Alkyl Shift

Chemical reactions are frequently carried out in a step-by-step method, with two or more different reactions occurring in succession. A balanced equation tells you what’s responding and what’s being generated, but it doesn’t tell you how the reaction happens. The reaction mechanism (also known as the reaction path) describes the exact, step-by-step procedure by which a reaction takes place.

The chemical reaction involves the formation of a reactive intermediate in the process of going from reactants to products.

Types of reactive intermediates:

There are six types of reactive intermediates; namely:

1. Carbocation

2. Carbanion

3. Free radical

4. Carbene

5. Nitrene

6. Benzyne

Carbocation Rearrangement

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

They have a positively charged carbon atom with three bonds, which is usually bound to alkyl groups and hydrogen atoms.

Carbocation reactions may have unexpected results. The nucleophile may add to a carbon other than the one with the leaving group in an SN1 (unimolecular nucleophilic substitution) reaction. 

Similarly, the double bond in the result of an E1 (unimolecular elimination) reaction could be in an unexpected location. Rearrangement reactions, in which the carbocation intermediate rearranges to generate a new carbocation, can often explain these observations.

Less stable carbocation (for example, 1o or 2o ) could perform a rearrangement reaction to form a more stable carbocation (2o or 3o). 1,2-hydride shifts and 1,2-alkyl shifts are the most common rearrangement processes.

1,2-Alkyl shift

A rearrangement reaction is a type of organic reaction that can occur in conjunction with a variety of other reactions, including substitution, addition, and elimination.

for example, if we have quaternary carbon next to secondary carbocation, the most probable scenario is an alkyl group migration (or just an alkyl shift).

A carbocation is a positively-charged carbon atom with six electrons. The carbocation is electron-deficient, requiring two electrons to complete the octet. This indicates that the carbocation has an empty orbital to which the C-C bond can donate a pair of electrons.

There are half bonds between the carbon being shifted and each of the two nearby carbon atoms in the transition state. We have a (more stable) tertiary carbocation as one bond lessens and the other elongates.

Carbocation-induced reactions can sometimes undergo rearrangement. Carbocation stability improves as you progress from primary to secondary to tertiary.

In the above examples shown, through a cyclic transition state involving a carbocation, a secondary carbocation is being converted into a tertiary carbocation which is more stable.

1,2- hydride shift

A 1,2-hydride shift occurs when a hydrogen atom in a carbocation migrates from a neighbouring carbon atom to the carbon atom with the formal charge of +1.

Hydride Shift reaction with SN1

SN1 stands for nucleophilic substitution reaction. The rate-determining step is represented by the slowest step in the reaction. Because this is an SN1 reaction, it is a unimolecular molecule, and Hughes-Ingold gave it this symbol. The reactions based on the electrophile are first-order reactions, whereas the reaction based on the nucleophile is zero-order. To stabilise the molecule, the hydrogen atom travels to a new place (hydride shift).

The strength of the nucleophile has no effect on this reaction; the atoms rearrange to form a stable carbocation, similar to the carbocation hydride shift. The leaving group is removed from the molecule, allowing the nucleophile to join it. The reaction is subsequently carried on by deprotonation, culminating in the SN1 reaction.

The reaction can be shown as;

1,3 hydride shift

At low temperatures, hydride changes can take place. Heating a cation’s solution, on the other hand, can easily and quickly increase the process of rearrangement.

 A 1,3-hydride shift interchanging the functionality of two different kinds of methyl is one way to account for a minor barrier.

Ring expansion reactions:

Shifting one of the alkyl groups in the ring is advantageous because it causes ring expansion and the development of less strained, five-membered groups in the ring.

The example can be shown as:

Conclusion:

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.

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.