Q6(a)(ii). C=O bands of butyric acid, its ethyl ester and its anhydride
The IR spectra of butyric acid and ethyl butyrate show sharp strong singlet absorption at 1725 cm⁻¹ and 1740 cm⁻¹, respectively. By contrast, the IR spectrum of butyric anhydride shows a doublet at 1750 cm⁻¹ and 1825 cm⁻¹. Why are these so different?
Model answer
The free solution for 2023 answers questions 1, 2, 3, 5 and 8, the ones picked in its time plan. Question 6 is answered in full in the Chemistry Paper II, 2021 to 2025 book, with every part of every paper.
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UPSC Civil Services (Main) Chemistry Paper II: Model Answers 2021 to 2025
- All eight questions of each paper, with the question printed above its answer
- Mechanisms drawn out and labelled on the scheme
- Topic-wise weightage and a topic-wise index
CoversIR carbonyl stretchacid anhydride doubletvibrational couplingsymmetric and antisymmetric stretchester C=O
All parts of Question 6
- 6(a)(i) Energy change of HCl + D₂ → DCl + HD10 marks
- 6(a)(ii) C=O bands of butyric acid, its ethyl ester and its anhydride5 marks
- 6(b)(i) Products A, B and C10 marks
- 6(b)(ii) Tacticity: isotactic, syndiotactic and atactic polymers5 marks
- 6(c)(i) Structure of C₁₆H₂₅NO from IR and ¹H NMR10 marks
- 6(c)(ii) Order of chemical shift of the lettered protons10 marks