See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Concept: When a secondary alkyl halide reacts with a strong base/nucleophile such as sodium acetylide (HC≡CNa), two competing pathways occur: SN2 (substitution) and E2 (elimination). The outcome depends on the steric environment and the strength of the base versus nucleophile character of the reagent. Step 1 – Identify the substrate: The starting material is 2-bromo-1,2,3,4-tetrahydronaphthalene (2-bromotetralin), a secondary cyclic alkyl bromide. Step 2 – Identify the reagent: Sodium acetylide (HC≡CNa) is both a strong base (pKa of acetylene ~25) and a good nucleophile. However, with secondary substrates, E2 elimination competes significantly with SN2 substitution. Step 3 – Assess SN2 vs E2: For secondary alkyl halides, sodium acetylide (being a strong, somewhat bulky base) can abstract a β-hydrogen leading to E2 elimination, giving an alkene, while also acting as a nucleophile to give the substitution product (alkyne). Because the substrate is secondary and the reaction is in Et2O (a less polar solvent favoring E2), elimination is favored but substitution still occurs to some extent. Step 4 – Identify the E2 product: E2 elimination from 2-bromotetralin removes a β-hydrogen adjacent to C2. The most substituted (Zaitsev) alkene formed would be the endocyclic double bond giving 1,2-dihydronaphthalene (Δ1,2 or Δ2,3 position in the ring). However, looking at the answer choices, option (b) shows the E2 product as the fully aromatic/tetralin structure (tetralin = 1,2,3,4-tetrahydronaphthalene). Wait – re-examining: option (a) shows E2 giving 1,2-dihydronaphthalene (80%) and SN2 giving the alkynyl product (20%). Option (b) shows SN2 alkynyl product (20%) plus what appears to be tetralin (80%). Step 5 – Re-examine option (b): The second structure in option (b) is drawn as 1,2,3,4-tetrahydronaphthalene (tetralin) without the double bond – but this would require reduction, not elimination. However, in the context of the answer choices the structure labeled 80% in (b) is actually 1,2-dihydronaphthalene (a partially unsaturated bicyclic compound with one double bond in the non-aromatic ring), not fully saturated tetralin. The E2 elimination from 2-bromotetralin gives the dihydronaphthalene (an allylic/conjugated product stabilized by the aromatic ring). This is the major product (80%) due to the secondary nature of the substrate and strong base conditions, while SN2 to give the ethynyl-tetralin is the minor product (20%). Step 6 – Why not other options: (a) is incorrect because it also shows the same ratio but the E2 product drawn might be positioned differently; (c) is wrong because 100% SN2 is unrealistic for a secondary halide with a strong base; (d) is wrong because aromatic substitution via this pathway is not expected. Step 7 – Conclusion: The major product is the E2 elimination product (1,2-dihydronaphthalene, 80%) and minor product is the SN2 substitution product (2-ethynyl-tetralin, 20%), corresponding to option (b). Therefore, the correct answer is B.