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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper II covers organic chemistry and spectroscopy.

UPSC Civil Services (Mains), Chemistry optional · 2023 · Paper II

Q1(b)(i). IUPAC name with stereodescriptors

Question

Write the IUPAC nomenclature of the above compound by assigning the stereochemistry. [structure given in the paper]

Answer in brief

(2R,3R)-3-Chloro-2-hydroxypent-4-enal.

Key points

  • Parent: pentanal, CHO = C-1; C=C is 4-ene; chloro before hydroxy.
  • CIP order: C-2: OH > C-3 > CHO > H; C-3: Cl > C-2 > C-4 > H.
  • Lowest priority H faces the viewer: anticlockwise is reversed, giving R.

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.

Model answer to Q1(b)(i), 2023 Paper II: solution pages 1 and 2
Solution pages 1 and 2 of 26
Read the whole 2023 solution2023 question paper

Topic notes: stereochemistry and conformational analysis (R/S, E/Z, Fischer and Newman projections, cyclohexane chair), with every Civil Services, IFoS and Geo-Scientist PYQ →

CoversIUPAC nomenclatureR/S configurationCIP sequence rulesstereodescriptors3-chloro-2-hydroxypent-4-enal

All parts of Question 1