Chemistry - GOC Question with Solution | TestHub

Methanol has an $\mathrm{s}{\mathrm{p}}^3$ carbon and no delocalisation: $\mathrm{C}-\mathrm{O}$ is 143 pm.

(A) In phenol the $\mathrm{s}{\mathrm{p}}^2$ carbon is smaller and the oxygen lone pair is delocalised into the ring, so the $\mathrm{C}-\mathrm{O}$ bond gets double-bond character and is about 136 pm. Shorter.

(B) In cyclohexanol the $\mathrm{C}-\mathrm{O}$ bond is a normal $\mathrm{s}{\mathrm{p}}^3$ $\mathrm{C}-\mathrm{O}$ bond (about 143 pm). Not shorter.

(C) In acetic acid the oxygen lone pair is delocalised into $\mathrm{C}=\mathrm{O}$, so $\mathrm{C}-\mathrm{OH}$ is about 136 pm. Shorter.

(D) The para $-\mathrm{N}{\mathrm{O}}_2$ pulls the lone pair further into the ring, so the $\mathrm{C}-\mathrm{O}$ bond is shorter than in phenol. Shorter.

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