Nucleophilicity versus Basicity

Chapter 8: Nucleophilic Substitution

Nucleophilicity and basicity are very similar properties in that species that are nucleophiles are usually also bases (e.g. HO- , RO-).

This is not too surprising since in the LEWIS sense, they are functioning as LONE PAIR donors (i.e. both are LEWIS BASES), compare the two pairs of reactions mechanisms shown below to convince yourself.

However, it can avoid confusion by keeping these two types of reactivity separated because there are important differences....

1. Nucleophilicity:
 

nucleophile reacting with a carbocation
Both these reactions depict a nucleophile reacting with an electrophilic C atom
nucleophile reacting to displace a leaving group (S<sub>N</sub>2)

Kinetically controlled reactions of lone pair donor with an electrophilic carbon (usually) atom resulting in the formation a new C-X bond.
2. Basicity:
simple acid - base reaction
Both these reactions depict a base reacting with an electrophilic H atom, a proton
nucleophile reacting to displace a leaving group (S<sub>N</sub>2)

Equilibrium (thermodynamic) reaction of the lone pair donor with a proton, forming a new H-X bond

The following general reaction mechanisms show you why it is important to appreciate the differences, since an anion that is reacting as a nucleophile will result in a substitution, but if it reacts as a base, then elimination will result, be it on a carbocation (SN1 vs E1) or with the loss of a leaving group (SN2 vs E2).
 
 

Substitutions
as a nucleophile, attacking a carbocation (S<sub>N</sub>1)
SN1
as a nucleophile displacing a leaving group (S<sub>N</sub>2)
SN2
Eliminations
as a base removing H+ from a carbcation (E1)
E1
as a base, removing H+ and loss of LG (E2)
E2