Organic Chemistry II: Alcohols, Phenols, and Ethers - SS2 Chemistry Past Questions and Answers - page 5

Ethers are organic compounds that contain an oxygen atom bonded to two alkyl or aryl groups. They exhibit distinct properties and characteristics compared to other organic compounds. Some key features of ethers include:

1.    Boiling Points and Solubility: Ethers generally have lower boiling points compared to alcohols of similar molecular weight. This is due to the absence of hydrogen bonding between ether molecules. Ethers are typically soluble in organic solvents but have limited solubility in water.

2.    Inertness: Ethers are relatively inert compounds and exhibit low reactivity. They are stable under normal conditions and are less prone to oxidation and other reactions compared to alcohols and other functional groups.

3.    Lewis Base Properties: Ethers can act as Lewis bases due to the presence of lone pairs of electrons on the oxygen atom. This property allows ethers to form complexes with Lewis acids, such as metal cations.

4.    Anaesthetic Properties: Certain ethers, such as diethyl ether (ethoxyethane), have anaesthetic properties and have been used historically as general anaesthetics. However, their use has declined due to safety concerns.

Examples illustrating the properties of ethers include the low boiling point of diethyl ether (-23 °C) compared to ethanol (78 °C) and its use as a nonpolar solvent in organic reactions.

Ethers can be synthesised through several methods. Three commonly used synthesis methods for ethers are:

1.    Williamson Ether Synthesis: The Williamson ether synthesis is a widely used method for the preparation of ethers. It involves the reaction of an alkoxide ion with an alkyl halide or an aryl halide. The steps of the Williamson ether synthesis are as follows:

●     Deprotonation: A strong base, such as sodium hydride (NaH), deprotonates the alcohol to form an alkoxide ion.

●     Nucleophilic Substitution: The alkoxide ion acts as a nucleophile, attacking the carbon atom of the alkyl halide or aryl halide, resulting in the formation of the ether.

This method is applicable for the synthesis of symmetrical ethers when the alkyl halide or aryl halide used has the same alkyl or aryl group on both sides.

2.    Acid-Catalyzed Dehydration of Alcohols: In the presence of an acid catalyst, alcohols can undergo dehydration reactions to form ethers. The steps of the acid-catalysed dehydration of alcohols are as follows:

●     Protonation: The alcohol is protonated by the acid catalyst, generating a good leaving group (water).

●     Elimination: The protonated alcohol undergoes elimination of a water molecule, resulting in the formation of the ether.

This method is useful for the synthesis of unsymmetrical ethers, where different alkyl or aryl groups are desired on each side of the oxygen atom.

3.    Reaction of Alkyl Halides with Sodium Alkoxides: Alkyl halides can react with sodium alkoxides to form ethers through a nucleophilic substitution reaction. The steps involved are as follows:

●     Deprotonation: The alkoxide ion, formed by the reaction of an alcohol with sodium metal, acts as a nucleophile by attacking the alkyl halide.

●     Nucleophilic Substitution: The alkoxide ion replaces the halide group in the alkyl halide, resulting in the formation of the ether.

This method is commonly used for the synthesis of symmetrical ethers.