See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Step 1 – Identify the starting material. The substrate is trans-4-chlorocyclohexane-1-thiol (or its equivalent: a cyclohexane bearing H and a leaving group Cl in a trans-diaxial arrangement at C1 and C4, with an SH implied by the molecular formula C6H10S of product A containing sulfur). Actually, re-reading: the starting material drawn shows H on wedge at C1 and Cl on wedge at C4 of cyclohexane. The molecular formula of product A is C6H10S, which has one degree of unsaturation beyond cyclohexane (C6H12), suggesting a ring or double bond involving S. The minor product is cyclohexene (an elimination product). The starting material must already contain sulfur — it is trans-4-chlorocyclohexanethiol (C6H11ClS). The structure drawn shows the cyclohexane with H (representing SH by convention in this problem context, i.e., the wedge H actually represents the SH group position) at C1 and Cl at C4. Step 2 – Reaction conditions. NaOH in aqueous EtOH is a classic condition for both SN2 substitution and E2 elimination. The minor product is cyclohexene (from E2 elimination of HCl). The major product A has formula C6H10S. Step 3 – Deduce the major product. With trans-4-chlorocyclohexanethiol and NaOH: NaOH deprotonates the thiol (SH) to give a thiolate anion (RS−), which is an excellent intramolecular nucleophile. The thiolate at C1 attacks C4 (bearing Cl) in an intramolecular SN2 reaction. For an intramolecular SN2 to occur, the nucleophile and leaving group must be on opposite faces (inversion at C4). In the trans isomer, C1-S and C4-Cl are trans (diaxial in chair), which allows the backside attack geometry for intramolecular SN2. This intramolecular cyclization forms a bicyclic thioether: the sulfur bridges C1 and C4 of cyclohexane, creating 7-thiabicyclo[2.2.1]heptane (a norbornane analog with S as the one-atom bridge). This matches option (b) and the molecular formula C6H10S (MW = 114, degrees of unsaturation = 2, consistent with bicyclic structure). Step 4 – Why other options fail. (a) 1-mercaptocyclohex-2-ene would require both substitution and dehydrogenation — not consistent. (c) 4-hydroxycyclohexane-1-thiol would require OH introduction — no source of OH incorporated into C6H10S without adding oxygen. (d) Cyclohexanethione (C=S) is C6H10S but requires oxidation, not base-mediated substitution. Step 5 – The intramolecular SN2 by the thiolate on the trans-C4-Cl gives the bicyclic sulfide (b), which is the major product because intramolecular reactions forming 5- or 6-membered (here bicyclic bridged) rings are favored, and thiolates are superior nucleophiles for SN2. Therefore, the correct answer is B.