What is a carbocation? Well cations are positively charged ions and “carbo” come from carbon so carbocations are positively charged carbon ions. They are intermediates in chemical reactions involving organic compounds.
Some older literature refers to carbocations as carbonium ions but the term is falling into disuse on the advice of the International Union for Pure and Applied Chemistry (IUPAC). Another generic name for carbocations is carbenium ions this term is not recommended by IUPAC.
The simplest carbocation is a carbon atom bonded to three hydrogen atoms. This is a primary carbocation. Other primary carbocations form bonds to two hydrogen atoms and to another carbon containing group or alkyl. This group can be a simple molecule such as CH3 giving the final ion the formula CH3-CH2+ or a long chain of carbon atoms.
Secondary carbocations involve the bonding of one hydrogen atom and two alkyl groups. The simplest secondary carbocation is CH3-CH+-CH3.
Finally, tertiary carbocations contain bonds with three alkyl groups. It requires more energy to form a primary carbocation than to form a secondary one, which in turn requires more energy than it requires to form a tertiary carbocation.
Alkyl groups bonded to a carbocation tend to push electrons away from themselves towards the ion. The presence of these extra electrons close to the ion stabilizes the carbocation. The more alkyl groups a carbocation has the more stable it becomes. Therefore primary carbocations are the least stable and tertiary carbocations the most stable. That said all carbocations are inherently unstable
A carbocation with a double bond to an alkyl group or benzyl is more stable than other carbocation.
In reactions carbocations are targets for negatively charged ions in particular the hydroxyl ion and halogen ions.
The chemical study of carbocation began in 1902. Scientists observed that adding concentrated sulfuric acid to the colorless organic chemical triphenylmethyl alcohol yielded a deep yellow solution. Another organic chemical triphenylmethyl chloride produced orange colored complexes when mixed with tin or aluminum chloride. These colored solutions resembled those of salts rather than organic compounds.
Because carbocations are intermediates in a chemical reaction, they are difficult to observe. George A Olah of the University of Southern California, Los Angeles, Ca. used nuclear magnetic resonance to observe the tert-butyl cation formed during the reaction between a superacid and tert-butyl fluoride. In 1994, Olah received the Noble Prize for chemistry for his work on carbocations.
The Chemguide http://www.chemguide.co.uk/
Chemistry Daily Chemistry Encyclopedia
IUPAC Gold Book http://goldbook.iupac.org/C00839.html