Question

Arrange the following compounds in increasing order of $\mathrm{C}—\mathrm{OH}$ bond length: Methanol, Phenol, $\mathrm{p}$-ethoxyphenol

• A. Phenol < methanol < $\mathrm{p}$-ethoxyphenol
• B. methanol < $\mathrm{p}$-ethoxyphenol < phenol
• C. Phenol < $\mathrm{p}$-ethoxyphenol < methanol
• D. methanol < phenol < $\mathrm{p}$-ethoxyphenol

Answer 1

The correct option is C

Phenol < $\mathrm{p}$-ethoxyphenol < methanol

Explanation for correct option:

(C)

• In methanol, there is no resonance hence $\mathrm{C}-\mathrm{OH}$ possess only single bond character.
• In phenol, there is resonance between $-\mathrm{OH}$ bond and phenyl ring so $\mathrm{C}-\mathrm{OH}$ possesses some partial double bond character.

• In $\mathrm{p}$-Ethoxyphenol, there is resonance between $-\mathrm{OH}$ bond, phenyl ring and also $-\mathrm{OEt}$ group, so the involvement of lone pair of oxygen of $-\mathrm{OH}$ group in $\mathrm{p}$-Ethoxyphenol is poor as compared to phenol due to the $-\mathrm{OEt}$ group involvement in resonance. So, the occurrence of partial double bond character $\mathrm{C}-\mathrm{OH}$ is high in phenol then $\mathrm{p}$-Ethoxyphenol.

• Since increasing double bond character decreases bond length hence the correct order will be:

Phenol < $\mathrm{p}$-ethoxyphenol < Methanol

Therefore, option(C) is correct.