Carbocation Stability

What is carbocation?

Carbocations are also known as carbonium ions. Typically, carbocation is a term used to refer to a positively charged carbon atom that is short of electrons. Carbocation has a positive charge because it lacks electrons. This means that the carbon can accept another two electrons. This sets apart a carbocation from other cations we usually get to see and, at the same time, makes it a Lewis acid.

The primary job of a carbocation is to end its state as a carbocation. It can do this in two ways:

• It can either get rid of its positive charge.
• It can take on a negative charge.

Both these methods involve providing the electrons that the carbon is lacking.

Depending on the valence of the charged carbon, carbocations are classified into two main categories. These are:

• Protonated carbenes
• Protonated alkane

Structure of carbocations

Carbocations are majorly planar in structure. The trivalent carbon is known to be sp2 hybridised. Carbocations usually have three main substituents that are responsible for making the carbon sp2 hybridised and also lend the overall molecule its trigonal planar geometry.

The carbon atom uses three hybridised orbitals to allow single bonding to three substituents. The three substituents are known for being oriented to form an equilateral triangle. All the substituents of the carbocation remain in the same plane and have a bond angle of 120° between them. The carbon atom in the carbocation is electron deficient, and it only contains six valence electrons used in the formation of three sigma covalent bonds with the substituents.

The remaining p-orbital, though, is empty. Since there are six valence electrons on carbon, all used in sigma bonds, this is why the p-orbital that stretches above and below the plane remains unoccupied.

Types of carbocations

There are three major types of carbocation. These include:

Primary carbocation: The carbon with a positive charge gets attached to one carbon of an aryl or alkyl.

Secondary carbocation: The carbon with a positive charge gets attached to two other carbon atoms.

Tertiary carbocation: The positively charged carbon atom gets attached to three other carbon atoms.

Overview of carbocation stability

Carbocation stability is an important feature to understand. It is essential to understand the stability of carbocations because there are many features of reactivity one needs to keep in mind, especially if it is related to nucleophilic substituents. There are numerous organic reactions that are commonly used in the preparation of organic compounds, including the formation of carbocations.

One example of carbocation stability is a tert-butyl carbocation, which is more stable than an isopropyl carbocation. Since alkyl groups, ethyl, methyl, and others, are weak electron-donating groups, they end up stabilising any nearby carbocations. This is how the substituted carbocations become more stable.

Factors influencing carbocation stability

Carbocations are at their most stable when the charge is on a tertiary carbon. At the same time, carbocations are the least stable when the charge is on a primary carbon. Carbocations tend to change or shift their positive charge to the most stable configuration. This process is known as carbocation rearrangement.

There are three major factors that influence carbocation stability. These include:

Resonance

As the number of resonances increases, the stability of carbocations also increases. Furthermore, the greater the number of resonating structures, the greater the stability of the carbocations. The delocalisation of the positive charge causes the electron loss to decrease further, thus increasing the overall stability.

Comparatively, it has been found that the resonance effect is a far greater influencing factor than substitution. The structures that have resonance are more stabilised than others. Another carbocation stability example would be cyclopropane carbocation, which is very stable because of the presence of dancing resonance. This is why tricyclo-propane carbocation is one of the most stable carbocations.

Electronegativity

Electronegativity refers to the ability of an atom to draw or attract electrons to itself. So the higher the electronegativity of an atom, the greater the number of electrons the atom attracts. Carbocation stability also gets directly influenced because of the electronegativity feature of the carbon atom with a positive charge. So when the electronegativity of the carbon atom goes up, the stability of the carbocation will go down, and vice versa. For example sp > sp2 > sp3. If we take the example of vinylic carbocation, the hybridisation of the positive carbon atom is sp, whose electronegativity is higher than the sp2 alkyl carbocation hybridised carbon. Because of this, the stability of the primary vinylic carbocation is less than that of a primary alkyl carbocation.

Hyperconjugation and inductive effect

When we increase the substitution, it results in an increase in hyperconjugation, which increases the instability. The greater the hyperconjugation, the higher the stability. So the overall carbocation stability depends on the total number of carbon atom groups linked to the carbon, which has a positive charge.

Conclusion

A carbocation is any positively charged molecule of carbon. Carbocations usually have three positive bonds and even several electrons in their atoms, and they’re in a trigonal planar shape. As carbocation bears the positive charge, they lack the electrons in their octet. As a result, they are also considered electrophiles. They usually make rapid bonds with any nucleophiles. Carbocations close to another carbon-carbon double or triple bond exhibit particular stability. The charge “moves” from atom to atom in this action known as “delocalisation.” Even primary carbocations, which are ordinarily exceedingly unstable, are amazingly simple to generate when close to a double bond, to the point that they will participate in S_N1 reactions.