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Saturday, 31 October

JEE Main 2021 · ChemistryMultiple choiceDiagram basedMediumDiagram reading

JEE Main 31 August 2021, Shift 2, Chemistry Q42

Question 42 of 90 in this shift, Chemistry question 12 of 30, Section A.

Arrange the following conformational isomers of n-butane in order of their increasing potential energy : [four Newman projections labelled I, II, III, IV]

The figure, in words

Four Newman projections (circle with three bonds meeting at the centre and three more bonds starting at the circle edge), labelled I-IV, with substituent labels as drawn. I: CH3 at top, H upper-left, H upper-right, H lower-left, H lower-right, CH3 at bottom. II: CH3 at top, CH3 upper-right, H left, H lower-left, H lower-right, H right (the top CH3 bond and the upper-right CH3 bond are drawn close together). III: CH3 at top, H upper-left, CH3 upper-right, H lower-left, H at bottom, H lower-right. IV: H at top, H upper-right, H left, H3C lower-left, CH3 lower-right, H right.
  1. (1)I < IV < III < II
  2. (2)II < III < IV < I
  3. (3)I < III < IV < IIOfficial answer
  4. (4)II < IV < III < I

Official answer

Option 3

NTA final key.

Chapter
Hydrocarbons
Topic
Alkanes and Free Radicals
Idea tested
Ethane and Butane Conformers

Same idea in other shifts

Asked 3× in all
  1. 25 Jun 2022, Shift 1 · Q72In the following structures, which one is having staggered conformation with maximum dihedral angle ?EasyDiagram basedHas a figure
  2. 3 Aug 2021, Shift 1 · Q50Which of the following is the most stable conformer?MediumDiagram basedHas a figure

Question text from the official JEE Main paper published by NTA; answer from the final answer key. Chapter, topic and idea tags, difficulty and skill are Lumi’s. This question needs a figure we do not reproduce; it is described in words above. Lumi analyses 108 of the 174 JEE Main shifts held from 2017 to 2026.