See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Step 1 – Identify the starting material structure. The starting material is Ph-CH(OH)-C≡C-CH=CH-OMe. It contains a propargylic alcohol (secondary OH on the carbon bearing the phenyl group), an internal alkyne, a conjugated vinyl ether terminus (CH=CH-OMe), and one existing C=C double bond. Step 2 – Reaction with H2/Pd-BaSO4 (Lindlar-type conditions, semi-hydrogenation). Pd-BaSO4 is a poisoned (partially deactivated) palladium catalyst used for semi-hydrogenation of alkynes to cis-alkenes. Under these conditions, the alkyne (C≡C) is selectively reduced to a cis (Z) alkene (C=C). The existing vinyl ether double bond (CH=CH-OMe) is generally less reactive under these mild conditions and may also be hydrogenated, but the key transformation is the alkyne → cis-alkene. After hydrogenation of the triple bond, product (A) is Ph-CH(OH)-CH=CH-CH=CH-OMe (a 1,3-diene system with the vinyl ether intact, or with the enol ether present). More precisely, (A) is Ph-CH(OH)-CH=CH-CH=CH-OMe, a diene with a terminal vinyl ether. Step 3 – Reaction of (A) with H3O(+) (acid hydrolysis). The vinyl ether group (enol ether: -CH=CH-OMe) undergoes acid hydrolysis. An enol ether on hydrolysis gives an aldehyde (or ketone). The terminus -CH=CH-OMe hydrolyzes to -CH2-CHO, but since it is part of a conjugated system, the hydrolysis of the vinyl ether -CH=CH-OMe yields -CH2-CHO. However, the full conjugated system Ph-CH(OH)-CH=CH-CH=CH-OMe upon acid treatment also causes dehydration of the allylic/propargylic alcohol (the Ph-CH(OH)- group loses OH under acidic conditions to extend conjugation) AND hydrolyzes the vinyl ether. Step 4 – Combined effect: dehydration of the alcohol and hydrolysis of the vinyl ether. The secondary benzylic/allylic alcohol Ph-CH(OH)- readily dehydrates under acidic conditions (H3O+) to give Ph-CH= extending conjugation. Simultaneously, the vinyl ether -CH=CH-OMe hydrolyzes to give -CH2-CHO (or via enol → aldehyde: -CHO at the terminal position). After dehydration of OH and hydrolysis of OMe enol ether: - Dehydration: Ph-CH(OH)- → Ph-CH= (adds one double bond) - The chain Ph-CH=CH-CH=CH-OMe after dehydration at the alcohol end becomes Ph-CH=CH-CH=CH-CH=... - Hydrolysis of vinyl ether -CH=CH-OMe → -CHO Putting it together: Starting from Ph-CH(OH)-CH=CH-CH=CH-OMe (compound A): - Acid-catalyzed dehydration of the benzylic alcohol gives Ph-CH=CH-CH=CH-CH=CH... but we must track carbons carefully. Carbon count in starting material: Ph-C1H(OH)-C2≡C3-C4H=C5H-OMe (5 carbons in chain). After semi-hydrogenation: Ph-C1H(OH)-C2H=C3H-C4H=C5H-OMe (A). Under H3O+: 1. Vinyl ether hydrolysis: C4H=C5H-OMe → C4H2-C5HO (i.e., -CH2-CHO), giving Ph-CH(OH)-CH=CH-CH2-CHO initially as enol tautomer, but the enol ether C4=C5-OMe hydrolyzes to give the aldehyde at C5 → -CH2-CHO. 2. The allylic/benzylic alcohol at C1 dehydrates: Ph-CH(OH)- loses H2O → Ph-CH= connecting into the diene. Result: Ph-CH=CH-CH=CH-CHO, which is Ph-(CH=CH)2-CHO. This is option (c): Ph-(CH=CH)2-CHO. Why other options fail: (a) retains the OH and has 6 carbons in chain – does not account for dehydration and correct hydrolysis product. (b) Ph-CH=CH-CHO has only 2 carbons between Ph and CHO – too short (only 3C chain vs 5C chain). (d) Ph-(CH=CH)3-CHO would require 7 carbons – too long for the 5-carbon chain. Therefore, the correct answer is C.