Chapter 12.4, Problem 12.18P

Interpretation Introduction

Interpretation:

The product that is expected for the given reaction has to be drawn.

Concept Introduction:

Alkynes are the compounds that contain a triple bond between two carbon atoms.  The carbon atom present in the triple bond is $sp$ hybridised.  Due to this, the geometry of alkyne will be linear.  Terminal alkyne are the one which has a proton attached to the triple bond and internal alkyne are the one in which there are no protons attached to the triple bond.

Hydrogenation reaction is one of the reactions that alkynes undergo.  Addition of hydrogen is the hydrogenation reaction.  This can be accomplished by using hydrogen molecule and metal catalyst.  Hydrogenation reaction is also known as reduction reaction.

When an alkyne undergoes hydrogenation reaction with hydrogen and catalyst such as platinum, the final product obtained will be alkane.  The intermediate formed in the hydrogenation reaction is alkene and this is further reduced to alkane.

Under the above conditions, the alkene cannot be isolated.  This is because the alkene is more reactive than the alkyne towards hydrogenation.

If the alkene has to be obtained from alkyne means, then a partially deactivated catalyst like Lindlar’s catalyst or sodium in liquid ammonia can be used.  Partially deactivated catalyst is known as poisoned catalyst.  When the alkyne is reduced using Lindlar’s catalyst, the alkene product obtained will be having cis configuration.  If the alkyne is reduced using sodium in liquid ammonia means the alkene obtained will be having trans configuration.