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Pinacol Pinacolone - Definition, Rearrangement Reaction, Mechanism Steps, Applications and Uses

 

Pinacol is a white solid organic diol compound. It is composed of two hydroxy groups (-OH) on adjacent carbon atoms, basically it is a vicinal diol. In contrast, pinacolone is a colorless liquid ketone with the molecular formula C6H12O. It smells like peppermint or camphor. The rearrangement of pinacol-to-pinacolone is called the pinacol-pinacol rearrangement.

Table of content

  • What is pinacol and pinacolone?
  • Rearrangement reaction
  • Pinacol pinacolone rearrangement reaction
  • Pinacol pinacolone rearrangement mechanism
  • Applications of pinacolone
  • Practice problems
  • Frequently asked questions

What is pinacol and pinacolone?

As the name implies, pinacol contains alcohol functional group and pinacolone contains ketone functional group. Pinacol is a white solid organic compound. Pinacol is a compound with two hydroxyl groups bonded to adjacent carbon atoms.

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Pinacolone is an organic compound containing ketone functional groups. It is a colorless liquid with a scent like peppermint or camphor.

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Rearrangement reaction

Rearrangement reactions occur when the carbon skeleton of a molecule is rearranged. The rearrangement gives rise to a structural isomer, which is associated in its chemical composition with the original molecule. The mechanism involves the migration of a substituent from one atom to another under the conditions of the reaction. The migration happens in the same molecule to preserve the total chemical composition in terms of the elements involved. .

The rearrangement reaction cannot be satisfactorily represented by the mere transfer of a discrete electron, which is represented by curved arrows when writing down the reaction. For a better representation, we have to consider the migration of an alkyl group along with a bond. This migration should happen fluidly for the least disturbance. Ionic bond breaking and forming do not occur in this reaction. Additionally, orbital interactions also provide a better theoretical model for pericyclic reactions than discrete electron transfers. In spite of all these discoveries, textbooks often employ the curved arrow method of discrete electron transfer to elucidate the reaction because one could arrive at the same result by migrating electrons. These methods may not be necessarily accurate.

Pinacol pinacolone rearrangement reaction

Pinacol pinacolone rearrangement is an organic reaction characterized by the formation of ketones by dehydration of alcohols. This reaction is possible when concentrated sulfuric acid, heat, and boiling chips catalyze the Pinacol rearrangement among 1,2-diols such as glycols/vicinal diols. Pinacol is an alcohol and is rearranged into ketones. This reaction follows the reaction mechanism of image to form a carbonyl group from the diol. The 2,4-DNP test is used to identify the pinacolone. This test helps to detect the presence of aldehydes and ketones in organic compounds. After adding a few drops of reagent to the product, an organic precipitate is formed, indicating the presence of carbonyl groups. 

 Detection of carbonyl groups in the product indicates that the pinacols are rearranged to form the pinacolone. The 2,4-DNP reagent has a lone pair of electrons. It becomes nucleophilic and attacks the carbonyl groups because they are electrophilic and leads to the formation of a compound 2,4-DNP (dinitrophenylhydrazone), which is an orange precipitate.

The pinacol-pinacolone rearrangement proceeds through the formation of a positively charged intermediate (carbocation). The methyl group of this intermediate moves from one carbon to another. This reaction can be given as:

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Pinacol pinacolone rearrangement mechanism

The rearrangement reaction of pinacol to pinacolone occurs through the following stepwise mechanism:

Step 1: Since the reaction is carried out in acidic conditions. In this step, a proton or hydrogen ion i.e. H+ is released from the acid. Usually, H2SO4 acid is considered. It breaks down to form an H+ cation and SO42- anion. The cation thus released is attached to the hydroxyl group present in pinacol. Therefore, the hydroxyl group of pinacol is protonated.

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Step 2: A carbocation is formed in this step. The water molecule detaches from the compound and results in the formation of a carbocation. This carbocation is of tertiary type and hence stable but it has a chance to get more stability.

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Step 3: Migration of the methyl group occurs in this step just to make the intermediate more stable. The methyl group shifts from one carbon atom toward another positively charged carbon atom.

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Step 4: This step is the step of deprotonation. The protonation which occurred in Step 1 is now headed towards deprotonation. The proton which got attached to pinacol initially now detaches from it, resulting in the formation of pinacolone as the final product.

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Uses of pinacolone

Mentioned below are some of the uses of pinacolone produced from pinacol:

1. It finds extensive use in herbicides, fungicides and pesticides. These chemicals are widely used in agriculture to prevent fungal diseases in crops. Fungal diseases are one of the most common factors behind crop failure if proper care is not taken.

2. It is beneficially used in the pharmaceutical industry. It is an active component in many topical antifungal medications. Such medications should be used cautiously to prevent severe reactions to the body or the area where it is applied. The medications derived from pinacolone are flammable and can only be procured through a doctor’s prescription. Due to the strong nature of the compound, it is not advisable to use it in excessive amounts.

Practice problems:

Q1. Which intermediate carbocation is more stable in pinacol pinacolone rearrangement?

(A) 1o carbocation (B) 2o carbocation

(C) 3o carbocation (D) 4o carbocation

Answer: (C) 

Solution: 3o Carbocation is more stable in pinacol pinacolone rearrangement.

Q2. What is the IUPAC name of pinacolone?

(A) 3,3-dimethyl-2-butanone. (B) 2,3-dimethyl-2-butanone.

(C) 3,3-dimethyl-2-pentanone. (D) 3,3-dimethyl-2-Hexanone.

Answer: (A) 

Solution: The IUPAC name of pinacolone is 3,3-dimethyl-2-butanone. The structure of pinacolone is 

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Q3. What is the IUPAC name of the simplest pinacol?

(A) 2,3-dimethyl-2,3-butanediol (B) 3,3-dimethyl-2,3-butanediol

(C) 2,3-dimethyl- 2,3-pentanediol (D) 2,3-dimethyl-2,3-Hexanediol

Answer: (A)

Solution: The IUPAC name of the simplest pinacol is 2,3-dimethyl-2,3-butanediol.The structure of pinacol is 

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Frequently asked questions:

Question 1. How to synthesize pinacol? 
Answer: Pinacol can be synthesized by the Pinacol coupling reaction of acetone. First, the ketone is heated with magnesium amalgam and then reduced with water.

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Question 2. What is migratory aptitude?
Answer: The ease with which any group undergoes nucleophilic 1,2-shift is known as its migratory aptitude. All the groups do not migrate with equal ease in 1,2-nucleophilic rearrangement.

Question 3. What is the order of migratory aptitude?
Answer:  This process is not limited to 1,2-methyl shifts. Carbon can have different types of groups attached to it. Some of the group may rearrange more readily than others. Depending on the reaction conditions and on the nature of the substrate, it could be deduced which group might migrate.

Generally, the order of migration of groups can be given as:

H > aryl > alkyl

Question 4. Compare the migratory aptitude of the p-methylphenyl and p-nitrophenyl group?
Answer: The migratory aptitude of the p-methylphenyl group will be more than p-nitrophenyl because the benzene ring is activated in case of p-methylphenyl and has more electron density than p-nitrophenyl. Hence it moves faster than the p-nitrophenyl group. 

Related Topics:

Ethanol

Phenol

Ethers-classification and preparation

Ketone

Ethers-physical and chemical properties

Methanol

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