See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Step 1 - Starting material and first reaction: The starting material is CH3-C(ONa)=CH2, which is the sodium enolate of methyl vinyl ketone (MVK) or equivalently the sodium salt of isopropenol (sodium isopropenoxide). It reacts with HC≡CH (acetylene) under the conditions shown. Step 2 - Reaction with HC≡CH: The sodium enolate (CH3-C(ONa)=CH2) acts as a nucleophile. Acetylene undergoes nucleophilic addition (vinylation). The sodium alkoxide adds to acetylene in a Reppe-type reaction, giving CH3-C(=CH2)-O-CH=CH2, i.e., isopropenyl vinyl ether. Step 3 - H+ treatment (acid hydrolysis): The vinyl ether CH3-C(=CH2)-O-CH=CH2 undergoes acid hydrolysis. Vinyl ethers hydrolyze under acidic conditions to give an aldehyde/ketone plus an enol. The vinyl ether hydrolyzes to give isopropenyl alcohol (which tautomerizes to acetone) and acetaldehyde, but more precisely the product is the mixed enol ether hydrolysis product. Actually, the vinyl ether R-O-CH=CH2 on hydrolysis gives ROH + CH3CHO. So we get isopropenol (CH3-C(OH)=CH2, which tautomerizes to acetone) and acetaldehyde? That does not lead to the answer. Let me reconsider. Step 2 revised - Reformatsky/Reppe reaction context: The sodium salt CH3-C(ONa)=CH2 (sodium isopropenoxide) reacts with acetylene. In base-catalyzed addition, the carbanion/enolate adds to acetylene to give: CH2=C(CH3)-O-CH=CH2 (isopropenyl vinyl ether) after the sodium salt. On acid workup (H+), the enol ether hydrolyzes: the vinyl ether portion (-O-CH=CH2) hydrolyzes to give -OH + CH3CHO equivalent, but the whole molecule is CH2=C(CH3)-O-CH=CH2. Acid hydrolysis of this vinyl ether gives CH2=C(CH3)-OH (isopropenol → acetone by tautomerism) and acetaldehyde. This still does not give a C5 diene. Step 2 re-revised: The sodium enolate of methyl vinyl ketone adds to acetylene. The enolate carbon (CH2=) attacks acetylene: CH2=C(CH3)-ONa + HC≡CH → product where the alpha carbon adds to acetylene: giving CH2=C(CH3)-CH=CH-ONa (1,4-addition product / sodium enolate). On H+ workup, the enol tautomerizes: CH2=C(CH3)-CH=CH-ONa + H+ → CH2=C(CH3)-CH2-CHO (4-methylpent-4-en-1-al?). No. Corrected mechanism: The enolate oxygen of CH3-C(ONa)=CH2 adds to acetylene giving CH3-C(=CH2)-O-CH=CH2 (isopropenyl vinyl ether). Acid hydrolysis (Claisen rearrangement is thermal, not acid). Under H+, vinyl ether hydrolyzes to give ketone: CH3-CO-CH3 (acetone) + CH3CHO. This still does not give C5 product. Final correct pathway: The enolate carbon attacks acetylene → CH2=C(CH3)-C≡CH (dehydration product after H+, giving 2-methylbut-1-en-3-yne). Then H2/Pd-BaSO4 (Lindlar-type semi-hydrogenation) reduces the triple bond to cis double bond: CH2=C(CH3)-CH=CH2 (2-methylbuta-1,3-diene = isoprene). Then Al2O3/Δ causes dehydration or isomerization. Isoprene on Al2O3/Δ would not change much, but answer (a) is H2C=CH-C(CH3)=CH2, which IS isoprene. So the final product after Al2O3/Δ is isoprene itself, or the Al2O3 step causes isomerization to give isoprene. Option (a) H2C=CH-C(=CH2)-CH3 is isoprene (2-methyl-1,3-butadiene). This matches the synthesis of isoprene from isopropenyl acetylene via semi-hydrogenation, and the Al2O3/Δ step may cause further isomerization confirming isoprene as product. Therefore, the correct answer is A.