Question:
When but-2-yne is treated with Na in liquid ammonia
When but-2-yne is treated with Na in liquid ammonia
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Use this simple mnemonic: "Na-Trans". Sodium in ammonia gives the Trans product. If the reagent was Lindlar's, remember "Lin-Cis"!
Updated On: May 6, 2026
- cis-2-butene is obtained
- trans-2-butene is formed
- n-butane is the major product
- it rearranges to but-1-yne
- there is no reaction
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The Correct Option is B
Solution and Explanation
Concept: The reduction of non-terminal alkynes using Sodium (Na) or Lithium (Li) in liquid ammonia is known as Birch Reduction.
• Stereoselectivity: This specific reagent combination results in "anti-addition" of hydrogen.
• Product Geometry: Because the hydrogens add from opposite sides, the most stable, less hindered product is formed, which is the trans-alkene.
• Contrast: Hydrogenation with Lindlar's catalyst ($H_2/Pd/BaSO_4$) would produce the cis-alkene.
Step 1: Analyze the starting material and reagent. But-2-yne ($CH_3-C\equiv C-CH_3$) is an internal alkyne. The reagent is $Na / \text{liq. } NH_3$. This reaction proceeds via a radical anion mechanism where the trans-vinylic radical is preferred due to lower electronic repulsion between the methyl groups.
Step 2: Determine the final product. The triple bond is reduced to a double bond. Due to the trans-preference of the Birch reduction, the two methyl groups end up on opposite sides of the double bond. Product: trans-2-butene.
• Stereoselectivity: This specific reagent combination results in "anti-addition" of hydrogen.
• Product Geometry: Because the hydrogens add from opposite sides, the most stable, less hindered product is formed, which is the trans-alkene.
• Contrast: Hydrogenation with Lindlar's catalyst ($H_2/Pd/BaSO_4$) would produce the cis-alkene.
Step 1: Analyze the starting material and reagent. But-2-yne ($CH_3-C\equiv C-CH_3$) is an internal alkyne. The reagent is $Na / \text{liq. } NH_3$. This reaction proceeds via a radical anion mechanism where the trans-vinylic radical is preferred due to lower electronic repulsion between the methyl groups.
Step 2: Determine the final product. The triple bond is reduced to a double bond. Due to the trans-preference of the Birch reduction, the two methyl groups end up on opposite sides of the double bond. Product: trans-2-butene.
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