UPSC Civil Services (Mains), Chemistry optional · 2023 · Paper II
Q1(b)(ii). Dimerisation ability of three triarylmethyl-type radicals
Question
Arrange the above radicals in ascending order of their dimerisation ability. [structures of three triarylmethyl-type radicals (A), (B), (C) given in the paper]
Answer in brief
(B) < (A) < (C): (p-FC₆H₄)₃C· < Ph₃C· < Ph₂(t-Bu)C·.
Key points
- Ph₃C·: α-C shielded by three-ring propeller; couples at para-C: quinoid dimer.
- (B): all para positions carry F: coupling site blocked; least dimer.
- (C): two rings delocalise: less resonance stabilisation; para-H free: most dimer.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Coverstriphenylmethyl radicalGomberg radicalradical dimerisationquinoid dimerradical stability
All parts of Question 1
- 1(a)(i) Acidity of cyclopentadiene5 marks
- 1(a)(ii) Hydrogen exchange in cyclopropyl and cyclopropenyl ketones5 marks
- 1(b)(i) IUPAC name with stereodescriptors5 marks
- 1(b)(ii) Dimerisation ability of three triarylmethyl-type radicals5 marks
- 1(c) Methyl iodide with sodium azide: DMF against methanol10 marks
- 1(d) Photochemical electrocyclic closure of two cyclonona-1,3-dienes10 marks
- 1(e)(i) 2-Chloro-6-methylcyclohexanone with methoxide5 marks
- 1(e)(ii) Name reaction that gives nitrogen as a by-product5 marks