Problem on condensation polymerisation

Question: Identify the correct functional group ‘X’ and the reagent or reaction condition ‘Y’ in the following scheme.

a)      X = COOCH₃, Y = H₂ / Ni / Heat

b)      X = CONH₂, Y = H₂ / Ni / Heat

c)       X = CONH₂, Y = Br₂ / NaOH

d)      X = CN, Y = H₂ / Ni / Heat

Solution: The end product of the following scheme is a condensation polymer. From here we can get a clue for the whole reaction. So let us recall the basics of condensation polymerisation.

1. ·         A condensation polymer is formed through a condensation reaction--where two molecules (monomers) join together and loose small molecules as by-products such as water or methanol.
2. ·         There are ester and amide linkages in the polymer.
3. ·         Monomers that are joined by condensation polymerization have two functional groups.

·         These functional groups can be a carboxylic acid and an amine which form an amide linkage

·         or a carboxylic acid and an alcohol which form an ester linkage

Now let us go back to the question and check each option whether it can fulfill the above requirement.

a)      If  X = COOCH₃, Y = H₂ / Ni / Heat

In the first step, compound will hydrogenate in the presence of catalyst Ni.

 
The obtained product is an alcohol. In the next step the alcohol will react with carboxylic acid and form an ester linkage.

 The final product, a polyester, is formed by condensation of two monomers having two different functional groups i.e. an alcohol and an acid. in this reaction a water molecule is also formed as a byproduct. The poly-ester has an ester (-COO-) linkage. Thus, option (a) is correct for the given scheme.

b)      If  X = CONH₂, Y = H₂ / Ni / Heat
In the first step, amide will hydrogenate in amine in the presence of catalyst Ni.

In the next step the amine will react with carboxylic acid and form an amide linkage.

The final product, a polyamide, is formed by condensation of two monomers having two different functional groups i.e. an amine and an acid. in this reaction a water molecule is also formed as a byproduct. The poly-amide has an amide (-HN-CO-) linkage.  Thus, option (b) is correct for the given scheme.

c)       If  X = CONH₂, Y = Br₂ / NaOH

In the first step, compound will react in the presence of sodium hydroxide and bromine. Basically it is a Hoffman Bromide Reaction.

[The Hofmann rearrangement is the organic reaction in which a primary amide (–CONH₂) group is converted into primary amino (– NH₂) group. ]

The obtained product is an amine. In the next step this amine will react with carboxylic acid and form an amide linkage.  

The final product is a polyamide which is form by condensation polymerisation. Thus, option (c) is correct for the given scheme.

d)      If  X = CN, Y = H₂ / Ni / Heat
In the first step, compound will hydrogenate in the presence of catalyst Ni.

The obtained product is an amine. In the next step this amine will react with carboxylic acid and form an amide linkage.

The final product is a polyamide which is form by condensation polymerisation. Thus, option (d) is correct for the given scheme.

Thus, all options are correct for the given scheme.