Structures of Functional Groups
NCERT Reference: Chapter 7 – Alcohols, Phenols and Ethers – Page 174
Quick Notes
- Alcohols and phenols contain the hydroxyl group (–OH) bonded to sp3 or sp2 hybridised carbon, respectively.
- Ethers contain the alkoxy group (–OR), where oxygen is bonded to two carbon atoms.
- The bonding in all these functional groups involves sigma (σ) bonds formed via sp3 hybrid orbitals of oxygen.
- The shape around oxygen is bent/angular due to lone pair–bond pair repulsion.
Full Notes
Bond Angles: Alcohols, Phenols, and Ethers
Bonding in Alcohols
In alcohols, the –OH group is bonded to a carbon atom via a sigma (σ) bond.
This bond is formed by overlap of an sp3 hybridised orbital of carbon with an sp3 hybridised orbital of oxygen.
The C–O–H bond angle in alcohols is slightly less than the tetrahedral angle (about 108.9° vs. ideal 109.5°), due to repulsion between the two lone pairs of electrons on the oxygen.
Bonding in Phenol
The –OH group in phenol is attached to a sp2 hybridised carbon of an aromatic ring.
Bonding in Ethers
In ethers, oxygen has:
- Two lone pairs and two bond pairs, arranged in a nearly tetrahedral geometry.
- The C–O–C bond angle is slightly greater than tetrahedral (e.g., 111.7°) due to repulsion between bulky –R groups.
Bond Lengths: Alcohols, Phenols, and Ethers
The C–O bond length in phenol is 136 pm, shorter than in methanol (142 pm).
This shortening is due to:
- Partial double bond character between the oxygen lone pair and the delocalised π-electrons of the aromatic ring.
- sp2 hybridisation of the carbon to which the oxygen is attached.
The C–O bond length (~141 pm) is similar to that in alcohols.
Summary
- Alcohols and phenols use an –OH group while ethers use an –OR group.
- Oxygen forms σ bonds using sp3 orbitals and the geometry around oxygen is bent.
- Alcohol C–O–H angles are slightly below 109.5°; ether C–O–C angles can be slightly above 109.5°.
- Phenol has a shorter C–O bond than methanol due to resonance and attachment to an sp2 carbon.