Rearrangement Reaction of Alkyl Carbocation

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Introduction

The intermediate species which possess a positively charged carbon are called carbocation or more commonly known as carbonium ions. In carbocation or carbonium ion, it has unhybridized 2p orbital and sp2 hybridised carbon. The bond angle in the carbocation is 120 degrees. If we talk about carbocation rearrangement reaction, it can be defined as the movement of the carbocation from an unstable state to a more stable state of carbocation with the help of different structural organizational shifts within the molecule”. Alkyl carbocation is a carbocation that has an alkyl group. They are known to be the most common carbocation.


Characteristics of a Carbocation

There are several characteristics of carbon cations, which are discussed below:

  1. Carbocation cation is positively charged ion.

  2. It is diamagnetic in nature i.e all electrons are completely filled in shells.

  3. Carbocations are trivalent in nature.

  4. They have six electrons in valence or outermost shells.

  5. They are produced due to heterolytic cleavage.

  6. All electrons in carbocations are bonded electrons.

  7. Hybridisation in carbocation is sp2 hybridisation.

  8. Carbocation has planar geometry.

  9. They act as electrophilic reagents i.e one that has an electron-withdrawing tendency.


Formation of Carbocations

There are several methods by which formation of carbocation occurs, some of them are discussed below:

1. By protonation of Alkene, Alkynes, Alcohols or Carbonyl Compounds

Under this reaction, alkene, alkyl, alcohols or carbonyl compounds undergo protonation reaction in several steps and lead to the formation of carbocations. 

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2. By Hydrolysis or Ionisation of Halides

Under this process, halides react with acidic halides to form carbocations.

3. Deamination of Amines by Nitrous Acid

Under this process, amides react with nitrous acid to form carbocations.


Stability of Carbocations

Stability of carbocation is explained by resonance or H--effect or I--effect.

The relative stability of carbocations is given below:

Triphenyl > benzyl > 30 > 20 > 10> Vinyl


Reactions Observed in Carbocations

  1. Combination with nucleophile: Carbocation reacts with nucleophilic molecules as they have a tendency to withdraw electrons to form alcohol.

  2. Elimination of protons to form alkene: Under this process, protons from carbocation are eliminated to form alkene molecules.

  3. Rearrangement to form more stable carbocation: In this process, carbocation undergoes rearrangement reaction to form a more stable carbocation.


Rearrangement Reaction to form more Stable Carbocation

Rearrangement in carbocation happens only when alcohol is converted into several carbocations. The process or reaction is termed as rearrangement reaction to form carbocations. Carbocation has a positive charge on the carbon atom of a molecule which is connected by three more groups to form a complete structure. Rearrangement of carbocations are of two types: one is alkyl shift and another one is the hybrid shift.


Alkyl shift

When a carbocation does not have a hydrogen atom that is present on the adjacent carbon atom, which is easily available or present to undergo rearrangement reaction, then this shift is known as alkyl shift. Sometimes, it also happens that hybrid shift is not able to form stable carbocation than in such cases, alkyl shift is applied. 


Mechanism of Alkyl Shift Rearrangement Reaction

During this, rearrangement reaction in the very first step electrons align themself from C---C bond. As soon as a pair of electrons is donated into the empty p-orbitals, one C---C bond starts to break itself to form a new C---C bond. 

The pair of electrons from the C-C bond must align with the empty p orbital on the carbocation ( this means they have to be aligned in the same plane). Then, as the pair of electrons from the C–C bond is donated into the empty p-orbital, one C–C bond begins to break and the new C–C bond begins to form. Partial bonds between the carbon being transferred and each of the two adjacent carbon atoms are there in the transition state. In the last step of the reaction, there is tertiary carbocation as an end product. 

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Hybrid Rearrangement Reaction

Rearrangement reaction involving hydrogen is called hybrid rearrangement reaction. The most common example is the 1,2-hydride shift.

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Yes or no based questions

1. is it necessary that such shifts occur only from the carbon atom adjacent to the positive charge?

Answer: Yes

2. Can both reduction and rearrangement reactions occur simultaneously?

Answer: No

FAQ (Frequently Asked Questions)

1. Why Rearrangement doesn’t occur at 1 butyl Carbocation? What is the product when 1butene reacts with HCl?

Answer: Immediately after the formation of the tertiary carbocation, no rearrangement can occur that will make it more stable.

2. Define Rearrangement reaction in Carbocation.

Answer: It can be defined as the movement of the carbocation from an unstable state to a more stable state of carbocation with the help of different structural organizational shifts within the molecule.

3. Explain the term Alkyl Shift.

Answer: When a carbocation does not have a hydrogen atom that is present on the adjacent carbon atom, which is easily available or present to undergo rearrangement reaction, then this shift is known as alkyl shift. Sometimes, it also happens that the hybrid shift is not able to form a stable carbocation. In such a case, alkyl shift is applied.