1,3-Butadiene reacts with one equivalent of HBr. Which pair correctly gives the major product at $- 8 0^{\circ} C$ and at $+ 4 0^{\circ} C$ (after equilibration)?

A1-bromobut-2-ene at -80 C; 3-bromobut-1-ene at +40 C
B4-bromobut-1-ene at -80 C; 1-bromobut-2-ene at +40 C
C3-bromobut-1-ene at -80 C; 1-bromobut-2-ene at +40 C
D1-bromobut-2-ene at both temperatures

Answer: C

Protonation at C-1 gives the allylic cation

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

(delocalised). At low temperature the faster 1,2-addition (kinetic control) gives 3-bromobut-1-ene. At higher temperature the products equilibrate and the more stable internal alkene, 1-bromobut-2-ene (1,4-adduct), predominates (thermodynamic control).

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1,3-Butadiene reacts with one equivalent of HBr. Which pair correctly gives the major product at −80∘C and at +40∘C (after equilibration)?

Answer: C

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1,3-Butadiene reacts with one equivalent of HBr. Which pair correctly gives the major product at $- 8 0^{\circ} C$ and at $+ 4 0^{\circ} C$ (after equilibration)?