Question

The Cope Rearrangement: A Pericyclic Reaction (Section 20.6B) The Cope rearrangement converts a 1,5-diene to give an isomeric 1,5-diene. The reaction takes place in a single step and involves the redistribution of six electrons in a cyclic transition state.

Short answer

Answer: The Cope Rearrangement converts a 1,5-diene molecule to its isomeric form by a single-step concerted mechanism involving six electrons moving in a cycle. This creates a cyclic transition state, which results in the formation of new double bonds and an isomeric 1,5-diene product.

Step-by-step solution

Identify the starting 1,5-diene

Consider the initial 1,5-diene molecule, where there are two double bonds separated by a single carbon atom.

Analyze the electron movements in the reaction

In the Cope Rearrangement, the reaction takes place through a concerted mechanism involving six electrons moving in a cycle, which creates a cyclic transition state. The movement of electrons can be represented as follows: 1. Electron pair (1) from a double bond moves towards the carbon atom next to it. 2. Electron pair (2) from the other double bond moves towards the single carbon atom in between the double bonds, forming a new double bond. 3. Electron pair (3) from the single bond connecting the carbon atom in between double bonds and the carbon atom next to it moves towards the carbon atom next to the other double bond, forming a new double bond.

Visualize the cyclic transition state

The cyclic transition state involves all the six electrons and the movement of electron pairs (1), (2), and (3) mentioned in step 2. It is important to note that the reaction occurs in a single step, and all the electron movements are simultaneous.

Identify the isomeric product

After the completion of the electron movements, an isomeric 1,5-diene is formed. The new 1,5-diene consists of two double bonds separated by a single carbon atom, just like the starting 1,5-diene. However, the positions of the double bonds have changed, resulting in a different isomer.

Recap the Cope Rearrangement

The Cope Rearrangement is a pericyclic reaction that involves the conversion of a 1,5-diene to its isomeric 1,5-diene in a single step through a cyclic transition state. The reaction involves the coordinated movement of six electrons between the carbon atoms in the molecule, resulting in new double bond formations and the final isomeric product. Finally, remember that understanding the Cope Rearrangement mechanism and analyzing the electron movements, transition state, and the isomeric product formation are essential to tackle similar pericyclic reactions.