a. Name the kind of sigmatropic rearrangement that occurs in each of the following reactions.
1.
2.

Verified step by step guidance

1. Step 1: Analyze the first reaction. The structure on the left shows a conjugated diene system with a methyl group at one end and an ethyl group at the other. Upon heating (indicated by the triangle symbol), the molecule undergoes a rearrangement where the positions of the methyl and ethyl groups are swapped. This is characteristic of a [1,5]-sigmatropic rearrangement, where a sigma bond migrates along the conjugated system.
2. Step 2: Understand the mechanism of the [1,5]-sigmatropic rearrangement. In this type of rearrangement, the sigma bond shifts from one position to another within the conjugated system, maintaining the overall connectivity of the molecule while redistributing substituents.
3. Step 3: Analyze the second reaction. The structure on the left is a bicyclic compound with a conjugated diene system. Upon heating, the molecule undergoes a rearrangement where the double bonds shift positions, and the substituents rearrange. This is characteristic of a Cope rearrangement, a specific type of [3,3]-sigmatropic rearrangement.
4. Step 4: Understand the mechanism of the Cope rearrangement. In this reaction, the sigma bond migrates across the conjugated system, resulting in a redistribution of the substituents and a shift in the positions of the double bonds. The reaction is thermally driven and proceeds through a cyclic transition state.
5. Step 5: Summarize the types of sigmatropic rearrangements observed. The first reaction is a [1,5]-sigmatropic rearrangement, while the second reaction is a [3,3]-sigmatropic rearrangement (Cope rearrangement). Both reactions are thermally induced and involve the migration of sigma bonds within conjugated systems.

Key Concepts

Sigmatropic Rearrangement

Sigmatropic rearrangements are a class of pericyclic reactions where a sigma bond and a pi bond are rearranged in a concerted process. These reactions involve the migration of a substituent across a double bond, resulting in a structural change of the molecule. Understanding the mechanism and the types of sigmatropic rearrangements is crucial for predicting the products of such reactions.

Types of Sigmatropic Rearrangements

Sigmatropic rearrangements are categorized based on the number of atoms involved in the migration. The most common types are [1,3] and [1,5] rearrangements, where the numbers indicate the positions of the migrating atom relative to the original bond. Recognizing these types helps in identifying the specific rearrangement occurring in a given reaction.

Woodward-Hoffmann Rules

The Woodward-Hoffmann rules provide a framework for predicting the stereochemical outcomes of pericyclic reactions, including sigmatropic rearrangements. These rules are based on the conservation of orbital symmetry and help determine whether a reaction is allowed or forbidden under thermal or photochemical conditions. Applying these rules is essential for understanding the feasibility of the rearrangements in the given reactions.