Assertion (A): At $40^{\circ} C$ , the addition of HBr to buta-1,3-diene gives mainly 1-bromobut-2-ene. Reason (R): 1-Bromobut-2-ene has a disubstituted $C = C$ , more stable than the monosubstituted $C = C$ of 3-bromobut-1-ene, and the adducts equilibrate at $40^{\circ} C$ .

ABoth A and R are true and R is the correct explanation of A
BBoth A and R are true but R is not the correct explanation of A
CA is true but R is false
DA is false but R is true

Answer: A

40^{\circ} C

the adducts interconvert through the allylic cation. The more stable 1,4-adduct (internal, more substituted alkene) accumulates. Thus A and R are true and R explains A.

Practice General Organic Chemistry III: Reaction Intermediates More on Reaction energy profiles, kinetic vs thermodynamic control

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Assertion (A): At 40 ∘C, the addition of HBr to buta-1,3-diene gives mainly 1-bromobut-2-ene. Reason (R): 1-Bromobut-2-ene has a disubstituted C=C, more stable than the monosubstituted C=C of 3-bromobut-1-ene, and the adducts equilibrate at 40 ∘C.

Answer: A

More questions on Reaction energy profiles, kinetic vs thermodynamic control

Assertion (A): At $40^{\circ} C$ , the addition of HBr to buta-1,3-diene gives mainly 1-bromobut-2-ene. Reason (R): 1-Bromobut-2-ene has a disubstituted $C = C$ , more stable than the monosubstituted $C = C$ of 3-bromobut-1-ene, and the adducts equilibrate at $40^{\circ} C$ .