Chemistry - GOC Question with Solution | TestHub

Compound (R) has a donor ($-\mathrm{OC}{\mathrm{H}}_3$) and an acceptor ($-\mathrm{CHO}$) on the $\mathrm{C}=\mathrm{C}$, so the dipolar resonance form lowers the rotation barrier.

(i) Acceptor only, no donor: higher barrier. (ii) Donor only: higher. (iii) $-\mathrm{N}(\mathrm{C}{\mathrm{H}}_3{)}_2$ is a stronger donor than $-\mathrm{OC}{\mathrm{H}}_3$: more single-bond character, lower barrier. (iv) $-\mathrm{N}{\mathrm{O}}_2$ is a stronger acceptor than $-\mathrm{CHO}$: lower barrier. (v) Plain alkene: highest barrier. (vi) $-\mathrm{CON}{\mathrm{H}}_2$ is a weaker acceptor than $-\mathrm{CHO}$: weaker push-pull, higher barrier.

Higher than (R): (i), (ii), (v), (vi) = 4.

Answer: 4.