Which of the following dienes can undergo the Diels-Alder reaction?

• A. cyclohexene
• B. $1_2-\mathrm{butadiene}$
• C. $1_3-\mathrm{butadiene}$
• D. $2-\mathrm{butyne}$

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the Diels-Alder reaction requirements. The Diels-Alder reaction is a [4+2] cycloaddition between a conjugated diene (with four π electrons) and a dienophile (with two π electrons). The diene must be able to adopt an s-cis conformation to react effectively.
2. Step 2: Analyze each compound to determine if it is a conjugated diene capable of adopting the s-cis conformation. Cyclohexene has only one double bond, so it is not a diene and cannot undergo the Diels-Alder reaction as a diene.
3. Step 3: Consider 1,2-butadiene. This compound has two double bonds, but they are cumulated (adjacent double bonds sharing a carbon), not conjugated. Cumulated dienes do not participate well in Diels-Alder reactions because they cannot adopt the required s-cis conformation.
4. Step 4: Examine 1,3-butadiene. This is a conjugated diene with alternating double and single bonds, allowing it to adopt the s-cis conformation necessary for the Diels-Alder reaction. Therefore, it can undergo the reaction.
5. Step 5: Look at 2-butyne, which contains a triple bond and is not a diene. It cannot act as a diene in the Diels-Alder reaction, so it is not suitable.