# cis- and trans-isomers in equilibrium

Rotation of groups about double bonds are usually restricted due to large torsional strain and it is due to this reason that cis- and trans-isomers have their independent existence. However, certain structural modifications in the compound makes the rotation of groups about double bond possible and hence, for such compound, cis- and trans-isomer exist together in equilibrium.

Identify which of the following compounds has such structural feature and its cis and trans isomers exist together in equilibrium:

(A) $$\ce{CH3-CH=CH-CHO}$$
(B) $$\ce{CH3-CH=CH-CH3}$$
(C) $$\ce{cyclohexyl-CH=CH-cyclohexyl}$$
(D) $$\ce{CH3-CH=NH}$$

Could someone explain the answer step-by-step and the theory behind it?

I think that rotation of groups about a double bond can be possible due to conjugation of double bonds, so the answer should be (A). But I am afraid I haven't completely understood the logic behind it.

• Have you considered keto-enol tautomers? – Karsten Theis Mar 31 at 11:14
• I would suggest D as the H atom can easily tunnel from one side on the N atom to the other. Tunnelling cannot happen in any of the other molecules as the groups are too massive. – porphyrin Apr 1 at 14:46
• – Buck Thorn Apr 12 at 14:18