identity of the solvent.
   
   
   2. Relative nucleophilicity (on a single substrate in a single solvent system):
   
   
   3. Methoxide ion is a good nucleophile (reacts rapidly with a given substrate):
   
   
   
   

CH 3 O− + CH 3 I rapid CH 3 OCH 3 + I−

4) Methanol is a poor nucleophile (reacts slowly with the same substrate under the

same reaction conditions):

CH 3 OH + CH 3 I veryslow CH 3 OCH 3 + I−

H

+

5) The SN2 reactions of bromomethane with nucleophiles in aqueous ethanol:

Nu− + CH 3 Br NuCH 3 + Br−

Nu = HS– CN– I– CH 3 O– HO– Cl– NH 3 H 2 O

Relative

reactivity 125,000 125,000 100,000 25,000 16,000 1,000^700  1

4. Trends in nucleophilicity:
   
   
   1. Nucleophiles that have the same attacking atom: nucleophilicity roughly
      parallels basicity.