Introduction: 

Heterolytic and homolytic bond fissions result in the formation of short-lived fragments called reaction intermediates. Among the important reactions, intermediates are carbonium ions.

Until the early 1970s, all carbocations were called carbonium ions. In present-day chemistry, a carbocation is any positively charged carbon atom, classified into two main categories according to the valence of the charged carbon such as three in the carbenium ions and five in the carbonium ions. The nomenclature proposed by G. A. Olah is characterized by a three-center two-electron delocalized bonding scheme and is essentially synonymous with so-called non-classical carbocations, which are carbocations that contain bridging C-C or C-H sigma bonds.

Carbocations are reactive intermediates in many organic reactions. This idea was proposed by Julius Stieglitz in 1899 and was further developed by Hans Meerwein in his 1922 study of the Wagner Meerwein rearrangement. Carbocations were also found to be involved in the SN1 reaction, and rearrangement reactions. Thus the reactive intermediate that is formed by heterolytic fission is called carbocations. The products of heterolytic fission are ions. Reactions that involve heterolytic fission take place at measurable rates. Heterolytic fission occurs most readily with polar compounds in polar solvents.

What are carbocations?

Organic ions which contain a positively charged carbon atom are called carbocations or carbonium ions. They are formed by heterolytic fission.

In heterolytic bond fission of a C-X bond in an organic molecule, if X is more electronegative than the carbon atom, the former takes away the bonding electron pair and becomes negatively charged (: X-). In this, an ion becoming a positive charge is also formed.

The positively charged species are called carbocations.

Structure of carbocation:

The charged carbon atom in carbon is a ‘’sextet “; it has only six electrons in its outer valence shell instead of the eight valence electrons that ensure maximum stability octet rule. Therefore, carbocations are often reactive, seeking to fill the octet of valence electrons as well as regain a neutral charge.

The positively charged carbon atom in a carbonium ion uses Sp2 hybrid orbitals to form three sigma bonds. An empty p orbital extends above and below the plane of the sigma bonds. This empty p orbital makes the carbon atom electron-deficient and gives it a positive charge. Thus a carbonium ion will combine with any substance like nucleophiles which can donate a pair of electrons. 

A carbocation is generally observed in an SN1 reaction, elimination reaction, etc.

Thus carbocation has a planar structure having all the three-sigma bonds in one plane with the bond angles of 120 degrees between them.

Classification of carbocations:

The different carbocations are named based on the number of carbon groups bonded to the carbon. The carbocations can be termed as methyl, primary, secondary or tertiary on the basis of how many carbon atoms are attached to them.

• Methyl carbocation: If no carbon atom is attached to the carbon with the positive charge it is simply called methyl carbocation.

• If one, two, or three carbon is attached to the carbon with the positive charge it is called the primary, secondary, and tertiary carbocation respectively.

• If there is a presence of a carbon-carbon double bond near the positively charged carbon it is termed allylic carbocation.

• In the same way, if the carbon with the positive charge is attached to a double bond, the carbocation is termed Vinylic carbocation.

• Whenever the carbon which consists of the positive charge is part of a benzene ring, then the carbocation is an aryl carbocation.

• If the carbon having a positive charge is immediately next to a benzene ring, it is termed a benzylic carbocation.

Stability of carbocations:

The stability of the carbocation can be estimated by the inductive effect and the hyperconjugation phenomenon that directly depends on the number of alkyl substituents adjacent to the central positive carbon atom. Thus the carbocation stability increases with the increase in the number of the alkyl groups.

This explains why tertiary carbocation is more stable than secondary which is, in turn, more stable than methyl carbocation.

Two factors are termed to play a role in this case:

• + I effect: Electron donating inductive effect of alkyl group the more alkyl group attached to the carbon the greater is the stability effect.

• Hyperconjugation: By hyperconjugation some charge delocalization occurs and the sigma bond of beta C-H. However, the presence of electron attracting groups adjacent to the carbon atom bearing a positive charge makes the carbocation less stable.

• Resonance: The stability of carbocations increases with the increasing number of resonances. The more the number of resonating structures is, the more is the stability of the carbocation. The reason for this is the delocalization of the positive charge. The electron deficiency is decreased due to the delocalization and thus it increases the stability.

Reactions of carbocations: 

The carbocations being very reactive and unstable undergo reactions as soon as they form to give stable products.

1. Combination with Nucleophile: The carbocation may combine with a species possessing an electron pair.

2. Elimination or loss of proton: The carbocation may lose a proton from an adjacent carbon 

atom.

3. Rearrangements of carbocations: 1,2 shift an alkyl or aryl group or hydrogen migrates with its electron pair to the positive center leaving another positive charge behind.

Conclusion:

Carbocations are carbon atoms in an organic molecule bearing a positive formal charge. Therefore they are carbon cations. Carbocations have only six electrons in their valence shell making them electron deficient. Thus, they are unstable electrophiles and will react very quickly with nucleophiles to form new bonds. Because of their reactivity with heteroatoms, carbocations are very useful intermediates in many common organic reactions.