Assertion (A): The 1,2-adduct of HBr and buta-1,3-diene formed at $- 80^{\circ} C$ does not rearrange to the 1,4-adduct if kept at that temperature, even though the rearrangement would lower the free energy. Reason (R): The 1,2-adduct is thermodynamically less stable than the 1,4-adduct.

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: B

A is true: rearrangement needs ionisation of the

C - Br

bond to the allylic cation, and this barrier cannot be crossed at

- 80^{\circ} C

. R is also true. But the relative stability only sets the direction of change, not whether the change is fast enough; the lack of rearrangement is a kinetic fact. So R does not explain A.

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

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Assertion (A): The 1,2-adduct of HBr and buta-1,3-diene formed at −80 ∘C does not rearrange to the 1,4-adduct if kept at that temperature, even though the rearrangement would lower the free energy. Reason (R): The 1,2-adduct is thermodynamically less stable than the 1,4-adduct.

Answer: B

More questions on Reaction energy profiles, kinetic vs thermodynamic control

Assertion (A): The 1,2-adduct of HBr and buta-1,3-diene formed at $- 80^{\circ} C$ does not rearrange to the 1,4-adduct if kept at that temperature, even though the rearrangement would lower the free energy. Reason (R): The 1,2-adduct is thermodynamically less stable than the 1,4-adduct.