Dehydration tertiary alcohols

The reaction gives poor yields of ethers with secondary and tertiary alcohols dehydration to form the corresponding olefin is a more favorable reaction. The reaction fails for the production of diaryl ethers from phenols. 

Diethyl ether and other simple symmetrical ethers are prepared industrially by the sulfuric acid-catalyzed dehydration of alcohols. The reaction occurs by SN2 displacement of water from a protonated ethanol molecule by the oxygen atom of a second ethanol. Unfortunately, the method is limited to use with primary alcohols because secondary and tertiary alcohols dehydrate by an El mechanism to yield alkenes (Section 17.6). 

Tertiary alcohols dehydrate most readily, primary alcohols least readily, and, unsurprisingly, secondary alcohols are intermediate. This relates to the relative stability of the intermediate carbocation. The temperature and concentration of the acid depends upon the type of alcohol. A primary alcohol, such as ethanol, requires concentrated acid and a very high temperature (180 degrees Celsius), while a tertiary alcohol, such as t-butyl alcohol, requires 20 percent sulfuric acid at 85 to 90 degrees Celsius. The process follows an El mechanism and produces the thermodyncimically more stable product. 

Ring enlargement of cyclobutanols.1 The reagent effects dehydration of secondary and tertiary alcohols. Dehydration of the tertiary cyclobutanol (1) results in dehydration and ring enlargement to give isolaurene (2) in quantitative yield. 

The Nohr-McDonald elimination reaction is a high temperature tertiary alcohol dehydration performed using a high boiling point aromatic solvent with zinc chloride and is described by the author. A second transorber, (I), was prepared by the author using the Nohr-McDonald elimination reaction and subsequently used to prepare the /3-cyclodextrin graft polymer as illustrated in Eq. 1 ... 

Reduction of (427) with lithium in liquid ammonia followed by trapping of the enolate with methyl iodide gave the pure 10/3-methyl compound, which was transformed into ( )-D-homotestosterone (428) by successive treatment with dilute aqueous acid at room temperature and with hot dilute aqueous methanolic sodium hydroxide. To synthesize ( )-progesterone, the carbonyl group of (428) was protected as its dioxolane, the alcohol oxidized, the derived 17a-ketone reacted with methylmagnesium bromide, the tertiary alcohol dehydrated, and the ketone deprotected to furnish the D-homo-17a-methyl-... 

Tertiary alcohols dehydrate at lower temperatures than secondary alcohols. 

Tertiary alcohols dehydrate by the El mechanism, t-Butyl alcohol is a typical example. The first step involves rapid and reversible protonation of the hydroxyl group. 

Yields of ethers from the acid-catalyzed intermolecular dehydration of alcohols are highest for symmetrical ethers formed from unbranched primary alcohols. Examples of symmetrical ethers formed in good yield by this method are dimethyl ether, diethyl ether, and dibutyl ether. From secondary alcohols, yields of ether are lower because of competition from acid-catalyzed dehydration (Section 10.6). In the case of tertiary alcohols, dehydration to an alkene is the only reaction. 

Secondary and tertiary alcohols dehydrate by the unimolecular elimination pathway (El), discussed in Sections 7-6 and 9-2. Protonation of the weakly basic hydroxy oxygen forms an alkyloxonium ion, now containing water as a good potential leaving group. Loss of H2O supphes the respective secondary or tertiary carbocations and deprotonation furnishes the alkene. The reaction is subject to all the side reactions of which carbocations are capable, particularly hydrogen and alkyl shifts (Section 9-3). 

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