JEE Main · 2020 · 08 Jan 2020 · Shift 1 · Alcohols Phenols and Ethers

The length of the $\mathrm{C}-\mathrm{OH}$ bond depends on how far the oxygen lone pair is drawn into resonance. In phenol the lone pair conjugates with the ring, giving the $\mathrm{C}-\mathrm{O}$ bond partial double-bond character and shortening it; in $p$-ethoxyphenol the electron-donating ethoxy group raises the ring's electron density and strengthens this resonance, shortening the $\mathrm{C}-\mathrm{OH}$ bond further still. Methanol has no such resonance, so its $\mathrm{C}-\mathrm{OH}$ is a pure single bond and the longest of the three. The increasing order of $\mathrm{C}-\mathrm{OH}$ bond length is thus phenol $<$ $p$-ethoxyphenol $<$ methanol.