Question:
Addition of HBr to 1.3-butadiene at 40°C yields:
Addition of HBr to 1.3-butadiene at 40°C yields:
Updated On: Feb 16, 2026
- 80% 1,2-addition product and 20% 1,4-addition product
- 80% 1,2-addition product and 20% 1,3-addition product
- 80% 1,4-addition product and 20% 1,2-addition product
- 80% 1,2-addition product and 20% 1,3-addition product
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The Correct Option is C
Solution and Explanation
To understand the addition of HBr to 1,3-butadiene at 40°C, we need to consider the concept of kinetic and thermodynamic control in organic reactions. 1,3-butadiene is a conjugated diene, meaning it has alternating single and double bonds, which allows for two possible modes of addition with HBr: 1,2-addition and 1,4-addition.
- 1,2-Addition Product:
- In 1,2-addition, HBr adds across the first double bond of the butadiene.
- Hydrogen (H2) attaches to the first carbon, and bromine (Br) attaches to the second carbon.
- This reaction is kinetically controlled at lower temperatures because it proceeds fastest and forms initially.
- 1,4-Addition Product:
- In 1,4-addition, HBr adds across the conjugated system.
- Hydrogen attaches to the first carbon, and bromine attaches to the fourth carbon after rearrangement of the double bonds.
- This product is more stable and favored at higher temperatures due to thermodynamic control because it leads to a more substituted, stable alkene.
At 40°C, the reaction is under thermodynamic control, which means the 1,4-addition product is predominant. Thus, 80% 1,4-addition product and 20% 1,2-addition product is formed as the major and minor products respectively.
This is because the 1,4-addition product is more stable and energetically favored at higher temperatures like 40°C, whereas the 1,2-addition is a kinetically controlled product that forms rapidly but is less stable.
Therefore, the correct answer is: 80% 1,4-addition product and 20% 1,2-addition product.
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