One mole of $HBr$ adds to 2-methylbuta-1,3-diene (isoprene) at $- 8 0^{\circ} C$ under kinetic control, with no peroxide. Assume the more stable allylic cation is formed and that bromide then adds to the cationic carbon adjacent to the point of protonation (1,2-addition). The major product is:

A3-Bromo-3-methylbut-1-ene
B1-Bromo-3-methylbut-2-ene
C3-Bromo-2-methylbut-1-ene
D4-Bromo-2-methylbut-1-ene

Answer: A

Protonation at C1 gives the tertiary allylic cation

(CH X_{3}) X_{2} C X^{+} - CH = CH X_{2}

, which is more stable than the alternative secondary allylic cation. Kinetic 1,2-attack at the tertiary carbon gives

(CH X_{3}) X_{2} CBr - CH = CH X_{2}

, 3-bromo-3-methylbut-1-ene. The 1,4-product is the 1-bromo-3-methylbut-2-ene (thermodynamic), and the other two come from the less stable cation or an anti-Markovnikov route.

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One mole of HBr adds to 2-methylbuta-1,3-diene (isoprene) at −80∘C under kinetic control, with no peroxide. Assume the more stable allylic cation is formed and that bromide then adds to the cationic carbon adjacent to the point of protonation (1,2-addition). The major product is:

Answer: A

More questions on Conjugated dienes and Diels-Alder reaction

One mole of $HBr$ adds to 2-methylbuta-1,3-diene (isoprene) at $- 8 0^{\circ} C$ under kinetic control, with no peroxide. Assume the more stable allylic cation is formed and that bromide then adds to the cationic carbon adjacent to the point of protonation (1,2-addition). The major product is: