Chemistry - GOC Question with Solution | TestHub

Rotation about a $\mathrm{C}=\mathrm{C}$ bond requires loss of pi overlap. The pure ethene double bond x has the greatest localized double-bond character. Hyperconjugation slightly reduces the localized bond order in propene z. Nitrogen lone-pair donation in y produces contributors with more $\mathrm{C}-\mathrm{C}$ single-bond character; the donor-acceptor system in w permits still greater delocalization. Therefore the qualitative rotational-barrier order is x > z > y > w, A. The source conclusion w > z > y > x reverses its own bond-order reasoning.