Question

why cis-2-butene is less stable than trans-2-butene?

Answer 1

In case of cis-2-butene two methyl group is on same side which is creating a steric bulk and hence destabilising it.

In trans -2 butene, two methyl groups are located at opposite position leads to less steric hindrance, hence trans-isomer is more stable than cis-isomer.

hope it helps you!