Question

Compare the reaction between 2,4,6-cycloheptatrienone and cyclopentadiene to the reaction between 2,4,6-cycloheptatrienone and ethene. Why does 2,4,6-cycloheptatrienone use two $π$ electrons in one reaction and four $π$ electrons in the other?
a.
b.

Accepted Answer

Step 1: Analyze the reaction between 2,4,6-cycloheptatrienone and cyclopentadiene. This reaction is a Diels-Alder reaction, where cyclopentadiene acts as the diene (providing four π electrons) and 2,4,6-cycloheptatrienone acts as the dienophile (providing two π electrons). The reaction forms a bicyclic compound with a new six-membered ring. Step 2: Analyze the reaction between 2,4,6-cycloheptatrienone and ethene. This reaction is a [2+2] cycloaddition, where ethene provides two π electrons and 2,4,6-cycloheptatrienone also contributes two π electrons. The reaction forms a four-membered ring fused to the cycloheptatrienone structure. Step 3: Understand why 2,4,6-cycloheptatrienone uses different numbers of π electrons in these reactions. In the Diels-Alder reaction, the conjugated π system of cyclopentadiene interacts with the π system of 2,4,6-cycloheptatrienone, allowing the use of four π electrons from the diene and two π electrons from the dienophile. In the [2+2] cycloaddition with ethene, only two π electrons from each reactant are involved. Step 4: Consider the electronic structure of 2,4,6-cycloheptatrienone. The molecule has a conjugated π system, but its reactivity depends on the type of reaction and the nature of the other reactant. In the Diels-Alder reaction, the extended conjugation of cyclopentadiene matches the π system of 2,4,6-cycloheptatrienone, while in the [2+2] cycloaddition, ethene's simpler π system limits the interaction to two π electrons. Step 5: Summarize the key difference. The number of π electrons used by 2,4,6-cycloheptatrienone depends on the type of cycloaddition reaction and the electronic compatibility of the reactants. The Diels-Alder reaction involves a [4+2] interaction, while the [2+2] cycloaddition involves a simpler interaction between two π electrons from each reactant.

Compare the reaction between 2,4,6-cycloheptatrienone and cyclopentadiene to the reaction between 2,4,6-cycloheptatrienone and ethene. Why does 2,4,6-cycloheptatrienone use two $π$ electrons in one reaction and four $π$ electrons in the other?
a.
b.

Verified video answer for a similar problem:

Key Concepts

Diels-Alder Reaction

Electrophilicity and Nucleophilicity

Conjugation and Resonance

Compare the reaction between 2,4,6-cycloheptatrienone and cyclopentadiene to the reaction between 2,4,6-cycloheptatrienone and ethene. Why does 2,4,6-cycloheptatrienone use two π\(\pi\) electrons in one reaction and four π\(\pi\) electrons in the other?a. b.

Verified video answer for a similar problem:

Key Concepts

Diels-Alder Reaction

Electrophilicity and Nucleophilicity

Conjugation and Resonance

Compare the reaction between 2,4,6-cycloheptatrienone and cyclopentadiene to the reaction between 2,4,6-cycloheptatrienone and ethene. Why does 2,4,6-cycloheptatrienone use two $π$ electrons in one reaction and four $π$ electrons in the other?
a.
b.