Alcohols, Phenols and Ethers - Structures of Functional Groups

Explain why the bond angle in alcohols like $\text{CH}_3\text{OH}$ is slightly less than the tetrahedral angle, whereas in ethers like $\text{CH}_3\text{OCH}_3$, it is slightly greater.

In $\text{CH}_3\text{OH}$, $\angle C-O-H = 108.9^\circ < 109.5^\circ$. In $\text{CH}_3\text{OCH}_3$, $\angle C-O-C = 111.7^\circ > 109.5^\circ$.

In alcohols, the repulsion between the two lone pairs on the oxygen atom pushes the $O-H$ and $C-O$ bonds closer together, decreasing the angle. In ethers, although lone pair repulsion is present, the steric hindrance (repulsion) between the two bulky alkyl groups ($-\text{CH}_3$) outweighs the lone pair effect, forcing the bond angle to expand beyond $109.5^\circ$.

Compare the $C-O$ bond length in $\text{CH}_3\text{OH}$ and $\text{C}_6\text{H}_5\text{OH}$.

$d(C-O)_{\text{phenol}} < d(C-O)_{\text{methanol}}$

The $C-O$ bond in phenol ($136 \text{ pm}$) is shorter than in methanol ($142 \text{ pm}$) because in phenol, the lone pair on oxygen undergoes resonance with the benzene ring, giving the $C-O$ bond partial double bond character. Additionally, the carbon in phenol is $sp^2$ hybridized (more $s$-character, more electronegative), making the bond shorter and stronger than the $sp^3$ carbon-oxygen bond in methanol.