Question

Draw structural formulas for the products of reaction of cyclopentadiene with each dienophile. (a) \(\mathrm{CH}_{2}=\mathrm{CHCl}\) (b) \(\mathrm{CH}_{2}=\mathrm{CHCOCH}_{3}\) (c) \(\mathrm{HC} \equiv \mathrm{CH}\) (d) CC(=O)C#CC(C)=O

Short answer

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Step-by-step solution

Identify the diene and dienophiles

The diene in this case is cyclopentadiene, a 5-membered ring with 2 double bonds. The dienophiles given are: (a) \(\mathrm{CH}_{2}=\mathrm{CHCl}\) (b) \(\mathrm{CH}_{2}=\mathrm{CHCOCH}_{3}\) (c) \(\mathrm{HC} \equiv \mathrm{CH}\) (d) CC(=O)C#CC(C)=O

Perform the Diels-Alder reaction with each dienophile

In a Diels-Alder reaction, the diene reacts with the dienophile to form a 6-membered ring. We'll do this for each dienophile: (a) Cyclopentadiene reacts with \(\mathrm{CH}_{2}=\mathrm{CHCl}\) to form the following product: [\(\mathrm{CH}_{2}(\mathrm{CH})_{3}\mathrm{CHCl}\)] (b) Cyclopentadiene reacts with \(\mathrm{CH}_{2}=\mathrm{CHCOCH}_{3}\) to form the following product: [\(\mathrm{CH}_{2}(\mathrm{CH})_{3}\mathrm{CHCOCH}_{3}\)] (c) Cyclopentadiene reacts with \(\mathrm{HC} \equiv \mathrm{CH}\) to form the following product: [\(\mathrm{CH}_{2}(\mathrm{CH})_{3}\mathrm{CH}=H\)] (d) Cyclopentadiene reacts with CC(=O)C#CC(C)=O to form the following product: [\(\mathrm{CH}_{2}(\mathrm{CH})_{3}\mathrm{CHC(C)=O}(\mathrm{CC}=\mathrm{O})\)]

Draw the structural formulas of the products

Now we can draw the structural formulas for each product: (a) Product with \(\mathrm{CH}_{2}=\mathrm{CHCl}\):  (b) Product with \(\mathrm{CH}_{2}=\mathrm{CHCOCH}_{3}\):  (c) Product with \(\mathrm{HC} \equiv \mathrm{CH}\):  (d) Product with CC(=O)C#CC(C)=O: 

Key concepts

Cyclopentadiene

Cyclopentadiene is a five-membered hydrocarbon ring compound that features two adjacent double bonds—making it an ideal diene for Diels-Alder reactions.

This compound tends to react readily due to the high ring strain present in the five-membered ring coupled with the electron-rich nature of its conjugated diene system. In the context of the Diels-Alder reaction, cyclopentadiene serves as a 4π-electron component that fuses with a dienophile to create a new six-membered ring in a concerted and stereospecific manner.

Understanding Ring Strain

Ring strain arises because the bond angles in cyclopentadiene are less than the ideal tetrahedral angle of 109.5°, resulting in increased kinetic reactivity.

Conjugated Diene System

The conjugated system enables overlap of p-orbitals, facilitating the reaction with dienophiles by allowing electrons to be shared across the entire diene pi-system.

Dienophiles

Dienophiles are critical components in Diels-Alder reactions; they are electron-poor alkenes or alkynes that eagerly participate with electron-rich dienes like cyclopentadiene.

In the given problem, we encounter different types of dienophiles: (a) an alkene with a chlorine substituent, (b) an alkene with an electron-withdrawing acyl group, (c) an alkyne, and (d) an electron-deficient molecule derived from chemical notation 'smiles'. Each of these dienophiles exhibits a unique reactivity profile based on their structural features, impacting the outcome of the reaction.

Alkene vs. Alkyne Dienophiles

Alkenes, with their pi-bonds, are standard dienophiles and react at a moderate pace, whereas alkynes are more reactive due to their linear geometry and more accessible π-electrons.

Structural Formulas

Structural formulas provide a visual representation of molecules, illustrating the exact connectivity of atoms within a compound.

In a Diels-Alder reaction, the structural formula allows chemists to deduce the stereochemistry and regiochemistry of the final product. Understanding how to read and draw these formulas is crucial for predicting and explaining the reaction outcomes in organic chemistry. The process involves identifying how the atoms and functional groups are rearranged post-reaction, which sheds light on the mechanism and provides insights into the relationship between structure and reactivity.

Illustrating Diels-Alder Products

For the described reactions, the structural formulas will display six-membered rings formed from the union of cyclopentadiene and each dienophile, reflecting new sigma bonds and, in some cases, remaining pi bonds or functional groups from the original dienophiles.