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Question 36

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

  1. Anti Conformation (Most Stable): This is a staggered position where the two bulky methyl (-CH3) groups are exactly 180° apart. This maximizes the distance between them, minimizing steric hindrance and making it the lowest energy state.
  2. Gauche Conformation (Staggered but Less Stable): The two methyl groups are rotated to be 60° apart. While still staggered to avoid overlapping bonds, the bulky methyl groups are close enough to cause some steric hindrance, making it less stable than the Anti form.
  3. Eclipsed Conformation (Unstable): The bonds on the front and back carbons align perfectly. This forces the electron clouds to repel each other, creating high torsional strain and lowering the molecule's stability.
  4. Total Eclipsed Conformation (Least Stable): This is the highest energy state. The two massive methyl groups are directly aligned (0° apart) and perfectly eclipse one another, causing maximum physical crowding and repulsion.

    Thus the correct order is I<III<IV<II. The right option is A.

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