Question

Which of the following molecules will show both hyperconjugation and mesomeric effect?

• A. $C{H}_{3}Cl$
• B. $C{H}_3-CH=C{H}_2$
• C. $C{H}_3-CH=CH-\underset{\underset{O}{||}}{C}-C{H}_3$
• D. $C{H}_2=CH-CH=C{H}_2$

Answer 1

The correct option is C $C{H}_3-CH=CH-\underset{\underset{O}{||}}{C}-C{H}_3$

$C{H}_3-CH=CH-\underset{\underset{O}{||}}{C}-C{H}_3$
In this compound, $-\underset{\underset{O}{||}}{C}-C{H}_3$ is an electron-withdrawing group which will show the $-M$ effect due to the conjugation of $C=C$ bond with the $C=O$ bond.
Also the leftmost $H$ atoms of $C{H}_3$ group can show hyperconjugation with the $C=C$ bond which is in conjugation with $C=O$ bond.
Hence option (c) structure can show both hyperconjugation as well as mesomeric effect.