アルコールからのエーテル: アルコール脱水とウィリアムソン エーテル合成

Acid-catalyzed alcohol dehydration begins with proton transfer from the acid catalyst to the hydroxyl group, forming an oxonium ion—a better leaving group than hydroxyl. A second alcohol molecule then attacks this oxonium ion via an SN2 mechanism, displacing water and forming a new oxonium ion. Final proton transfer to water yields the ether product.

Secondary and tertiary alcohols preferentially dehydrate to alkenes rather than ethers because elimination competes favorably with substitution. Additionally, using different primary alcohols yields a mixture of ethers, making the method unsuitable for asymmetrical ether preparation. These limitations make alcohol dehydration practical only for symmetrical ethers from primary alcohols.

Williamson ether synthesis is a two-step process that deprotonates an alcohol with a strong base like sodium hydride to form an alkoxide ion, which then undergoes an SN2 reaction with an alkyl halide to form the ether. This method is more versatile because it produces asymmetrical ethers and avoids the mixture problems and elimination side reactions associated with alcohol dehydration.

Methyl and primary alkyl halides are preferred substrates because they are less sterically hindered and exhibit high SN2 reactivity. Secondary and tertiary alkyl halides are more hindered and undergo elimination instead of substitution, reducing ether yield. The nucleophilic alkoxide ion attacks the carbon bearing the halide most effectively when steric hindrance is minimal.

The alkoxide ion, formed by deprotonating an alcohol with a strong base, acts as the nucleophile in the SN2 reaction. It attacks the carbon of the alkyl halide, displacing the halide as a leaving group and forming the ether product. The alkoxide's strong nucleophilicity and basicity make it ideal for this substitution reaction.

Temperature significantly influences whether alcohol dehydration produces ethers or alkenes. At lower temperatures like 413 K with sulfuric acid, ethanol dehydration yields ethoxyethane. At higher temperatures around 443 K, the same reaction produces ethene instead. This temperature dependence reflects the competing pathways of substitution and elimination.

Alcohol dehydration uses an acid catalyst to activate the hydroxyl group and proceeds via an SN2 mechanism where one alcohol acts as substrate and another as nucleophile. Williamson synthesis uses a strong base to deprotonate the alcohol first, then the resulting alkoxide nucleophile attacks an alkyl halide via SN2. Both follow SN2 mechanisms but differ in activation strategy and substrate requirements.