The major product $[\mathrm{B}]$ in the following sequence is:

JEE Main · 2020 · 08 Jan 2020 · Shift 2 · Hydrocarbons

The major product

[B]

in the following sequence is:

(A)(see structure)
(B)(see structure)
(C)(see structure)
(D)(see structure)
✓ Correct

Answer: (D)

Solution

Hydroboration-oxidation with

BH X_{3}

then

H X_{2} O X_{2} / OH X^{-}

adds water across the double bond with anti-Markovnikov orientation, placing the

- OH

on the less substituted carbon.

PBr X_{3}

then swaps that

- OH

for bromine, and the bulky base potassium tert-butoxide forces an E2 elimination. Because the base removes a hydrogen to give the more substituted, Zaitsev alkene, the sequence returns the more highly substituted alkene as the major product.

JEE Main · 2020 · 08 Jan 2020 · Shift 2 · Single correct option

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