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  Reactions of Ethers Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Reactions of Ethers

Ethers are extensively used as solvents for a range of organic compounds and reactions, suggesting that they are comparatively unreactive themselves. Certainly, with the exception of the alkanes, fluorocarbons and cycloalkanes, ethers are possibly the least reactive, general class of organic compounds. The inert nature of the ethers comparative to the alcohols is undoubtedly because of the nonexistence of the reactive O-H bond.
The most general reaction of ethers is cleavage of the C-O bond by strong acids. This may take place by SN1 or E1 mechanisms for 3º-alkyl groups or by an SN2 mechanism for 1º-alkyl groups. Some illustrations are displayed in the diagram. The conjugate acid of the ether is an intermediate in all these reactions, just like conjugate acids were intermediates in certain alcohol reactions.

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The first two reactions proceed by a sequence of SN2 steps where the bromide or iodide anion displaces an alcohol in the first step and in the second then converts the conjugate acid of that alcohol to an alkyl halide. Because SN2 reactions are favored at least hindered sites, the methyl group in illustration no.1 is cleaved first. The 2º-alkyl group in illustration no.3 is probably cleaved by an SN2 mechanism, but SN1 alternative cannot be ruled out. The phenol produced in this reaction does not react further, because SN2, SN1 and E1 reactions do not occur on aromatic rings. The last illustration displays the cleavage of a 3º-alkyl group by a strong acid. Acids that are having poorly nucleophilic conjugate bases are frequently chosen for this purpose so that E1 products are favored. The reaction displayed here is the reverse of the tert-butyl ether preparation explained earlier.
Ethers in which oxygen is bonded to 1º- and 2º-alkyl groups in the presence of air (gaseous oxygen)are subject to peroxide formation. This reaction exists an additional hazard to the make use of these flammable solvents, because peroxides decompose explosively when heated or struck. The technique of peroxide formation is believed to be free radical in nature (note that molecular oxygen has two unpaired electrons).

R-O-CH(CH3)2   +   O2 →  R-O-C(CH3)2-O-O-H   a peroxide

Reactions of Epoxides

Epoxides (oxiranes) are three-membered cyclic ethers which are easily prepared from alkenes by reaction with per-acids. Because of the large angle strain within this small ring, epoxides go through acid and base-catalyzed C-O bond cleavage more easily than do larger ring ethers. Among the following illustrations, the first is unexceptional except for the fact that it takes place under milder conditions and more quickly than other ether cleavages. The second and third illustrations clearly displayed the exceptional reactivity of epoxides, because unstrained ethers exist in identical reactant or as solvent do not react. The aqueous acid employed to work up the 3rd reaction, following the Grignard reagent cleavage of the ethylene oxide, just neutralizes the magnesium salt of the alcohol product.

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