Organic Chemistry| Carbocations: Stability, Formation and Reactions

A carbocation is a species where a carbon atom bonds to three carbon atoms and has a positive charge. Carbocations are electron deficient species and therefore very reactive and unstable.

The simplest carbocation is the methyl cation CH₃⁺:

Fig. 1: The methyl cation is the simplest carbocation. It has only six electrons in the outer shell and is electron deficient

The methyl cation has only three bonds to the positively charged center, only six electrons in the outer shell and is electron deficient. It is planar trigonal with an empty p orbital.

The methyl cation is so unstable that is rarely formed even in the gas phase. Most stable carbocations have extra stabilization mainly from electron donating functional groups but even these relatively stable cations cannot be detected in normal solutions by NMR.

Carbocations are characterized as primary, secondary or tertiary depending upon the number of bonds that the carbon atom with the positive charge has with other carbon atoms (Fig. 2). As the methyl or in general the alkyl groups are electron-donating tertiary carbocations show extra stability and different reactivity than the primary.

Primary carbocations have a carbon atom with a positive charge that has only one bond with another carbon atom

Secondary carbocations have a carbon atom with a positive charge that has two bonds with  two carbon atoms.

Tertiary carbocations have a carbon atom with a positive charge that has three bonds with three carbon atoms.

Fig. 2: Primary, secondary and a tertiary carbocation. The methyl (alkyl in general) group is electron-donating and stabilizes the carbocations. The most stable of the three is the tertiary carbocation.

A few other interesting cases of carbocations are the following:

Allylic carbocations: Carbocation where the carbon atom with the positive charge is immediately adjacent to a carbon-carbon double bond.

Benzylic carbocations: Carbocation where the carbon atom with the positive charge is immediately adjacent to a benzene ring.

Vinylic carbocations: Carbocation where the carbon atom with the positive charge is part of an alkene.

Aryl carbocations: Carbocation where the carbon atom with the positive charge is part of a benzene ring.

Fig. 3: Allylic, vinylic, benzylic and aryl carbocations (where R = alkyl)

Carbocations are by their very nature unstable species. However, there are certain factors that stabilize them such as:

1. Resonance
2. Inductive effects 
3. The # of carbons atoms attached to the C+ atom

A detailed description on carbocation stability is given on the post entitled “Carbocations: Factors affecting Stability”.

Dr. George Andrew Olah, a distinguished professor at the University of Southern California, received the Nobel prize in 1994 for his research regarding the generation-stabilization and reactivity of carbocations in superacids.