A carbocation is a reactive intermediate that has a carbon atom bearing a positive charge and three bonds to that carbon instead of four.
Since all carbocations carry a positive charge on a carbon atom - there's an easy way to remember this in the name. A cation is a positive ion and the carbo- indicates a carbon atom. There are different types of carbocations and their structure determines the relative stability and reactivity of the carbocations.
A primary carbocation is attached to only one other alkyl group. In a general structural formula you may see the alkyl group denoted as an ‘R’ group.
Figure 1. Primary carbocation general formula and examples of primary carbocations.
In a secondary carbocation the carbon carrying a positive charge is attached to 2 other alkyl groups. R and R’ represent different alkyl groups which may be the same or different.
Figure 2. Secondary carbocation general formula and examples of secondary carbocations.
A tertiary carbocation is a positively charged carbon that is attached to 3 alkyl groups. R, R’ and R’’ are alkyl groups and may be the same or different.
Figure 3. Tertiary carbocation general formula and examples of tertiary carbocations.