Ethers may be synthesized in principly two ways:
We have studied preparation of alkenes by the removal of water from an alcohol molecule with the aid of Sulfuric Aicd or Phosphoric Acid. This is an intramolecular dehydration and results in an elimination reaction producing an alkene. Water can also be removed between two separate molecules of alcohol to produce an ether. This involves an initial protonation of one molecule of the alcohol followed by an SN2 style substitution of another alcohol molecule on the protonated molecule.(See Fig 1 below)
This preparation of ethers is rather severely limited to primary alcohols since secondary alcohols lead to a large elimination product to form an alkene, and tertiary alcohols lead exclusively to alkene products due to elimination. The alkenes form because of the loss of water by the protonated secondary and tertiary alcohols to form the very stable carbocations which then lose a beta carbon to produce the relatively substituted alkene.(See Fig 2). There is a better way of producing ethers that do not involve these problems.
The Williamson synthesis is a good way to prepare unsymmetrical ethers where the alkyl groups are not the same. The reaction involves an SN2 reaction in which an alkoxide ion replaces a halogen,sulfonyl, or sulfate group. Usually, alkyl halides are used. The alkoxide can be prepared by the reaction of the corresponding alcohol with an active metal such as metallic Sodium or the use of a metal hydride like NaH acting upon the alcohol. The resulting alkoxide salt is then reacted with the alkyl halide (sulfonate or sulfate) to produce the ether in an SN2 reaction. See Fig 3 below)
Since the reaction occurs using an SN2 pathway, the usual limitations apply. The alkyl halide is best to be a primary halide with secondary giving substantial elimination product and a tertiary halide yielding exclusively elimination product. In addition, a low temperature will increase the yield of the substitution product over that of the elimination product.
Epoxides, also called oxiranes, are organic structures that contain a three atom cyclic ether consisting of two Carbons and an Oxygen. These three membered ring structures are very reactive because of the great deal of ring strain reminescent of cyclopropane reactivity and ring strain. Epoxides can be formed by reacting an alkene with a peroxyacid, a carboxylic acid with an extra Oxygen atom in its structure.. Peracetic acid, CH3COOOH, can be used. The problem with most peroxy acids however is their tendency to decompose. One particular stable peroxy acid is Magnesium MonoPeroxyPthalic Acid (MMPP). The peroxy acid inserts the extra Oxygen atom between the sp2 carbons of the alkene creating an Oxygen bridge and the epoxide structure. This is a syn addition since the creation of the Oxygen bridge is on the same side of the alkene molecule. This formation of the epoxide is called an Epoxidation.(See Fig 4 below)
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R. H. Logan, Instructor of Chemistry, Dallas County Community College District, North Lake College.
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Revised: 7/20/97
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