See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Acid-catalyzed dehydration of alcohols proceeds via carbocation intermediates, and the major product is determined by carbocation stability and Zaitsev's rule (most substituted/stable alkene). Step 1: Protonation of the alcohol. Under H+/heat, the -OH of 2-cyclohexylethanol is protonated to give a good leaving group (water). Step 2: Loss of water to form a carbocation. Initially, a primary carbocation would form at the terminal carbon (-CH2+ adjacent to the ring), which is very unstable. This undergoes a 1,2-hydride or 1,2-alkyl shift. Step 3: Ring expansion via 1,2-carbon (alkyl) shift. The primary carbocation at the exocyclic carbon triggers a 1,2-shift of the adjacent C-C bond of the cyclohexane ring. This ring expansion converts the primary carbocation into a more stable secondary carbocation now incorporated into a seven-membered ring — but more relevantly, the shift produces a tertiary carbocation at the ring junction, effectively giving a cyclohexyl carbocation with exocyclic character. Actually, more precisely: The substrate is cyclohexyl-CH2-CH2-OH. After protonation and loss of water, a primary carbocation forms at C1 of the ethyl chain (i.e., -CH2(+)-CH2- attached to cyclohexane). A 1,2-hydride shift from the ring carbon to this primary carbocation gives a secondary carbocation directly on the ring carbon bearing the ethyl group (-CH(+) on the ring with -CH3 or a secondary carbocation on the carbon alpha to ring). Step 4: Re-evaluating the mechanism: The carbocation that forms after 1,2-hydride shift from the exocyclic CH2 gives a tertiary-like carbocation on the ring carbon (secondary in ring + adjacent substituent). Specifically, the ring carbon bearing the -CH2- group becomes the carbocation center (secondary carbocation at the ring, with the -CH3 group now pendant). Step 5: Elimination (E1). Loss of a proton from the adjacent carbon gives the most stable (most substituted) alkene. The resulting alkene is cyclohexylideneethane — i.e., a cyclohexane ring with an exocyclic double bond (=CHCH3), which is 1-ethylidenecyclohexane. This is a trisubstituted alkene and is more stable than the terminal alkene (vinylcyclohexane) by Zaitsev's rule. Why other options fail: - (a) Vinylcyclohexane: This would result from simple dehydration without rearrangement, giving a less substituted (terminal) alkene — not the major product. - (c) 1-methylcyclohex-1-ene: This would require ring contraction or a different rearrangement pathway, not consistent with the straightforward 1,2-hydride shift mechanism here. - (d) 4-methylcyclohex-1-ene: Same issue as (c); does not follow from the expected carbocation rearrangement. The major product is 1-ethylidenecyclohexane (cyclohexane ring with =CHCH3 exocyclic double bond), option (b), because the 1,2-hydride shift converts the primary carbocation to a secondary ring carbocation, and subsequent E1 elimination gives the more substituted exocyclic trisubstituted alkene. Therefore, the correct answer is B.