Question

Ordinarily the barrier to rotation about a carbon-carbon double bond is quite high but in compound P double bond between two rings was observed by NMR to have a rotational energy barrier of only about 20 cal/mol, showing that it has lot of single bond character.
The reason for this is

• A. Double bond having partial triple bond character because of resonance
• B. Double bond undergo flipping
• C. Double bond is having high single bond character because of aromaticity gained in both three and five membered rings.
• D. +I effect of $n{C}_3{H}_7$ groups makes double bond having partial single bond character.

Answer 1

The correct option is C Double bond is having high single bond character because of aromaticity gained in both three and five membered rings.


In the ionic form of the given compound, we have 6$\pi$ electrons in 5 membered ring and 2$\pi$ electrons in 3 membered ring. Both obeys (4n+2) pi Huckel's rule rule so they are aromatic and give more stability to the compound in ionic form. Hence the double in compound will have more single bond character and have a rotational energy barrier of 20 cal/mol.