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?
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12$\begingroup$ Basicity is simply a special kind of nucleophilicity, namely towards protons. So, both terms are quite related but in organic chemistry the term nucleophilicity is usually used with respect to carbon. The reason why basicity and nucleophilicity correlate rather well but not completely comes from the differences between the electrophilic properties of carbon atoms and protons (using the HSAB principle one might describe the difference like this: ... $\endgroup$– PhilippNov 27, 2014 at 17:30
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3$\begingroup$ ... protons are hard electrophiles and carbon centers are rather soft electrophiles, so hard nucleophiles are better bases than soft nucleophiles and soft nucleophiles react better with carbon). $\endgroup$– PhilippNov 27, 2014 at 17:31
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4 Answers
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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?
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:
$$\ce{CH3I ->[Nu-][H2O] CH3Nu}$$
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 $\equiv 1$).
Basicity is a thermodynamic phenomenon.
Basicity is based on the position of equilibrium:
$$\ce{B + HSol <=> BH+ + Sol-}$$
answered Nov 27, 2014 at 19:46
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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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$\begingroup$ So is it always the case that a strong base is a good nucleophile? e.g the hydroxide ion is strongly basic and it's a good nucleophile? $\endgroup$– RobChemNov 28, 2014 at 17:24
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$\begingroup$ No, not really. Read the other answer for a more specific explanation of the phenomena. $\endgroup$– entropidNov 28, 2014 at 17:26
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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.
