See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: The dehydration of 2-butanol under conc. H2SO4 proceeds via an E1 mechanism through a secondary carbocation intermediate (sec-butyl carbocation: Me-CH2-CH(+)-CH3). Loss of a proton from either the CH3 or CH2 group gives alkene products. Step 1 - Identify the possible products: From the sec-butyl carbocation (Me-CH2-C(+)H-CH3), elimination of H+ can give: - 1-butene (CH2=CH-CH2-CH3): loss of H from the terminal CH2 group - cis-2-butene (cis CH3-CH=CH-CH3): loss of H from the CH3 group, cis geometry - trans-2-butene (trans CH3-CH=CH-CH3): loss of H from the CH3 group, trans geometry Zaitsev's rule predicts 2-butene isomers are major products over 1-butene. Step 2 - Assign energy levels a, b, c: From the energy profile diagram, the three products at the right end are at three different energy levels: - Level a (highest energy) = least stable product = 1-butene (less substituted, less stable) - Level b (intermediate energy) = cis-2-butene (more substituted but cis has steric strain making it less stable than trans) - Level c (lowest energy) = most stable product = trans-2-butene (most substituted and no steric strain) Step 3 - Identify product (b): Product b corresponds to the intermediate energy level, which is cis-2-butene. The cis isomer is more stable than 1-butene (more substituted double bond) but less stable than trans-2-butene (due to steric repulsion between the two methyl groups on the same side). Step 4 - Why other options fail: - (a) 1-butene: This is product 'a' (highest energy = least stable) - (c) trans-2-butene: This is product 'c' (lowest energy = most stable) - (d) iso-butene: This would require a methyl shift (1,2-hydride or methyl shift) which does not occur readily here and is not shown as a product on the diagram Therefore, the correct answer is B.