Chemistry - GOC Question with Solution | TestHub

(A) The amide $\mathrm{C}-\mathrm{N}$ bond has partial double-bond character because of N lone pair delocalisation into $\mathrm{C}=\mathrm{O}$ (barrier about 70-80 kJ/mol). The amine $\mathrm{C}-\mathrm{N}$ bond rotates almost freely (about 12 kJ/mol). Correct.

(B) In ethylene the $\mathrm{C}=\mathrm{C}$ is a full π bond (barrier about 270 kJ/mol). In the push-pull alkene the donor $-\mathrm{N}(\mathrm{C}{\mathrm{H}}_3{)}_2$ and the acceptor –CH=O make the $\mathrm{C}=\mathrm{C}$ partly a single bond in the dipolar resonance form, which lowers the barrier a great deal. Correct.

(C) The central bond of butadiene has some double-bond character through conjugation, so rotation (s-trans to s-cis) costs about 25 kJ/mol against about 12 kJ/mol for ethane. Correct.

(D) Sulphur is a better π-acceptor partner for the N lone pair than oxygen in the dipolar form ($\mathrm{C}=\mathrm{S}$ is more polarisable), so the thioamide has the greater $\mathrm{C}-\mathrm{N}$ double-bond character and the higher barrier. The statement has the order reversed. Wrong.

Answer: (A), (B), (C).