Carbocation rearrangement

Types of reactive intermediates:

There are 6 types of reactive intermediates:

1. Carbocation: 

It is a 6 electron species, which is electron deficient in nature. It is hybridised and has a planar structure having a bond angle of .

2. Carbanion

It has 8 electrons in the valence shell. It is electronically rich. It has a tetrahedral structure and the carbon atom is hybridised.

3. Free radical

A species with one or more unpaired electrons is known as a free radical. The total magnetic moment is 0 in species where all electrons are coupled.

4. Carbene

Carbenes are divalent carbon species that are neutral and contain two non-binding electrons. They are connected to two monovalent atoms or groups. Carbenes, like carbocations, are highly reactive chemical entities with short half-lives due to the core carbon atom’s valence shell having only six electrons and hence a strong tendency to complete its octet by obtaining two more electrons. 

Carbenes behave as Lewis acids or electrophiles as a result of this.

5. Nitrene

A nitrene is the nitrogen equivalent of a carbene in chemistry. Because the nitrogen atom only has 6 electrons accessible, it is classified as an electrophile. A nitrene is a reactive intermediate that is used in a variety of chemical processes.

6. Benzyne

an unsaturated cyclic hydrocarbon formed from benzene and similar to it, but with one of the benzene’s double bonds substituted by a triple bond.

Carbocation:

A Carbocation (formerly known as a carbonium ion) is a positively charged ion with one carbon atom. In a Carbonium ion, the charged carbon atom is a “sextet,” meaning it only possesses six electrons in its outermost or outer energy level.

Structure of carbocation

In a carbocation, there are six valence electrons. As a result, they have an electron shortage and act as Lewis acids.

A carbocation has the following characteristics:

• A carbocation’s carbon-bearing positive charge is hybridised and has an unhybridized orbital.

• It has a planar structure, with all three covalent bonds on planes separated by a bond angle of .

• Carbon is trivalent when it is positively charged.

• The positive carbon is electron deficient because it has a sextet of electrons.

• The order of carbocation stability is carbocation.

Stability of carbocation:

1. Inductive effect: The influence of electron-withdrawing or electron-donating groups elsewhere in the molecule on electron density in one part of the molecule.

The electron-donating groups showing effect stabilise the carbocation and the groups showing negative inductive effect ( ) effect destabilised the carbocation.

The more electron-donating groups are, the more the carbocation is stabilised.

2. Hyperconjugation: it is a stabilisation interaction that occurs when electrons in a -bond (typically ) engage with an adjoining vacant or partially filled p-orbital, resulting in increased stability. 

The electrons are stabilised due to orbital contact, which causes them to reside in a lower energy orbital.

3. Resonance effect: A carbocation’s stability is increased by conjugation with numerous bonds or a lone pair of electrons.

Carbocation rearrangement

Carbocation rearrangements are quite frequent in organic chemistry reactions, and they are described as the migration of a carbocation from an unstable state to a more stable one by various structural reorganizational “shifts” within the molecule.

The molecules can also undergo unimolecular substitution ( ) or unimolecular elimination once they’ve been rearranged ( ).

The 1,2-hydride or 1,2-alkyl shifts delivered at each phase cause carbocations to rearrange, resulting in a more stable carbocation.

For example: 

It is known that a tertiary carbocation is more stable than a secondary carbocation due to the positive inductive effect.

The following example shows the carbocation rearrangement to attain a more stable configuration.

This is an example of a 1,2-alkyl shift.

Name reactions:

Pinacol-pinacolone rearrangement:

Vicinal–diols (1,2-diols) can be reorganised into carbonyl compounds when they are treated with acids (aldehydes or ketones). 

This reaction involves the formation of a tertiary carbocation.

This reaction involves a 1,2 alkyl shift.

The reaction can be shown as:

Conclusion

Carbocations are intermediates that are reactive.  Carbocations have a sextet of electrons and are electron deficient. Super acids can produce stable carbocations.

A reaction intermediate is a small molecules entity formed by the reaction of reactants (or previous intermediates) to produce the immediately visible products of a chemical reaction.