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Organic Chemistry/Ethers, Sulfides, Epoxides, and Sulfur Functional Groups/Reactions of Ethers

Ethers, Sulfides, Epoxides, and Sulfur Functional Groups

Contents

Reactions of Ethers

Ethers are nonreactive to a wide range of reagents, which makes them very useful in organic synthesis.

Ethers, which are organic molecules with the general formula R−O−R{\rm {R{-}O{-}R}}, will also autoxidize with oxygen gas to form organic peroxides, which are very dangerous and explosive.

Ethers cleave in the presence of a haloacid (HBr, HCl, HI) to form alkyl halides (RX). This is known as acidic cleavage. Acidic cleavage is the breaking of a covalent bond using an acid. This reaction works with all ethers except for aryl ethers, such as phenyl ethers. Like alcohols, alkyl ethers are cleaved into alkyl halides using strong acids such as HI and HBr via a standard nucleophilic substitution reaction. In this reaction, the oxygen of the ether is protonated by the strong acid, creating a good leaving group. The nucleophilic halide then cleaves the ether bond, creating an alkyl halide and an alcohol. The alcohol is converted to an alkyl halide following the same mechanism. If either side of the ether is an aryl group, the aryl group will convert to an alcohol but will not further convert to an alkyl halide.

Summary of Ether Cleavage Reactions

Type of Alkyl Group (R) on Ether (R−O−R{\rm {R{-}O{-}R}}) Product from Cleavage with Haloacid (HBr, HCl, HI)
Methyl Methyl halide
Ethyl Ethyl halide
Propyl Propyl halide
Any alkyl group Alkyl halide
Benzene Phenol
Naphthalene Naphthol
Any aryl group Aryl alcohol

Cleavage of ethers by haloacids will convert any alkyl group of that ether into its corresponding alkyl halide. An aryl group of an ether is cleaved into an aryl alcohol.

For example, cyclohexyl methyl ether will cleave in the presence of hydrogen iodide (HI) to form cyclohexyl iodide and methyl iodide. But isopropoxybenzene will cleave in the presence of hydrogen bromide to isopropyl bromide and phenol. The phenyl ring is converted to phenol but cannot be converted into phenyl bromide because of the nonreactivity of the benzene ring in nucleophilic substitution reactions.

Acid Cleavage of Ethers

Acid cleavage of cyclohexyl methyl ether with hydroiodic acid (HI) will form methyl iodide and cyclohexyl iodide. Acid cleavage of isopropoxybenzene with hydrogen bromide (HBr) forms isopropyl bromide and phenol. The phenol is not converted to phenyl bromide because of its nonreactivity to nucleophilic substitution reactions.
Autoxidation is a spontaneous oxidation at room temperature in the presence of oxygen. Autoxidation of ethers is a slow but spontaneous process, and it occurs at ambient temperatures. Excess oxygen reacts at the C−H{\rm {C{-}H}} bond adjacent to the ethereal oxygen, which is the oxygen of the ether group. The reaction is a radical chain mechanism and forms hydroperoxides, ROOH. Radical chain mechanisms have three steps: initiation, propagation, and termination. Initiation begins with the cleavage of the O2 molecule into two free radicals. Each free radical reacts with the stable molecules and creates new free radicals, which feeds new reactions. Eventually, all the free radicals react with one another, forming stable molecules that no longer react, and the mechanism terminates.

Autoxidation of Ether

Autoxidation of ether is a slow and spontaneous process that forms hydroperoxides.
<Dehydration, Williamson Synthesis, and Alkoxymercuration-Demercuration>Preparations of Epoxides