Question

Arrange the following compounds in increasing order of C-OH bond length :
methanol, phenol, p-ethoxyphenol

• A. phenol < methanol < p-ethoxyphenol
• B. methanol < p-ethoxyphenol < phenol
• C. methanol < phenol < p-ethoxyphenol
• D. phenol < p-ethoxyphenol < methanol

Hint:

In phenols the lone pair on O is delocalised into the ring, giving the C-O bond partial double-bond character (shorter); an electron-donating group on the ring reduces this delocalisation.

Answer 1

The Correct Option is D

Methanol: the C-OH carbon is sp³, there is no resonance, so the C-O bond is a pure single bond and the longest.
Phenol: the oxygen lone pair delocalises into the ring (+M effect), the C-O bond acquires partial double-bond character and is shorter.
p-Ethoxyphenol: the -OEt group is a strong electron donor and pushes electron density into the ring, so the -OH oxygen donates its lone pair less readily; the double-bond character of C-OH is less than in phenol, so its C-OH bond is slightly longer than in phenol but still shorter than in methanol.
Increasing C-OH bond length: phenol < p-ethoxyphenol < methanol.
Answer: (4) phenol < p-ethoxyphenol < methanol