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Alcohol Structure and Hydroxyl Group
1.0Introduction to Alcohols and the Hydroxyl Group
Alcohol is a group of organic compounds featuring the presence of a hydroxyl group (-OH) attached to a saturated carbon atom. Alcohol generally has the formula R-OH, where 'R' is an alkyl or substituted alkyl group. It is the hydroxyl group that serves as the functional group, determining the characteristic chemical and physical properties of alcohols.
2.0Structure of the Hydroxyl Group
A hydroxyl group contains an oxygen atom bonded to a hydrogen atom. In alcohol, this oxygen atom is also bonded to a carbon atom by a single bond. The oxygen atom in the hydroxyl group has two electron pairs that are not shared (non-bonding or lone pairs). These lone pairs will cause the molecule to have a bent (or V-shaped) molecular geometry about the oxygen atom. The bond angle between the H-O-C atoms is approximately 108.9°, which is close to ideal tetrahedral angle of 109.5°. The slight difference in the bond angle from the ideal value is a result of the repulsive effect of the two lone pairs on the oxygen atom.
3.0Classification of Alcohols
Alcohols are classified based on the number of other carbon atoms directly bonded to the carbon atom bearing the hydroxyl group.
- Primary (1o) Alcohol: The carbon atom bonded to the -OH group is attached to only one other carbon atom. Example: Ethanol
- Secondary (2o) Alcohol: The carbon atom bonded to the -OH group is attached to two other carbon atoms. Example: Propan-2-ol
- Tertiary (3o) Alcohol: The carbon atom bonded to the -OH group is attached to three other carbon atoms. Example: 2-Methylpropan-2-ol
4.0Structure of Alcohol
Bonding and Hybridization
- Oxygen in the hydroxyl group is sp³ hybridized.
- Two orbitals form sigma bonds with:
- Hydrogen atom.
- Carbon atom of the alkyl group.
- The remaining two orbitals contain lone pairs of electrons.
Molecular Geometry
- The C–O–H bond angle measures approximately 104.5° (similar to tetrahedral).
- Alcohols are capable of forming hydrogen bonds between molecules, resulting in higher boiling points than ethers and alkanes with similar molecular weight.
5.0Types of Alcohols (Based on Carbon Atom Bearing –OH)
- Primary (1°): –OH attached to carbon bonded with one other carbon (CH₃CH₂OH).
- Secondary (2°): –OH attached to carbon bonded with two other carbons (CH₃CHOHCH₃).
- Tertiary (3°): –OH attached to carbon bonded with three other carbons ((CH₃)₃COH).
6.0Structure of Phenol
Phenols are compounds in which the hydroxyl group (–OH) is directly attached to a benzene ring (an aromatic sp² carbon).
Bonding
In the –OH group, the oxygen atom is sp³ hybridized. One of the orbitals overlaps with hydrogen, one overlaps with the aromatic carbon, and two are reserved for lone pairs of electrons. Because of resonance effects, one of the lone pairs on oxygen can delocalize into the benzene π-system, stabilizing phenol.
Molecular Structure
- The C–O bond length in phenol is shorter than in alcohols because of partial double bond character from resonance.
- Phenol molecules also form hydrogen bonds, increasing boiling points.
Resonance in Phenol
Phenol exhibits resonance where the negative charge on oxygen delocalizes into the benzene ring, increasing acidity compared to alcohols.
7.0Structure of Ether
Ethers are organic compounds with the general formula R–O–R′, where oxygen is bonded to two alkyl or aryl groups.
Bonding and Hybridization
- Oxygen is sp³ hybridized.
- Two orbitals form sigma bonds with alkyl/aryl groups.
- Two orbitals contain lone pairs.
- The bond angle around oxygen is approximately 111°, slightly larger than tetrahedral due to the repulsion of bulky alkyl groups.
Molecular Properties
- Ethers do not form strong intermolecular hydrogen bonds (except with water).
- They have lower boiling points than isomeric alcohols of the same molecular mass.
- Ethers are relatively inert but can undergo cleavage by strong acids (HI, HBr).
8.0Types of Ethers
- Simple Ethers (Symmetrical): R–O–R (e.g., Diethyl ether, CH₃CH₂–O–CH₂CH₃).
- Mixed Ethers (Asymmetrical): R–O–R′ (e.g., Ethyl methyl ether, CH₃–O–C₂H₅).
- Aromatic Ethers: At least one aryl group attached (e.g., Anisole, C₆H₅OCH₃).