Question

Draw the structures of organic compounds A and B. Indicate stereochemistry where applicable.

Answer 1

Concepts and reason

This problem is based on the concept of alkynes. Alkynes are the class of organic compounds containing carbon-carbon triple bonds. Alkyne is converted into higher alkyne on reaction with haloalkane in the presence of a base. The reduction of alkyne to alkene can be made in the presence of Lindlar's catalyst.

Fundamentals

Alkynes that have hydrogen attached to triple bonded carbon are termed as terminal alkynes. The other alkynes are referred to as nonterminal alkynes or internal alkynes. Non-terminal alkynes are more stable and have more boiling points than the corresponding terminal alkynes.

 

(1) Ethyne, which is a terminal alkyne, reacts with sodamide to give acetylides, which in reaction with haloalkane produce higher alkyne, which is pent-1-yne.

The reaction of acetylene with propyl bromide in the presence of sodamide is written as follows:

Part 1 The structure of organic compound \(A\) is as follows:

$$ \mathrm{H} \longrightarrow \mathrm{C} \equiv \mathrm{C} \longrightarrow \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} $$

(2) 

The hydrogenation of alkynes can be easily carried out using deactivated catalysts such as Lindlar's catalyst to produce cis alkene. The Lindlar's catalyst is \(\mathrm{Pd} / \mathrm{CaCO}_{3}\) in lead acetate poisoned with a small amount of quinoline. The acid-catalyzed reaction of water with alkenes results in the formation of alcohol. The alcohol formed here is 2 pentanol. The addition of water molecule takes place according to Markonikov's rule, which states that highly substituted alkenes are predominantly formed.

The reaction of pent-1-yne with Lindlar's catalyst is as follows:

The alkene formed from the above reaction gives alcohol on hydrolysis as follows:

Part 2 The structure of organic compound \(B\) is as follows: