Identify the product (P) in the following reaction: (CH3)2CH-CH(OH)-CH3 + HBr -> P (the alcohol is 3-methylbutan-2-ol)

NEET 2023 Phase 1 · ChemistryPrevious Year Question

Identify the product (P) in the following reaction: $(CH X_{3}) X_{2} CH - CH (OH) - CH X_{3} + HBr P$ (the alcohol is 3-methylbutan-2-ol)

Answer: (A) (A) $CH X_{3} - CBr (CH X_{3}) - CH X_{2} - CH X_{3}$ (2-bromo-2-methylbutane). Protonation of the $- OH$ and loss of water gives a secondary carbocation at C2.

A.(A) $CH X_{3} - CBr (CH X_{3}) - CH X_{2} - CH X_{3}$ (2-bromo-2-methylbutane)✓
B.(B) $CH X_{3} - CH = CH - CH X_{3}$ (but-2-ene)
C.(C) $(CH X_{3}) X_{2} CH - CHBr - CH X_{3}$ (2-bromo-3-methylbutane)
D.(D) $(CH X_{3}) X_{3} C - CH X_{2} Br$ (1-bromo-2,2-dimethylpropane)

Correct Answer

(A) (A) $CH X_{3} - CBr (CH X_{3}) - CH X_{2} - CH X_{3}$ (2-bromo-2-methylbutane)

Solution & Explanation

Protonation of the $- OH$ and loss of water gives a secondary carbocation at C2. A 1,2-hydride shift from the adjacent tertiary C-H produces the more stable tertiary carbocation, which is then captured by $Br X^{-}$ . The rearranged product is the tertiary bromide $CH X_{3} - CBr (CH X_{3}) - CH X_{2} - CH X_{3}$ (2-bromo-2-methylbutane). This $S X_{N} 1$ route with carbocation rearrangement explains why C (unrearranged) is not the major product.

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Identify the product (P) in the following reaction: (CHX3​)X2​CH−CH(OH)−CHX3​+HBr​P (the alcohol is 3-methylbutan-2-ol)

Solution & Explanation

More NEET PYQ solutions

Identify the product (P) in the following reaction: $(CH X_{3}) X_{2} CH - CH (OH) - CH X_{3} + HBr P$ (the alcohol is 3-methylbutan-2-ol)