Question

Propose a mechanism for the following pinacol rearrangement catalyzed by horon trifluoride ethyl echerate. O C 1 C C C 2 ( C C C C C 2 ) C C 1 Synthesis

Short answer

Question: Describe the step-by-step mechanism of the pinacol rearrangement of the given molecule, catalyzed by boron trifluoride ethyl etherate. Answer: The pinacol rearrangement of the given molecule involves three main steps. First, carbocation formation occurs in the presence of boron trifluoride ethyl etherate. Second, a 1,2-migration of a methyl group takes place from the adjacent carbon to the carbocation center. Lastly, the elimination of boron trifluoride ethyl etherate and a proton transfer lead to the formation of a ketone group.

Step-by-step solution

Formation of carbocation

In the presence of boron trifluoride (BF3) ethyl etherate, a Lewis acid, the carbocation formation occurs. The boron trifluoride etherate coordinates with the hydroxyl group oxygen, which weakens the bond between oxygen and its adjacent carbon atom, creating a carbocation on this carbon atom.

Methyl group migration

From the carbocation center, a 1,2-migration of a methyl group occurs, which moves one of the methyl groups from the adjacent carbon to the carbocation center. This is the essential step of the pinacol rearrangement. In this step, the bond between the migrating methyl group and its original carbon atom breaks, while a new bond forms between the methyl group and the carbocation center.

Elimination of boron trifluoride ethyl etherate

After the migration of the methyl group, the boron trifluoride ethyl etherate is eliminated, and a proton is transferred from the oxygen atom to the adjacent carbon atom. This creates a ketone group in place of the hydroxyl group present in the starting molecule. O=c1ccccc1 Thus, the final product will be a ketone, and the initial molecule undergoes the pinacol rearrangement through these steps under the catalysis of boron trifluoride ethyl etherate.

Key concepts

Carbocation Formation

In chemistry, carbocation formation is a pivotal process particularly important in the pinacol rearrangement reaction. A carbocation is essentially a positively charged carbon atom (C⁺), which is typically highly reactive and an intermediate in many organic reactions.

During a pinacol rearrangement, a Lewis acid, such as boron trifluoride (BF₃) ethyl etherate, plays a crucial role. It initiates the process by bonding to a hydroxyl (OH) group in the molecule. This interaction weakens the O-H bond and leads to the loss of a water molecule, causing a carbocation to form at the carbon atom that was previously bonded to the hydroxyl group.

The stability of the formed carbocation is a driving force of the reaction, and it dictates the path and the rate at which the subsequent steps of the rearrangement will proceed. Stability can often be enhanced through adjacent electron-donating groups or resonance stabilization.

Lewis Acid Catalysis

The concept of Lewis acid catalysis is fundamental to understanding the mechanics of the pinacol rearrangement. Lewis acids are species that can accept an electron pair. In the context of the pinacol rearrangement, the Lewis acid is boron trifluoride ethyl etherate, which acts as a catalyst. Catalysts are substances that increase the rate of a reaction without being consumed.

How does it work? As a catalyst, the boron trifluoride ethyl etherate does not undergo a permanent chemical change, which allows it to be used catalytically. It forms a temporary complex with the hydroxyl group's oxygen in the starting material. This complex formation is essential as it paves the way for carbocation formation, which is a precursor to the rest of the pinacol rearrangement steps.

By understanding Lewis acid catalysis, students can better comprehend the role of catalysts in facilitating chemical transformations while not being a part of the final product.

Methyl Group Migration

The methyl group migration represents a fascinating aspect of the pinacol rearrangement. It is a type of skeletal rearrangement where a methyl group (CH₃) moves from one carbon atom to an adjacent carbocation center. This migration is a key characteristic of the pinacol rearrangement and is essential for the overall process.

In simple terms, here's what happens: After the formation of the carbocation, the neighboring methyl group shifts to the positively charged carbon atom. This is known as a 1,2-migration and is driven by the pursuit of a more stable molecular structure. The reaction typically favors the migration of the group that leads to the most stable carbocation intermediate.

Understanding this migration is critical as it not only helps in the synthesis of complex molecules but also in comprehending the intrinsic movement of groups within a molecule during chemical reactions.

Ketone Synthesis

The culmination of the pinacol rearrangement is ketone synthesis. Ketones are organic compounds characterized by a carbonyl group (C=O) attached to two carbon atoms. In the pinacol rearrangement, the hydroxyl group of the starting molecule is replaced by a ketone functionality.

The process involves several steps, starting with carbocation formation, followed by methyl group migration, which sets the stage for the final transformation. Once the rearrangement is completed, the Lewis acid catalyst is released, and the molecule undergoes a proton shift, resulting in the formation of a ketone.

This synthesis of ketones is a valuable transformation in organic chemistry because ketones are versatile intermediates and final products in both industrial and laboratory synthesis. They can be used to create a wide range of materials, including pharmaceuticals and polymers, illustrating the practical importance of the pinacol rearrangement process in chemical synthesis.