Chapter 8: Nucleophilic Substitution

Nucleophiles

Nucleophile means "nucleus loving" which describes the tendency of an electron rich species to be attracted to the positive nuclear charge of an electron poor species, the electrophile .

The nucleophilicity expresses the ability of the nucleophile to react in this fashion.

In general terms this can be appreciated by considering the availability of the electrons in the nucleophile. The more available the electrons, the more nucleophilic the system.
Hence the first step should be to locate the nucleophilic center.  At this point we will be considering Nu that contain lone pairs and may be anionic, however the high electron density of a  C=C is also a nucleophile.
 

A collection of important nucleophiles are shown to the left.

Nucleophilicity trends (compared with basicity)

1. Across a row in the periodic table nucleophilicity (lone pair donation) C^- > N^- > O^- > F^- since increasing electronegativity decreases the lone pair availability. This is the same order as for basicity.
2. If one is comparing  the same central atom, higher electron density will increase the nucleophilicity,

e.g. an anion will be a better Nu (lone pair donor) than a neutral atom such as HO^- > H₂O. This is the same order as for basicity.
3. Within a group in the periodic table, increasing polarisation of the nucleophile as you go down a group enhances the ability to form the new C-X bond and increases the nucleophilicity, so I^- > Br^- > Cl^- > F^-. The electron density of larger atoms is more readily distorted i.e. polarised, since the electrons are further from the nucleus.

Note that is the opposite order to basicity (acidity increases down a group) where polarisability is much less important for bond formation to the very small proton.