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🥼organic chemistry review

Pinacol Rearrangement

Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025

Definition

The Pinacol rearrangement is a type of organic reaction that involves the rearrangement of a pinacol compound, which is a 1,2-diol, to form a ketone or aldehyde. This rearrangement is an important tool in organic synthesis for the formation of carbon-carbon bonds and the creation of more complex molecules.

5 Must Know Facts For Your Next Test

  1. The Pinacol rearrangement is initiated by the protonation of one of the hydroxyl groups, leading to the formation of a carbocation intermediate.
  2. The more stable carbocation intermediate then undergoes a 1,2-migration of one of the alkyl or aryl groups to the other carbon, forming a new carbon-carbon bond.
  3. The Pinacol rearrangement is often used in the synthesis of ketones and aldehydes, which are important functional groups in organic chemistry.
  4. The stereochemistry of the starting pinacol compound is retained in the product of the Pinacol rearrangement.
  5. The Pinacol rearrangement is a useful tool for the construction of more complex organic molecules, as it allows for the formation of new carbon-carbon bonds.

Review Questions

  • Describe the mechanism of the Pinacol rearrangement, including the formation of the carbocation intermediate and the migration of the alkyl or aryl group.
    • The Pinacol rearrangement begins with the protonation of one of the hydroxyl groups of the pinacol compound, forming a carbocation intermediate. This carbocation is stabilized by the migration of one of the alkyl or aryl groups from the adjacent carbon, resulting in the formation of a new carbon-carbon bond. The migrating group is typically the more stable of the two, leading to the formation of a ketone or aldehyde product. The stereochemistry of the starting material is retained in the final product.
  • Explain the importance of the Pinacol rearrangement in organic synthesis and how it can be used to construct more complex organic molecules.
    • The Pinacol rearrangement is a valuable tool in organic synthesis because it allows for the formation of new carbon-carbon bonds, which are essential for the construction of more complex organic molecules. By rearranging a pinacol compound, the Pinacol rearrangement can be used to introduce new functional groups, such as ketones and aldehydes, that can be further manipulated or incorporated into larger, more intricate structures. This reaction is particularly useful in the synthesis of natural products and other biologically active compounds, where the ability to form new carbon-carbon bonds is crucial for the creation of diverse molecular scaffolds.
  • Analyze how the stereochemistry of the starting pinacol compound is retained in the product of the Pinacol rearrangement, and discuss the significance of this stereospecificity in organic synthesis.
    • The Pinacol rearrangement is a stereospecific reaction, meaning that the stereochemistry of the starting pinacol compound is preserved in the final product. This is because the migration of the alkyl or aryl group during the rearrangement occurs in a concerted, intramolecular fashion, without the involvement of any external nucleophiles or electrophiles that could potentially disrupt the existing stereochemistry. The retention of stereochemistry is important in organic synthesis because it allows for the precise control and manipulation of molecular architecture, enabling the synthesis of complex, stereochemically defined target molecules. This stereospecificity is particularly valuable in the synthesis of natural products and other biologically active compounds, where the three-dimensional structure of the molecule is often crucial for its function and activity.