Which of the following is the thermodynamic product formed during the reaction of 1,3-butadiene with HBr under conditions of high temperature and long reaction time?

• A. 1-bromo-1-butene
• B. 1-bromo-2-butene
• C. 1-bromo-2-butene (trans isomer)
• D. 3-bromo-1-butene

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the difference between kinetic and thermodynamic control in addition reactions to conjugated dienes like 1,3-butadiene. Under low temperature and short reaction time, the kinetic product forms faster, while under high temperature and long reaction time, the thermodynamic product is favored due to its greater stability.
2. Step 2: Identify the possible products from the addition of HBr to 1,3-butadiene. The reaction can proceed via 1,2-addition (adding H and Br across carbons 1 and 2) or 1,4-addition (adding H and Br across carbons 1 and 4), leading to different isomers.
3. Step 3: Recognize that the 1,2-addition product corresponds to 1-bromo-2-butene, while the 1,4-addition product corresponds to 3-bromo-1-butene. The 1-bromo-1-butene is less common in this context.
4. Step 4: Analyze the stability of the products. The thermodynamic product is the more substituted and more stable alkene, which is typically the trans-1-bromo-2-butene formed via 1,2-addition under thermodynamic control.
5. Step 5: Conclude that under high temperature and long reaction time, the reaction mixture equilibrates to favor the thermodynamic product, which is the trans-1-bromo-2-butene.