The major product formed in the reaction given below will be : (Reactant: a hydrindane (cyclohexane fused to cyclopentane) carrying a \(-\mathrm{CH_2NH_2}\) group on the five-membered ring; reagent: NaNO\(_2\), aq. HCl, 0-5\(^\circ\)C.)
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A primary aliphatic amine gives an unstable diazonium salt that loses N\(_2\) to a primary carbocation; ring expansion (alkyl shift) then gives a more stable secondary carbocation, which is trapped by water.
\(\mathrm{R-CH_2NH_2 + HNO_2 \rightarrow R-CH_2N_2^+ \rightarrow R-CH_2^+ + N_2}\) (aliphatic diazonium salts are unstable even at 0-5\(^\circ\)C). The primary carbocation \(\mathrm{CH_2^+}\) is attached to a carbon of the cyclopentane ring. A ring C-C bond migrates to the cationic carbon (Demjanov-type ring expansion), enlarging the five-membered ring to a six-membered ring and moving the positive charge to a ring carbon (secondary carbocation, more stable). Water captures this carbocation to give an alcohol. Result: the fused 5-6 ring system becomes a fused 6-6 system (decalin) bearing an OH group at the 2-position: decalin-2-ol, the structure in option (3). Options (1) and (2) contain NO\(_2\) (not formed), and option (4) is the unexpanded hydrindanol. Answer: (3) decahydronaphthalen-2-ol (decalin-2-ol)
