7. Alcohols, Phenols, Ethers

Bond length

Based on the provided text, the analysis of the carbon-oxygen (C–O) bond lengths in alcohols, phenols, and ethers reveals distinct differences due to the nature of the carbon atom attached to the oxygen and resonance effects.

Here is the step-by-step analysis of the changes, reasons, and overall trend:

1. Alcohols (e.g., Methanol)

• Bond Characteristics: In alcohols, the oxygen of the –OH group is attached to the carbon atom via a single sigma ($\sigma$) bond. This bond is formed by the overlap of an $sp^3$ hybridized orbital of carbon with an $sp^3$ hybridized orbital of oxygen.

• Bond Length: The standard C–O bond length in an aliphatic alcohol like methanol acts as the baseline (approximately 142 pm, as inferred from its comparison to ethers).

2. Phenols

• Bond Characteristics: In phenols, the –OH group is attached to an $sp^2$ hybridized carbon atom of an aromatic ring.

• Bond Length: The C–O bond length in phenol is 136 pm, which is slightly shorter than the bond length in methanol.

• Reasons for the Shorter Bond:

  1. Conjugation and Resonance: The unshared (lone) electron pair on the oxygen atom is in conjugation with the aromatic ring. This delocalization imparts a partial double bond character to the carbon-oxygen bond, naturally pulling the atoms closer together and shortening the bond.

  2. Hybridization: The carbon atom in the aromatic ring is $sp^2$ hybridized. An $sp^2$ hybridized carbon has more s-character than an $sp^3$ carbon, making it slightly more electronegative and smaller, which forms a shorter and stronger bond with oxygen.

3. Ethers (e.g., Methoxymethane)

• Bond Characteristics: Similar to aliphatic alcohols, the oxygen atom in typical dialkyl ethers is attached to $sp^3$ hybridized carbon atoms.

• Bond Length: The C–O bond length in ethers is 141 pm, which is almost exactly the same as the C–O bond length found in alcohols.

Summary of the Trend

Phenol (136 pm) < Alcohol ($\approx$ 141-142 pm) $\approx$ Ether (141 pm)

While alcohols and ethers have nearly identical C–O single bond lengths due to their $sp^3$ hybridized carbons, phenol has a noticeably shorter bond because of its $sp^2$ hybridized carbon and the partial double bond character created by resonance.