The reaction of 1,3 -butadiene with HBr is shown below. At 40^∘ C the major product is the 1 . 4

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The reaction of 1,3 -butadiene with HBr is shown below. At...

The reaction of 1,3 -butadiene with HBr is shown below. At $40^{\circ} \mathrm{C}$ the major product is the 1 . 4-addition product; however, at $-80^{\circ} \mathrm{C}$ the major product is the 1,2 -addition product. Which of the two products has a lower activation energy for formation? (a) 1,4 -addition product. (b) 1,2 -addition product. (c) The products have same activation energy. (d) The relative activation energy cannot be determined.

The reaction of 1,3 -butadiene with HBr is shown below. At $40^{\circ} \mathrm{C}$ the major product is the 1 . 4-addition product; however, at $-80^{\circ} \mathrm{C}$ the major product is the 1,2 -addition product.
Which of the two products has a lower activation energy for formation?
(a) 1,4 -addition product.
(b) 1,2 -addition product.
(c) The products have same activation energy.
(d) The relative activation energy cannot be determined.

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Key Concepts

Activation Energy

Activation energy is the minimum amount of energy that reacting molecules must possess for a reaction to occur. It determines the rate at which products are formed, as reactions with lower activation energies generally proceed more quickly than those with higher barriers.

Kinetic versus Thermodynamic Control

Kinetic control refers to reaction conditions where product formation is governed by the rate at which a product is formed (lower activation energy) rather than its long-term stability. Under kinetic control, the product formed fastest—even if less stable—is favored, especially at lower temperatures. In contrast, thermodynamic control favors the formation of the most stable product, though its pathway may involve a higher activation energy and require higher temperatures or longer reaction times.

Temperature Effects on Reaction Pathways

Temperature plays a crucial role in determining which reaction pathway is favored. At lower temperatures, reactions typically favor pathways with lower activation energies (kinetic control), while at higher temperatures, the products that are more thermodynamically stable may become predominant as there is enough energy to overcome higher activation barriers.

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