[Chemistry Class Notes] on Carbocation Stability Pdf for Exam

The primary job of the carbocation is to stop being a carbocation and it consists of two approaches. Either by getting rid of the positive charge or by gaining a negative charge. Both methods involve distributing the missing electrons to the electrons that lack carbon. It is essential to differentiate a carbocation from other cations.

A carbocation is short of electrons and has a positive charge, where the carbon itself is capable of accepting another two. This makes it a lewis acid and makes a carbocation different from other cations.

Definition of Carbocation Stability

Until the year the 1970s, we all knew carbocations as carbonium ions. We usually look at carbocation as the positively charged carbon atom that it is. According to the valence of the charged carbon, we can classify it into two main categories. These two categories are protonated carbenes and Protonated alkane.

The heterolytic cleavage in an organic molecule where the carbon donates the shared pair of electrons to the leaving group. Which, in turn, results in the development of a positive charge on the carbon atom. Such species where covalency of carbon is three are called carbonium ions or carbocations (e.g., CH₃⁺). The carbon atom of the carbocation is sp² hybridized, and it uses the three hybridized orbitals for single bonding to three substituents; the remaining p-orbital is empty. However, the pentacoordinate positively charged species such as CH₅⁺is called carbonium ions. G. A. Olah was the first person to propose this nomenclature. We can synthesize some stable carbocation as solid salts, and we can also synthesize some into a solution (free or ion pair). Now we will look at the classification of carbocations.

Three Types of Carbocations

• Primary Carbocation: The carbon bearing the positive charge is attached to one carbon of an alkyl or aryl group.

• Secondary Carbocation: Here, carbon with a positive charge is attached to two other carbons.

• Tertiary Carbocation: In this case, the positively charged carbon is attached to three carbon atoms.

Factors Influencing the Stability of Carbocations

• Resonance: With the increasing number of resonances, the stability of carbocations increases. More number of resonating structures results in more stability of the carbocation. The delocalization of the positive charge is the main reason. Due to delocalization, the electron deficiency is decreased and the stability is increased.

In comparison, the resonance effect is a more dominating factor than substitution. The structures with resonance are better stabilized than others. Cyclopropane carbocation is remarkably very stable because of dancing resonance. Hence, tricyclo-propane carbocation is the most stable carbocation.

• Hyperconjugation and Inductive Effect: Increasing substitution results in an increase of hyperconjugation and thus an increased instability. The higher the hyperconjugation, the higher will be its stability. The Carbocation stability as a whole depends on the number of carbon groups linked to the carbon that carries the positive charge.

• Electronegativity: Electronegativity represents the ability of an atom to attract electrons. The more the electronegativity is, the more is the electron attraction towards the atom. The stability of the carbocation gets affected directly due to the electronegativity of the carbon that has a positive charge. If the electronegativity of the carbon atom increases, the stability of the carbocation decreases. sp > sp² > sp³. In the vinylic carbocation, the hybridization of the positively charged carbon is sp, whose electronegativity is greater than the sp² alkyl carbocation’s hybridized carbon. Due to this, the stability of a primary vinylic carbocation is lesser than a primary alkyl carbocation.

Classification of Carbocation

The carbocation is termed as methyl, primary, secondary or tertiary based on the number of carbon atoms attached to it:

• Methyl carbocation: If there is no carbon attached with the positively charged carbon it is called methyl carbocation.

• If one carbon is attached to the positively charged carbon it is called the primary carbocation. 

• If two carbon is attached to the positively charged carbon it is called secondary carbocation. 

• If three-carbon is attached to the positively charged carbon it is called the tertiary carbocation.

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

• Similarly, if the positively charged carbon is attached to a double bond, the carbocation is called vinylic carbocation. Here, hybridization of the positively charged carbon is sp and geometry is linear.

• Whenever the positively charged carbon is part of a benzene ring, then the carbocation is said to be aryl carbocation.

• If the positively charged carbon is immediately next to a benzene ring, it is named a benzylic carbocation.