JEE Main 2021 · 25 July · morning · Single correct · +4 / −1
is a repeating unit for :
- (a) Novolac
- (b) Buna-N
- (c) Acrilan
- (d) Neoprene
Answer and solution
Answer (a) Novolac
Solution
Chemistry ma’am · 3 steps
Spot the phenol rings joined by CH₂ bridges: that is phenol and formaldehyde, so it is Novolac.
- Read the unit. The repeating unit has two benzene rings, each carrying an -OH group. So each ring is a phenol unit. The rings are joined through -CH₂⁻ bridges at the positions next to -OH (the ortho positions).
- Where CH2 comes from. The -CH₂⁻ bridge comes from formaldehyde, HCHO. Phenol and formaldehyde condense in the presence of acid, losing water, to give a linear chain of phenol rings linked by methylene groups. This linear polymer is Novolac.
Polymer Monomer(s) Phenol ring with -CH₂⁻ links? Novolac phenol + formaldehyde yes Buna-N 1,3-butadiene + acrylonitrile no, has C=C and -CN Acrilan acrylonitrile no, has -CN on a carbon chain Neoprene chloroprene no, has -Cl and C=C - So. Only Novolac is built from phenol and formaldehyde, so the repeating unit shown belongs to (a) Novolac.
The idea to keep. Benzene rings with -OH joined by -CH₂⁻ always mean phenol–formaldehyde resin. Linear chain is Novolac; heat it with more HCHO to cross-link it into Bakelite.
Answer from the NTA final answer key. Solution by PrepWiser.