Write a reaction for preparation of Nylon-6.(ii) The salt of Sc3+ ion is colourless and the salt of Mn3+ ion is coloured. Explain. [ Z of Sc = 21 and Z of Mn = 25 ]

Answer:

(i) Nylon-6 is made by heating caprolactam with a little water at about 533–543 K. The ring opens to give 6-aminohexanoic acid units, which polymerise by forming amide bonds:

n caprolactam (seven-membered cyclic amide, –NH–(CH₂)₅–CO– in a ring) —(H₂O, 533–543 K)→ –[NH–(CH₂)₅–CO]_n– (nylon-6)

(ii) Colour of transition metal ions arises from d–d transitions: an electron in a lower-energy d orbital absorbs visible light and jumps to a higher-energy d orbital, and the complementary colour is seen. This needs partly filled d orbitals.

• Sc (Z = 21): [Ar]3d¹4s²; Sc³⁺: [Ar]3d⁰. With no d electron, no d–d transition is possible, so Sc³⁺ salts are colourless.
• Mn (Z = 25): [Ar]3d⁵4s²; Mn³⁺: [Ar]3d⁴. It has four unpaired d electrons; d–d transitions absorb visible light, so Mn³⁺ salts are coloured.