A cation (or more simply a positive charge) is formed on one of the carbon atoms of an organic molecule, often as an intermediate of a reaction. This carbocation or carbonium ion is highly reactive and will attract a negatively charged ion, for example an OH- (hydroxide) ion.

A good example of the formation of a carbocation is in electrophillic substitution undergone by members of the homologous series of alkenes. Here the double bond posessed by the alkene molecule represents a high area of electron density. Any positive charge on an electrophile will be attracted to this area of high density - for example the delta-positive hydrogen ion of sulphuric acid (H2SO4).

The electrophile attaches itself to one of the carbon atoms, in the process breaking the C=C double bond. This results in one of the carbons having only three bonds - a violation of the valency rules. Such a positive charge usually results in a rapid further reaction, in the case of the alkenes to form an alcohol.

The positioning within the molecule of the formed carbocation on one of the carbon atoms is not a random event. More than one type of carbocations exist and their relative chance of occuring varies. A primary carbocation is a positive charge on a carbon atom which is itself only bonded by a single bond to one other carbon atom. Logically a secondary carbocation is therefore a positive charge on a carbon atom which is itself singly bonded to two other carbon atoms.

Secondary carbocations are more stable due to their ability to 'borrow' charge from their greater number of neighbouring carbon atoms. As a result 97% of carbocations formed are secondary, with the remaining 3% being primary carbocations. Interestingly tertiary carbocations, whilst possible, are highly uncommon outside unusual laboratory experiments.