Buta-1,3-diene adds HBr faster at C-1,C-2 than at C-1,C-4 at -80 degC. The best explanation is that

in the allylic cation more positive charge sits on the secondary C-2 and the bromide ion pair is close to it, so attack there has the lower barrier

the 1,2-adduct is thermodynamically more stable than the 1,4-adduct

protonation at C-2 gives a different, more stable cation that leads only to the 1,2-adduct

bromide ion cannot reach C-4 at any temperature

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Buta-1,3-diene adds HBr faster at C-1,C-2 than at C-1,C-4 at $- 80^{\circ} C$ . The best explanation is that

Ain the allylic cation more positive charge sits on the secondary C-2 and the bromide ion pair is close to it, so attack there has the lower barrier
Bthe 1,2-adduct is thermodynamically more stable than the 1,4-adduct
Cprotonation at C-2 gives a different, more stable cation that leads only to the 1,2-adduct
Dbromide ion cannot reach C-4 at any temperature

Answer: A

Protonation at C-1 gives

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

, in resonance with

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

. The secondary terminus carries more positive charge and is nearer the counter-ion, so the 1,2-adduct forms faster (kinetic). The 1,4-adduct is the more stable one, not the faster one.

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Buta-1,3-diene adds HBr faster at C-1,C-2 than at C-1,C-4 at −80 ∘C. The best explanation is that

Answer: A

More questions on Reaction energy profiles, kinetic vs thermodynamic control

Buta-1,3-diene adds HBr faster at C-1,C-2 than at C-1,C-4 at $- 80^{\circ} C$ . The best explanation is that