The carbon–oxygen bond length in phenol (136 pm) is slightly less than that in methanol. Which of the following is one reason for this?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Partial double bond character due to conjugation of an unshared electron pair of oxygen with the aromatic ring
In phenol the oxygen is bonded to an sp2 hybridised carbon of the ring, and one unshared electron pair of oxygen is in conjugation with the ring. This gives the C–O bond some double bond character, so it is a little shorter than the C–O bond of methanol, where oxygen is joined to an sp3 carbon.
Why the other options are incorrect
Option (1) Repulsion between the two unshared electron pairs on the oxygen atom — Incorrect. This explains why the C–O–H bond angle in alcohols is slightly less than the tetrahedral angle; it does not shorten the C–O bond.
Option (2) Intermolecular hydrogen bonding between the –OH groups of phenol molecules — Incorrect. Hydrogen bonds hold different molecules together and explain the high boiling points of alcohols and phenols; they do not change a bond inside the molecule.
Option (3) Repulsive interaction between the two bulky groups attached to the oxygen atom — Incorrect. This explains why the C–O–C bond angle in ethers is slightly greater than the tetrahedral angle.
NCERT (2026-27 edition):
7.3 Structures of Functional Groups – “…In phenols, the –OH group is attached to sp2 hybridised carbon of an aromatic ring. The carbon– oxygen bond length (136 pm) in phenol is slightly less than that in methanol. This is due to (i) partial double bond character on account of the conjugation of unshared electron pair of oxygen with the aromatic ring … and (ii) sp2 hybridised state of carbon to which oxygen is attached.”