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Question 36

What will be the major product of following sequence of reactions?

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Solution

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Solution: Major Product Sequence

Step 1: Alkylation of the Terminal Alkyne

The starting material is a terminal alkyne: n-Bu-C≡CH.

  • Reaction with n-BuLi: strong organolithium base deprotonates the terminal acetylenic proton to form a nucleophilic carbanion:
    n-Bu-C≡C- Li+
  • Reaction with n-C5H11Cl: The carbanion performs an SN2 nucleophilic substitution on the primary alkyl halide, extending the chain into an internal alkyne:
    n-Bu-C≡C-C5H11

Step 2: Stereoselective Reduction

The internal alkyne is treated with Lindlar's Catalyst (Pd/CaCO3 poisoned with quinoline) and H2.

  • Lindlar's catalyst is a heterogeneous, deactivated catalyst that reduces alkynes selectively into alkenes without reducing them all the way to alkanes.
  • The mechanism occurs via syn-addition (both hydrogen atoms deliver to the same face of the triple bond simultaneously).
  • This stereospecific addition yields exclusively a cis-alkene (Z-alkene) geometry.
Correct Option Match:
The product must contain the n-Bu and -C5H11 groups positioned on the same side of the newly formed carbon-carbon double bond carbon axis. This matches structural geometry variant A.

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