See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: When a compound with two basic nitrogen atoms reacts with 1 mole of HCl, protonation occurs preferentially at the more basic (higher pKa) nitrogen. Step 1: Identify the two nitrogen atoms in 1,2,3,4-tetrahydroisoquinoline. The molecule contains two secondary amine nitrogens: N1 (benzylic, adjacent to the aromatic ring — an aryl-alkyl amine, less basic due to partial delocalization of lone pair into the aromatic ring) and N2 (a purely aliphatic secondary amine, more basic). Step 2: Compare basicities. The N2 nitrogen in the aliphatic portion of the ring is a simple dialkyl amine (higher pKa, more basic), while N1 is benzylic/aryl-adjacent and therefore less basic because the aromatic ring withdraws electron density via resonance/induction, reducing the availability of the lone pair. Step 3: With only 1 mole of HCl, protonation occurs at the more basic nitrogen — N2 (the aliphatic nitrogen, lower in the ring structure). This gives the ammonium salt with the positive charge on N2 and Cl- as counterion, while N1 remains as a free secondary amine (NH). Step 4: This matches option (b), which shows N1 with NH (free amine) and N2 protonated to NH2+ with Cl- counterion. Why other options fail: - Option (a) shows protonation at N1 (the less basic, aryl-adjacent nitrogen) — incorrect, as the more basic N2 is protonated preferentially. - Option (c) shows electrophilic aromatic chlorination, which does not occur under simple HCl treatment without a Lewis acid catalyst. - Option (d) shows nucleophilic substitution placing Cl on an aliphatic carbon, which is not the reaction of an amine with HCl. Therefore, the correct answer is B.