Chapter 12.4, Problem 12.19P

Interpretation Introduction

Interpretation:

The given reaction sequence has to be completed.

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.

Alkynes Synthesis:

Alkynes can be prepared from germinal dihalide or vicinal dihalide by treating with a very strong base.

Two equivalents of base is required because the substrate is a dihalide.  If the alkyne formed is a terminal alkyne, then three equivalents of base has to be used resulting in salt formation of the alkyne.  This on treatment with water gives the terminal alkyne.  The general reaction can be given as shown below,

Substitution reaction of Alkynes:

The terminal alkynes can be deprotonated upon treatment with a strong base such as sodium amide to give an alkynide ion.  Alkynide ions are very strong nucleophiles and can take part in ${\text{S}}_{\text{N}}\text{2}$ reaction, when treated with electrophile.  The electrophile may be only primary halide because elimination reaction will be favored in case of secondary and tertiary halide.

If the starting alkyne is acetylene, then the installation of the alkyl groups has to be done one at one time.  This means that one group is installed on one side and then the second substitution takes place in next step.

Hydrogenation:

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.