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?




The two are related, in that most nucleophiles are (Lewis) bases and vice versa. Some good nucleophiles are also strong bases, e.g. $\ce{HO-}$. However, a species can be a good nucleophile and a weak base, e.g. $\ce{I-}$; or a species can be a weak nucleophile and a strong base, e.g. $\ce{t-BuO-}$. How can we separate this behavior?



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> Nucleophilicity is a kinetic phenomenon.
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[Nucleophilicity](http://en.wikipedia.org/wiki/Nucleophile#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:


$$\ce{CH3I ->[Nu-][H2O] CH3Nu}$$


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 $\equiv 1$).



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> Basicity is a thermodynamic phenomenon.
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Basicity is based on the position of equilibrium:


$$\ce{B + HSol <=> BH+ + Sol-}$$


 30

There is a difference indeed: **basicity** is a particular kind of **nucleophilicity**. A **nucleophile** is a chemical species that donates an electron pair to an electrophile. A nucleophile can also be called a **base** when this donation occurs towards a particular electrophile, which is an hydrogen ion (a proton). 


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$\ce{H2O}$ has a $\mathrm{p}K\_\mathrm{a}$ of approximately 15, giving us $\ce{OH-}$ a reasonably strong base but also good nucleophile which you'll find competing between elimination and substitution reactions. Now $\ce{H2}$ has a pka of 30 which makes its conjugate base ($\ce{H-}$) an even stronger base. $\ce{CH4}$ which has a $\mathrm{p}K\_\mathrm{a}$ of 50, and its conjugate base ($\ce{CH3-}$) which is an extremely strong base, in fact it's strong enough to deprotonate any acid below it. 


Think of a strong base being equal to or stronger than $\ce{OH-}$. Furthermore, the higher the $\mathrm{p}K\_\mathrm{a}$ of the acid the greater the strength of the conjugate base.


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When the size of a negatively charged molecule is small then it behaves as a nucleophile, but when size of a negatively charged molecule is big then it behaves as a base. This is because of crowding around the central atom of other reactant species.


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