Q.Alkynes on reduction with sodium in liquid ammonia form trans alkenes. Will the butene thus formed on reduction of 2-butyne show the geometrical isomerism?

The key idea is Geometrical Isomerism.

1. Reduction of 2-butyne ($CH_3-C \equiv C-CH_3$) with sodium in liquid ammonia yields trans-2-butene ($CH_3-CH=CH-CH_3$).
2. Geometrical isomerism (cis-trans isomerism) occurs in alkenes when there is restricted rotation around the carbon-carbon double bond, and each carbon atom of the double bond is attached to two different groups.
3. In trans-2-butene, the two carbon atoms of the double bond are each bonded to a hydrogen atom (H) and a methyl group ($CH_3$). …

Geometrical isomerism, also known as cis-trans isomerism, is a type of stereoisomerism that arises due to restricted rotation around a bond, most commonly a carbon-carbon double bond. For an alkene to exhibit geometrical isomerism, two crucial conditions must be met:

1. There must be a carbon-carbon double bond.
2. Each carbon atom involved in the double bond must be attached to two different groups. If either carbon has two identical groups, geometrical isomerism is not possible.

The question describes a specific reaction: the reduction of an alkyne (2-butyne) using sodium in liquid ammonia. This is a well-known reaction condition that is stereoselective.

Let's break down the problem step-by-step:

1. Identify the reactant: The reactant is 2-butyne.

   2-butyne is a four-carbon alkyne with a triple bond between the second and third carbon atoms. Its structure is:

   $\text{CH}_3-\text{C}\equiv\text{C}-\text{CH}_3$

2. Understand the reaction conditions: The reduction is carried out with sodium ($\text{Na}$) in liquid ammonia ($\text{NH}_3$).

   This is a dissolving metal reduction, often referred to as a Birch reduction when applied to alkynes. A key characteristic of this reaction is its stereoselectivity: it exclusively reduces alkynes to trans alkenes. This means the two groups attached to the carbons of the original triple bond will end up on opposite sides of the newly formed double bond.

3. Determine the product of the reaction: Applying the reaction to 2-butyne.

   When 2-butyne is reduced with $\text{Na}/\text{liq. NH}_3$, the triple bond is converted into a double bond, and the product formed is trans-2-butene.

   The structure of trans-2-butene is:

4. Check for geometrical isomerism in the product (trans-2-butene): Now we need to evaluate if trans-2-butene meets the conditions for geometrical isomerism.
   • Condition 1: Presence of a carbon-carbon double bond? Yes, trans-2-butene clearly has a $\text{C}=\text{C}$ double bond.
   • Condition 2: Are the two groups on each carbon of the double bond different? Let's examine each carbon of the double bond:
     • The first carbon of the double bond (C2) is attached to a methyl group ($\text{-CH}_3$) and a hydrogen atom ($\text{-H}$). These two groups are different. …