Question

Novolac on heating with formaldehyde undergoes ____ to form an infusible solid mass called ______.
A. Polymerization, melamine
B. Vulcanization, resin
C. Cross-linking, bakelite
D. Condensation, polystyrene

Answer 1

Hint: We have to remember that the polymer is a complex and giant molecule that is made of low molecular weight compounds to form macromolecules. The word polymer is from the Greek, poly means “many” and mer means “part” , that is many units have interlinking. An infusible solid is usually a polymer, that has a very high melting point and is very hard.

Complete step by step answer:
We must remember that the Novolac (or Novolak) is a phenol-formaldehyde resin, with a formaldehyde to phenol molar ratio of less than one. It is a low molecular weight compound. It is a linear type structure. Novolac is prepared from formaldehyde and phenol and we can write the chemical equation for this preparation as,

We have to remember that Acid-catalysis is used to complete the polymerization. It is an amorphous thermoplastic polymer. In novolac, the units of phenolic are mainly linked by methylene or ether groups.
Novolac (or novolak) on heating with formaldehyde undergoes cross-linking to form an infusible solid mass called bakelite. Bakelite is a cross-linked polymer but novolac is a linear polymer. We can write a reaction of novolac with formaldehyde as,

The above reaction starts from o-hydroxy phenol and formaldehyde to give ortho-and/or para-hydroxymethyl phenol derivatives, which is further reacted with formaldehyde and to form compounds having rings joined to form bakelite.

So, the correct answer is Option C.

Note: We have to remember that the novolac is an important use of photoresists, high temperature resin and for making phonographic records. It is also used for making electrical switches, combs, various utensils handles are made by novolac and also binder for carbon bonded refractories.