For the addition of HBr to buta-1,3-diene, which of the following statements are correct? Select all that apply.

AThe 1,2- and 1,4-adducts arise from two different carbocation intermediates
BThe 1,2-adduct can rearrange to the 1,4-adduct on warming through the same allylic cation
CAt about $40^{\circ} C$ the 1,4-adduct 1-bromobut-2-ene is the major product (thermodynamic control)
DAt about $- 80^{\circ} C$ the 1,2-adduct 3-bromobut-1-ene is the major product (kinetic control)

Answer: B, C, D

Both adducts come from one resonance-stabilised allylic cation,

CH X_{3} - CH X^{+} - CH = CH X_{2}

and

CH X_{3} - CH = CH - CH X_{2} X^{+}

, attacked by bromide at two positions. At low temperature the faster-forming 1,2-adduct dominates, and at higher temperature equilibration through the same cation gives the more stable 1,4-adduct. The last statement is therefore false.

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For the addition of HBr to buta-1,3-diene, which of the following statements are correct? Select all that apply.

Answer: B, C, D

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