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JEE Main · 2021 · 31 Aug 2021 · Shift 2 · General Organic Chemistry

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Arrange the following conformational isomers of n-butane in order of their increasing potential energy :

I (see structure)
II (see structure)
III (see structure)
IV (see structure)
Structure I
I
Structure II
II
Structure III
III
Structure IV
IV
  1. (A)I < III < IV < II✓ Correct
  2. (B)I < IV < III < II
  3. (C)II < III < IV < I
  4. (D)II < IV < III < I

Answer: (A)

Solution

For n-butane the potential energy of a conformation rises with the strain it carries. The anti form (I), with the two methyl groups apart, is free of steric and torsional strain and is lowest. The gauche form (III), with the methyls staggered apart, carries one gauche methyl-methyl interaction. The form in which a methyl eclipses a hydrogen (IV) has torsional eclipsing strain, and the fully eclipsed form (II), with the two methyls eclipsing each other, has the greatest strain. The increasing order of potential energy is I < III < IV < II.

JEE Main · 2021 · 31 Aug 2021 · Shift 2 · Single correct option

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