Obviously it depends on the context whether you would call a particular species a nucleophile or a base but are the two terms largely synonymous or is there a difference?
Answer
The two are related, in that most nucleophiles are (Lewis) bases and vice versa. Some good nucleophiles are also strong bases, e.g. HOX−. However, a species can be a good nucleophile and a weak base, e.g. IX−; or a species can be a weak nucleophile and a strong base, e.g. t-BuOX−. How can we separate this behavior?
Nucleophilicity is a kinetic phenomenon.
Nucleophilicity is most often defined based on the relative rate of the reactions of nucleophiles with a standard substrate in a standard solvent.
For example, a standard reaction might look like:
CHX3INuX−→HX2OCHX3Nu
The nucleophilicity will be related to the relative rate constant of reaction with the nucleophile (relative to the rate constant of the reaction with water ≡1).
Basicity is a thermodynamic phenomenon.
Basicity is based on the position of equilibrium:
B+HSol−⇀↽−BHX++SolX−
No comments:
Post a Comment