In our discussion of inductive and mesomeric effects, we have covered the concept that certain groups donate or withdraw electrons.  This has the effect of making areas of molecules electron rich or poor, and this in turn will clearly influence their reactivity with other molecules.

An example of an electron rich molecule is the phenoxide ion shown above.  Clearly, this is most likely to react with positively charged species or species with areas of electron deficiency.

A similar example to this is the sulphonation of benzene (also electron rich):

Reagents such as sulphur trioxide which tend to react with species at areas of high electron density are called electrophiles.

An electrophile is something that is "electron-loving" and therefore has itself an electron deficiency:

Here, the sulphur atom is clearly electron deficient, as it is surrounded by three inductively withdrawing, more electronegative oxygen atoms. 

The counterpart of the electrophile is the nucleophile, which we will discuss next.