Classification of Carbocation

The carbon cations are exceedingly reactive and unstable due to an incomplete octet, to name a few traits. Carbocations, in plain terms, do not have eight electrons and so they do not satisfy the octet rule.

In carbocation, the carbon is sp2 hybridised forming a trigonal planar geometry. The outer electronic configuration of carbon contains 6 electrons and the empty p orbital shows the electron deficiency. Due to this, it is referred to as an electron-deficient species, commonly known as an electrophile.

A carbocation is commonly seen in SN1 reactions, elimination reactions, and other chemical reactions.

Classification of carbocation

The amount of carbon groups linked to the carbon determines the name of the carbocation. On the basis of how many carbon atoms are connected to it, the carbocation can be classified as methyl, primary, secondary, or tertiary:

1.    Methyl carbocation: When no carbon is linked to the positive-charged carbon, it is simply referred to as methyl carbocation.

2.    The primary carbocation, secondary carbocation, and tertiary carbocation, respectively, are formed when one, two, or three carbons are connected to the carbon with the positive charge.

3.    An allylic carbocation is defined as a carbon-carbon double bond next to a carbon with a positive charge.

4.    Similarly, if a positively-charged carbon is connected to a double bond, the carbocation is known as a vinylic carbocation. The hybridisation of the positive-charged carbon is sp, and the geometry is linear.

5.    When the carbon with the positive charge is found in a benzene ring, the carbocation is called an aryl carbocation.

6.    A benzylic carbocation is defined as carbon with a positive charge that is immediately next to a benzene ring.

Surprisingly, there is another type of carbocation known as pyramidal carbocation in addition to these two. The ions in this type are made up of a single carbon atom that prefers to hover over a four- or five-sided polygon that looks like a pyramid. The charge on the four-sided pyramidal ion will be +1, whereas the charge on the five-sided pyramid will be +2.

Formation of Carbocation

The carbocations can be created using one of the two basic procedures listed below:

1.    Cleavage of a carbon bond.

2.    Electrophilic addition

1. Cleavage of a Carbon Bond

When the link between carbon and the atoms bound to it is cleaved, the leaving group removes the shared electrons. As a result, the carbon atom is electron-poor. Therefore, a positive charge develops, resulting in the formation of a carbocation. The lower the value of activation energy, the greater will be the tendency of bond cleavage or creation of a more stable carbocation.

A carbocation is generated as a reaction intermediate in numerous organic reactions, such as the SN1 and E1 reactions.

2. Electrophilic addition

An electrophile attacks an unsaturated point (double or triple bond) in electrophilic addition, causing the pi bond to break, resulting in the production of a carbocation. The lower the activation energy and the faster the addition, the more stable the carbocations are. The reaction of HBr (an electrophile) with propene (CH3CH = CH2) demonstrates electrophilic addition to a pi bond.

It should be mentioned that secondary carbocation formation is preferred over primary carbocation formation because secondary carbocation is more stable due to resonance. This is in line with Markovnikov’s Rule as well. Alkenes, alkynes, and benzene rings are all known to undergo electrophilic addition reactions.

Carbocations are extremely reactive because of their electron shortage, empty orbital, and incomplete octet. As a result, its stability is contingent on octet completeness and the reduction of electron shortage.

Formation of carbocation examples

The following processes can help a carbocation maintain its stability:

1.    Nucleophilic addition

2.    Pi bond formation

3.    Rearrangement

1. Nucleophilic addition

A carbocation is an electron-deficient octet with a positive charge and an incomplete octet. The inclusion of a nucleophile stabilises the positive charge, resulting in the development of a new covalent bond. The carbocation is stabilised as a result of this. Because the carbocation is so reactive, even a weak nucleophile can connect to it. This is a highly common mechanism of carbocation stabilisation.

2. Pi bond formation

To eliminate its positive charge and complete its octet, the carbocation can accept electrons from surrounding hydrogen. A new pi connection can thus be established. Any base, in general, must remove the hydrogen atom. Because of the high reactivity of the carbocations, even a weak base like water or the iodide ion can aid in deprotonation. When deprotonation takes place, two types of products are generated. The main product is the more stable chemical.

3. Rearrangement

The carbocation can take electrons from nearby hydrogen to remove its positive charge and complete its octet. As a result, a new Pi connection can be formed. In general, any base must remove the hydrogen atom. Because of the carbocations’ high reactivity, even a weak base like water or the iodide ion can help in deprotonation. When protons are deprotonated, two types of products are produced. The chemical that is more stable is the major product.

Conclusion

A carbocation must be distinguished from other types of cations. Thus, a carbocation has a positive charge since it is devoid of electrons, implying that the carbon can accept two more. This distinguishes it as a Lewis acid, as well as a carbocation from the other cations we usually encounter.