What is Pinacol – Pinacolone Rearrangement | Pinacol – Pinacolone Rearrangement kya hai?

Molecular Rearrangement reaction & Types of rearrangement reactions:

The reactions involving the migration of an atom ( like H) or group (like methyl, ethyl, etc) from one atom to another (i.e. change in the bonding sequence within a molecule) are known as molecular rearrangements or rearrangement reactions.

Rearrangements are divided into two types, intermolecular and intramolecular rearrangement reactions. Pinacol – Pinacolone rearrangement is an example of an intramolecular rearrangement reaction.

In the intramolecular process, the group that migrates is not completely detached from the system in which rearrangement is taking place. In contrast, in the intermolecular process, the migrating group is first detached and later re-attached at another site.

A rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged with a carbocation intermediate to give a structural isomer of the original molecule.

Explain the Principle of Pinacol – Pinacolone Rearrangement with a mechanism and one application.

Principle of Pinacol Pinacolone Rearrangement

Acid-catalyzed conversion of vicinal diol, 1, 2 – diol, or vicinal glycol to give highly substituted ketone is known as Pinacol – Pinacolone rearrangement.

The pinacol rearrangement is the acid-catalyzed dehydration of glycols, which converts the glycol into an aldehyde or a ketone.

It is one of the simplest systems where an alkyl group migrates, with its bonding pair, to an electron-deficient carbon atom.

General Reaction

Example

Mechanism of Pinacol Pinacolone Rearrangement

Pinacol Pinacolone Rearrangement takes place via four steps,

Step I:- Protonation

In step-I protonation take place with the help of acid. The catalyst is either mineral acids (dilute H2SO4 or H3PO4) or Lewis acids (AlCl3, BF3, and ZnCl2) or any electrophilic reagent (PCl5). Proton gets attached to either of the oxygen atoms.

Step II:- Formation of carbocation

In this step, the water molecule eliminates with the formation of a carbocation. Tertiary carbocation formation takes place.

Step III – Rearrangement of the carbocation.

It is an important step in this reaction. Intramolecular rearrangement takes place between two adjacent carbon atoms with a shifting of the alkyl or aryl group. It is also called as 1-2 shift. Carbocation shifting to the adjacent position is also observed in this step.

If the glycol is unsymmetrically substituted then there is a choice (regioselective choice) of preferential protonation in the hydroxyl group. Of the two hydroxyl groups, the one that forms the more stable carbenium ion is protonated preferentially.

The migrating groups depend on various factors like, electron-donating & electron withdrawing factors, steric hindrance, and other strain factors on the rearrangement during migration.

Stereoelectronic factors and conformational effects also play an important role in determining whether a particular migration is favoured.

The reaction conditions (i.e. type of acid, concentration, solvent, and temperature) also make influence the course of the rearrangement reaction.

The usual migration was observed generally as Aryl>H > alkyl. But the migration of H is often unpredictable. In some cases, migration of hydrogen is preferred to that of aryl and in other cases, migration of alkyl is preferred to that of H.

Step IV:- Deprotonation

In the final step, deprotonation leads to the formation of stable pinacolone.

Applications of Pinacol Pinacolone Rearrangement.

Pinacol Pinacolone rearrangement is a very important process in organic chemistry.

Tetramethyl ethylene glycol is heated with H₂SO₄ giving methyl t- butyl ketone. This haloform reaction gives trimethyl acetic acid.

Pinacolone is used in manufacturing Pesticides, Fungicides, and Herbicides.

It plays an important role as an intermediate for biologically active compounds like antiviral, antibacterial, antifungal, and antituberculous drugs. Pinacolone is also used to prepare the cyanoguanidine drug – pinacidil. Another use of Pinacolone is its use in Stiripentol, which is used in the treatment of epilepsy.

Pinacolone is also used to produce triadimefon which is used in the agriculture industry. It is also used to prepare pinacidil, as well as naminidil.

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