There are four major categories into which organic reactions fall. These are:
To deal with 1. first, substitution usually occurs on carbon. There are two sub-categories of this type: nucleophilic, and electrophilic.
This is an example of electrophilic substitution (nitration of benzene):
Below is a characteristic example of nucleophilic substitution:
NC– is the nucleophile in this case.
Addition reactions are also sub-categorised into nucleophilic and electrophilic reactions. They can also be radical. For example, the addition to C-C double bonds is usually an electrophilic or radical process. e.g.:
This can either be initiated by attack of H+ or Br– on the double bond. However, the addition reactions to carbonyl groups (e.g. in either aldehydes or ketones) are nucleophilic. e.g.:
Elimination reactions are essentially addition reactions running backwards. e.g.:
Rearrangements can occur via intermediates that closely resemble cations, anions or radicals. However, normally we find that rearrangements occur via carbocations. During a rearrangement, as the name indicates, it is possible to change the carbon skeleton significantly. e.g.: the Pinacol rearrangement
All these reactions will be dealt with in more detail elsewhere.