Tuesday, January 29, 2019

organic chemistry - What's the difference between a nucleophile and a base?


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:


CHX3INuXHX2OCHX3Nu


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+HSolBHX++SolX



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