Question

Ordinarily the barrier to rotation about a carbon-carbon double bond is quite high but compound A was observed by NMR to have a rotational barrier of only about 20K cal/ mole is due to:

• A. double bond having partial triple bond character because of resonance
• B. double bond undergo flipping
• C. double bond having very high single bond character becauyse of aromaticity gained in both 3 & 5 membered ring.
• D. +I effect of $n{C}_3{H}_7$ groups makes double bond having partial single bond character.

Answer 1

Answer: (C)

double bond having very high single bond character becauyse of aromaticity gained in both 3 & 5 membered ring.

As rotational barrier of double bond is quite high because of sidewise overlapping of p-orbital leads to the formation of $\pi$ bond. But in case of $3$ membered and $5$ memberred ring to gain aromaticity each carbon must be $s{p}^2$ hybridized.

As there is one positive charge and double bonds present in the ring which makes it aromatic.

(Refer to Image)

Therefore due to aromaticity, all carbon bond is partially single and partially double. Due to aromaticity double bond having very high single bond character because of aromaticity gained in both $5$ membered and $3$ membered ring.