Chapter 6

Nucleophilic substitution Reactions
 

Structure Function Correlation with Nucleophile
  • In SN2 reaction stronger the nucleophile faster would be the reaction.
  • Strength of a nucleophile can be determined by the following general guidelines-
 
  1. A nucleophile with negative charge is more powerful than its conjugate acid.

 

 Example: NH2¯ >NH3, OH ¯ >H2O,
    1. Nucleophilicity generally follows similar order as basicity

 

 Example: R3C ¯  > R2N ¯  > RO ¯  > F ¯
    NH2¯ > RO ¯ >R2NH ¯ > ArO ¯ > NH3 > Pyridine > F ¯ > H2O > ClO4¯
    1. Going down in a group, nucleophilicity increase while basicity decrease
  Example:  I ¯ >Br ¯ > Cl ¯  > F ¯(less solvation, Higher polarization)

 

   S¯ > O¯
    1. More free nucleophile, more nucleophilicity

 

 Example 1: Rate of reaction using NaOH can be largely enhanced by specifically solvating cation Na+. (use of crown ether).
 
  Example 2:  NaOHDMSO > NaOHwater(basicity)
  In water Na+ and HO¯ both are solvated while in DMSO Na+ is solvated preferably than HO¯ leaving HO¯ as free nucleophile.