In the following reaction sequence, the product Y is
Show Hint
- $\text{NaNH}_2$ deprotonates terminal alkynes, and when an alkyl halide is present in the same molecule, it readily undergoes intramolecular cyclization to form a ring.
- Lindlar's catalyst always performs selective syn-addition of hydrogen to convert alkynes into cis-alkenes.
Step 1: Understanding the Concept:
The reaction involves two distinct steps:
1. Intramolecular Alkylation: Sodium amide ($NaNH_2$) is an extremely strong base. It deprotonates the terminal alkyne to form a nucleophilic acetylide ion, which can then attack an electrophilic site (like an alkyl halide) within the same molecule to form a ring.
2. Partial Hydrogenation: Lindlar's catalyst is a poisoned palladium catalyst that reduces alkynes selectively to alkenes. Because the reduction occurs via syn-addition, the product is always the cis-alkene (Z-isomer). Step 2: Detailed Explanation: Step 1: Formation of X
The starting material has a 14-carbon chain: 12 methylene groups ($-CH_2-$) and 2 alkyne carbons. It has a bromine atom at one end and a terminal alkyne at the other.
$NaNH_2$ deprotonates the terminal hydrogen to form an acetylide carbanion: $Br-(CH_2)_{12}-C \equiv C^-$.
This carbanion then performs an intramolecular nucleophilic substitution ($S_N2$) on the carbon bearing the bromine atom.
The chain closes to form a ring. Total carbons in ring = $12 + 2 = 14$.
Product X is cyclotetradecyne (a 14-membered cyclic alkyne). Step 2: Formation of Y
The cyclic alkyne X is treated with $H_2$ over Lindlar's catalyst ($Pd/CaCO_3/Quinoline$).
This reagent reduces the triple bond to a double bond. Since the hydrogen atoms are added to the same face of the triple bond (syn-addition), the resulting alkene is the cis-isomer.
Product Y is cis-cyclotetradecene.
In Figure B, the double bond is shown in the cis-configuration (Z), where the hydrogen atoms are on the same side. Step 3: Final Answer:
The cyclization and subsequent syn-reduction result in a 14-membered cyclic cis-alkene. The correct option is (B). Step 4: Final Verification:
Check the number of carbons (14) and the cis-double bond. Both match Figure B.
