Question

When warmed in dilute sulfuric acid, 1-phenyl-1,2-propanediol undergoes dehydration and rearrangement to give 2-phenylpropanal. (a) Propose a mechanism for this example of a pinacol rearrangement (Section 10.7). (b) Account for the fact that 2-phenylpropanal is formed rather than its constitutional isomer, 1-phenyl-1-propanone.

Short answer

Answer: The pinacol rearrangement mechanism involves three main steps: 1) Protonation of the 1,2-diol, 2) Rearrangement of the cation intermediate, and 3) Deprotonation of the rearranged cation to form the carbonyl compound. In the case of 1-phenyl-1,2-propanediol, the phenyl group migration during the rearrangement step leads to a more stable carbocation due to resonance with the phenyl ring. Thus, the formation of 2-phenylpropanal is favored over 1-phenyl-1-propanone.

Step-by-step solution

Understand the Pinacol Rearrangement Mechanism

The pinacol rearrangement is an organic reaction that involves the acid-catalyzed rearrangement of a 1,2-diol (such as 1-phenyl-1,2-propanediol) to give a carbonyl compound (in this case, 2-phenylpropanal). The general mechanism involves three main steps: 1. Protonation of the 1,2-diol by the acidic conditions 2. Rearrangement of the cation intermediate 3. Deprotonation of the rearranged cation to give the final carbonyl compound

Apply the Pinacol Rearrangement Mechanism to This Example

Now let's apply this general mechanism to the specific case of 1-phenyl-1,2-propanediol: 1. Protonation: The oxygen atom on the 1-phenyl-1,2-propanediol molecule gets protonated by the available sulfuric acid, forming an oxonium ion. 2. Rearrangement: A 1,2-migration occurs where the bond between the carbon and the substituent (phenyl group) migrates to the adjacent carbocation carbon, creating a new positive charge on the phenyl-substituted carbon. 3. Deprotonation: Finally, a water molecule deprotonates the protonated carbonyl carbon, resulting in the formation of the final pinacol rearrangement product, 2-phenylpropanal.

Explain the Formation of 2-Phenylpropanal over 1-Phenyl-1-Propanone

During the rearrangement step of the mechanism, the migration of the phenyl group leads to the formation of a new carbocation, which is stabilized by resonance with the phenyl ring. This rearrangement is preferred because the resulting carbocation is more stable compared to the alternative migration of the methyl group. If the methyl group had migrated, the carbocation would not have the stabilization effect provided by resonance with the phenyl ring, and therefore, the resulting carbocation would be less stable. Thus, based on the stability of the intermediate carbocations, the formation of 2-phenylpropanal is favored over the formation of its constitutional isomer, 1-phenyl-1-propanone.

Key concepts

1,2-diol

A 1,2-diol is a type of organic compound characterized by having two hydroxyl (-OH) groups attached to adjacent carbon atoms in a carbon chain. In the context of pinacol rearrangement, the 1,2-diol plays a crucial role in the initial stage of the reaction. When subjected to acid, the diol is protonated on one of its hydroxyl groups. This protonation is an essential step as it transforms the stable diol into a more reactive oxonium ion. The oxonium ion is a key intermediate that sets the stage for subsequent rearrangements. The presence of the two hydroxyl groups allows for a unique set of reactions, leading ultimately to the transformation into a carbonyl compound. In the given example, 1-phenyl-1,2-propanediol is the starting compound undergoing rearrangement to produce 2-phenylpropanal.

carbonyl compound

Carbonyl compounds are a broad class of organic molecules typically characterized by a carbon atom double-bonded to an oxygen atom, which is known as the carbonyl group (\( >C=O \)). In the pinacol rearrangement, the end goal is to convert the initial diol into a carbonyl compound, such as an aldehyde or ketone. The rearrangement achieves this by rearranging the molecular structure and eliminating water. In this specific reaction of 1-phenyl-1,2-propanediol, the target carbonyl compound is 2-phenylpropanal, an aldehyde with enhanced stability attributed to its formation. The transformation from a diol to a carbonyl compound involves the migration of a group (in this case, a phenyl group) and the loss of a hydroxyl group, ultimately providing a stable and desired product.

carbocation stability

Carbocation stability is an important factor that influences the direction and products of many organic reactions, including pinacol rearrangements. Carbocations, which are positively charged carbon atoms, can be stabilized by several mechanisms like hyperconjugation and resonance. In the context of the pinacol rearrangement of 1-phenyl-1,2-propanediol, after the protonation step, the molecule undergoes rearrangement which involves the formation of a carbocation. Here, the phenyl group migrates instead of a methyl group due to the resulting carbocation's stability through resonance with the aromatic ring. This resonance stabilization makes the pathway to forming 2-phenylpropanal more favorable. If the methyl group were to migrate, the resultant carbocation would lack resonance stability, making this pathway less likely. Hence, the inherent stability of the carbocation intermediate significantly dictates the final product's structure.

acid-catalyzed rearrangement

Acid-catalyzed rearrangements are reactions that, with the help of an acid, change the structure of a molecule through the shifting of bonds. In the pinacol rearrangement, an acid such as sulfuric acid plays a pivotal role. The reaction starts with the protonation of the diol, resulting in a more reactive oxonium ion. Once the oxonium ion is formed, the acid effectively lowers the energy barrier, making rearrangements possible. The presence of the acid ensures that the molecule has sufficient energy to allow for the migration of groups and the formation of carbocations. Consequently, this results in the elimination of water and the transformation of the diol into a more stable carbonyl compound, like 2-phenylpropanal in this case. The catalytic action of the acid optimizes the reaction pathway and determines the reaction's efficiency, resulting in a smooth conversion from the original structure to the desired product.