Predict the structure of \(A\):
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 2-methylpropan-2-ol, (CH3)3C–OH
Alkenes react with water in the presence of an acid catalyst to form alcohols (acid catalysed hydration). With an unsymmetrical alkene, the addition follows Markovnikov’s rule: H goes to the doubly bonded carbon that already has more hydrogens, and OH to the other one.
The alkene here is 2-methylpropene, \( \ce{(CH3)2C=CH2} \). The mechanism has three steps:
\( \ce{H^{+}} \) (as \( \ce{H3O^{+}} \)) adds to the \( \ce{CH2} \) carbon, giving the more stable tertiary carbocation \( \ce{(CH3)3C^{+}} \);
water attacks the carbocation, giving \( \ce{(CH3)3C-OH2^{+}} \);
this ion loses \( \ce{H^{+}} \) to form the alcohol.
\( \ce{(CH3)2C=CH2 + H2O} \xrightarrow{\ce{H^+}} \ce{(CH3)3C-OH} \)
So A is 2-methylpropan-2-ol (tert-butyl alcohol).
Why the other options are incorrect
Option (1) 2-methylpropan-1-ol, (CH3)2CH–CH2OH — Incorrect. This is the anti-Markovnikov product; it would need a primary carbocation, which is far less stable. It is obtained by hydroboration–oxidation.
Option (3) ethane, 2CH3–CH3 — Incorrect. Hydration adds H and OH; it neither reduces the alkene nor breaks its carbon chain.
Option (4) methanol + propan-2-ol — Incorrect. Splitting the molecule at the C=C bond happens only in oxidative cleavage (ozonolysis); acid hydration keeps the four-carbon skeleton.
NCERT (2026-27 edition):
From Alkenes – “(i) By acid catalysed hydration: Alkenes react with water in the presence of acid as catalyst to form alcohols. In case of unsymmetrical alkenes, the addition reaction takes place in accordance with Markovnikov’s rule.”