Why is the `C-O-H` bond angle in alcohols slightly less than the tetrahedral angle whereas the C-O-C bond angle in ether is slightly greater?

Options

A. of repulsion between the two bulky R groups .
B. O atom in both alcohols and ethers is `sp^(3)`-hybridised .
C. lone pair-lone pair repulsion is greater than bond pair - bond pair repulsion .
D. none of these

Answer

To understand why the `C-O-H` bond angle in alcohols is slightly less than the tetrahedral angle, while the `C-O-C` bond angle in ethers is slightly greater, we can analyze the molecular geometry and the factors affecting these angles.

### Step-by-Step Solution:

1. **Understanding Tetrahedral Geometry**:
- The ideal bond angle in a tetrahedral geometry is 109.5 degrees. This is the angle formed when a central atom is bonded to four substituents.

2. **Bonding in Alcohols**:
- In alcohols (e.g., CH3OH), the central carbon atom is bonded to three hydrogen atoms and one hydroxyl (-OH) group. The oxygen atom in the -OH group has two lone pairs of electrons.
- The presence of these lone pairs affects the bond angles due to lone pair-bond pair repulsion. Lone pairs occupy more space than bonding pairs, which can compress the bond angles between the bonded atoms.

3. **Effect of Lone Pair Repulsion**:
- In alcohols, the `C-O-H` bond angle is slightly less than 109.5 degrees due to the repulsion between the lone pairs on the oxygen and the hydrogen atom. This results in a bond angle that is less than the ideal tetrahedral angle.

4. **Bonding in Ethers**:
- In ethers (e.g., CH3OCH3), the central oxygen atom is bonded to two carbon atoms. There are no lone pairs on the oxygen in this case.
- The `C-O-C` bond angle in ethers is slightly greater than 109.5 degrees. This is because the two carbon atoms are larger and create more steric repulsion compared to the hydrogen atoms in alcohols.

5. **Steric Effects**:
- The larger size of the carbon atoms in ethers leads to increased steric repulsion between the two carbon groups. This repulsion pushes the `C-O-C` bond angle slightly greater than the tetrahedral angle.

6. **Conclusion**:
- Therefore, the `C-O-H` bond angle in alcohols is slightly less than the tetrahedral angle due to lone pair repulsion, while the `C-O-C` bond angle in ethers is slightly greater due to steric repulsion between the larger carbon groups.