UPSC Civil Services (Mains), Chemistry optional · 2024 · Paper II
Q2(a)(ii). 1,2- and 1,4-addition of HBr to butadiene
Question
Consider the following reaction: CH₂=CH–CH=CH₂ + HBr → CH₃–CH(Br)–CH=CH₂ + CH₃–CH=CH–CH₂Br. At −80 °C, the 1,2-addition product predominates while at −45 °C, the 1,4-addition product is preferred. Justify.
Model answer
The free solution for 2024 answers questions 1, 3, 5, 6 and 8, the ones picked in its time plan. Question 2 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
Covers1,2- vs 1,4-additionkinetic controlthermodynamic controlallylic carbocationbutadieneHBr addition
All parts of Question 2
- 2(a)(i) kH/kD = 7.1 for 2-phenylethyl bromide with ethoxide10 marks
- 2(a)(ii) 1,2- and 1,4-addition of HBr to butadiene5 marks
- 2(b)(i) Benzaldehyde with diethyl malonate and pyridine10 marks
- 2(b)(ii) Structure of X; name; proof of syn-elimination5 marks
- 2(c)(i) Products of three reactions15 marks
- 2(c)(ii) 2-Methylpent-2-enoic acid by the Reformatsky reaction5 marks