Ethers are organic compounds in which an oxygen atom is bonded to two alkyl groups, two aryl groups, or one alkyl and one aryl group. Their general formula is R–O–R′, where R and R′ may be the same or different. Ethers are widely used as solvents and are important intermediates in organic synthesis.
1. Preparation by Dehydration of Alcohols
When alcohols are heated with concentrated sulfuric acid at about 413 K (140°C), two molecules of alcohol combine to form an ether molecule with the elimination of a water molecule. This reaction is known as intermolecular dehydration of alcohols.
- The reaction occurs through protonation of the alcohol followed by nucleophilic attack by another alcohol molecule.
- It is most suitable for preparing symmetrical ethers.
- At higher temperatures (around 443 K), dehydration leads to the formation of alkenes instead of ethers.
Mechanism of Ether Formation by Dehydration of Alcohols
This method is mainly suitable for the preparation of symmetrical ethers and works best with primary alcohols.
Step 1: Protonation of Alcohol
The alcohol molecule reacts with concentrated sulphuric acid. The oxygen atom of the alcohol accepts a proton (H⁺), forming a protonated alcohol. This makes the alcohol a better leaving group.
Step 2: Nucleophilic Attack by Another Alcohol Molecule
A second alcohol molecule attacks the carbon atom of the protonated alcohol. Water is eliminated during the process. An oxonium ion is formed.
Step 3: Deprotonation
The oxonium ion loses a proton to form the ether. Thus, an ether molecule is produced and the acid is regenerated.
2. Preparation by Williamson Ether Synthesis
Williamson ether synthesis is the most important and versatile method for preparing ethers. In this method, an alkoxide ion reacts with an alkyl halide through a nucleophilic substitution reaction (SN₂) to produce an ether.
- The reaction proceeds through nucleophilic substitution (SN₂), in which the alkoxide ion attacks the carbon atom bonded to the halogen, leading to the formation of the ether and the removal of the halide ion.
- Aryl halides do not undergo Williamson ether synthesis because the C–X bond in aryl halides has partial double-bond character and is resistant to nucleophilic substitution.
Mechanism of Williamson Ether Synthesis (SN₂ Reaction)
Williamson Ether Synthesis is the most important laboratory method for preparing both symmetrical and unsymmetrical ethers.
Step 1: Nucleophilic Attack
The alkoxide ion (RO−) acts as a nucleophile and attacks the carbon atom bonded to the halogen in the alkyl halide (R′X). The attack occurs from the side opposite to the leaving halogen atom.
Step 2: Formation of Ether
As the alkoxide ion forms a bond with the carbon atom, the halide ion (X−) leaves simultaneously. This results in the formation of the ether molecule.