UPSC Civil Services (Mains), Chemistry optional · 2022 · Paper II
Q8(b)(ii). λmax of 1,3-butadiene and 1,3,5-hexatriene
Question
Explain why 1,3-butadiene exhibits a lower λmax for π → π* transitions compared to that of 1,3,5-hexatriene.
Answer in brief
Longer conjugation narrows the HOMO–LUMO gap: butadiene absorbs at 217 nm, hexatriene at 258 nm, since λ = hc/ΔE.
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Coversπ → π* transitionHOMO–LUMO gapconjugationbathochromic shiftHückel MO
All parts of Question 8
- 8(a)(i) Carbonyl stretching frequency of two acetylcyclohexenes5 marks
- 8(a)(ii) Dehydration of 1,2-dimethylcyclohexanol; telling the alkenes apart by IR10 marks
- 8(b)(i) λmax of two polyenes by Woodward–Fieser rules10 marks
- 8(b)(ii) λmax of 1,3-butadiene and 1,3,5-hexatriene5 marks
- 8(c)(i) McLafferty rearrangement; fragmentation of butyl butyrate10 marks
- 8(c)(ii) C₂H₂BrCl with J = 16 Hz; two dienones from UV data10 marks