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37.1.2.15 Method 15: Synthesis from Alcohols Using Brønsted Acids

DOI: 10.1055/sos-SD-037-00052

Tsukamoto, M.; Kitamura, M.Science of Synthesis, (20083772.

With a catalytic amount of Brønsted acid, an ether is formed from two molecules of alcohol via dehydration.[‌1‌,‌2‌] Acid activates the hydroxy group as an oxonium ion, which that works as a better leaving group in the nucleophilic attack by the second molecule of alcohol. The method is applicable to preparing some special symmetrical ethers such as diethyl ether,[‌230‌] diallyl ether,[‌230‌,‌231‌] and bis(diphenylmethyl) ether.[‌232‌] Sulfuric,[‌230‌] hydrochloric,[‌231‌] and phosphoric acids[‌232‌] have been used in homogeneous reactions. Unsymmetrical ethers (e.g., 127) are usually formed successfully in combinations of a primary alcohol (e.g., 126) and an alcohol (e.g., 125) possessing alkyl groups generating stable carbocations, such as tert-butyl[‌233‌] and substituted benzyl groups (Scheme 45).[‌232‌,‌234‌] Reaction of 3-hydroxycyclohex-2-enone (128) and ethanol gives the ethyl ether 129 in good yield.[‌235‌] Hydrobromic acid, generated in situ by nucleophilic attack of alcohol 130 on alkyl bromide 131 (130/131 10:1) at high temperature, catalyzes formation of symmetrical ethers 132 (Scheme 45).[‌236‌]

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