Question

Cis- decalin has lower energy than trans decalin.
What is the reason behind the above statement?

Answer 1

Observe the structure provided in the Hint

Reason

In trans-decalin, only the rigid (e, e) conformer is poosible. Flipping to (a, a) orientation is impossible, because the diaxial bonds point 180 away from each other and cannot be bridged by only four C atoms to complete the second ring. In cis-conformation, fusion is (e, a) and flipping can give the equilibrating (a, e) conformer. Therefore, cis-form is flexible.
The trans conformer (e, e) has lower energy than cis conformer (e, a)