Question
Most stable conformation of  $n-$ butane is

Answer

b
The lowest energy conformation will be the one, in which the two methyl groups are as far apart as possible i.e., $180^{\circ}$ away form each other. This conformation will be maximum staggered and is called anti conformation. Other conformations can be obtained by rotating one of the $C _2$ or $C _3$ carbon atoms through an angle of $60^{\circ}$ as shown ahead.

The order of stability of these conformations is, Anti $\,>\,$ Skew or Gauche $\,>\,$ Eclipsed $\,>\,$ Fully eclipsed.

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