Question

Addition of HBr to 1.3-butadiene at 40°C yields:

• A. 80% 1,2-addition product and 20% 1,4-addition product
• B. 80% 1,2-addition product and 20% 1,3-addition product
• C. 80% 1,4-addition product and 20% 1,2-addition product
• D. 80% 1,2-addition product and 20% 1,3-addition product

Answer 1

The Correct Option is C

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. 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.
2. 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.