Dehydration of 2-methyl-2-butanol

There is a strong similarity between the mechanism shown in Eigure 5.12 and the one shown for alcohol dehydration in Eigure 5.6. The main difference between the dehydration of 2-methyl-2-butanol and the dehydrohalogenation of 2-bromo-2-methylbutane is the source of the carbocation. When the alcohol is the substrate, it is the conespond-ing alkyloxonium ion that dissociates to fonn the carbocation. The alkyl halide ionizes directly to the carbocation. 

Write a stepwise mechanism for the formation of 2-methyl-2-butene from the dehydration of 2-methyl-2-butanol is sulfuric acid. 

The dilute sulfuric acid catalyzed dehydration of 2-methyl-2-butanol (r-amyl alcohol) proceeds readily to give a mixture of alkenes that can be analyzed by gas chromatography. The mechanism of this reaction involves... 

From your knowledge of the dehydration of tertiary alcohols, which olefin should predominate in the product of the dehydration of 2-methyl-2-butanol and why ... 

Problem 13.15. Give the equation for the dehydration of 2-methyl-2-butanol. If more than one product is formed, show which hydrogen is lost in the formation of each product and indicate the major product. 

Two isomeric alkenes are formed in the dehydration of 2-methyl-2-butanol. What are these two alkenes Are they constitutional isomers or stereoisomers ... 

The dehydration of an alcohol is an elimination reaction that requires breaking the carbon-oxygen bond and a carbon-hydrogen bond on an adjacent, P-carbon atom. Thus, dehydration of alcohols having two nonequivalent P-carbon atoms adjacent to the OH-bearing carbon atom can form a mixture of products. In such cases, the more substituted alkene is the major product. For example, the dehydration of 2-methyl-2-butanol yields 2-methyl-2-butene as the major product. The isomeric 2-methyl-1-butene forms in a substantially smaller quantity. 

However, dehydration of 2-methyl-1-butanol with H2SO4 forms 2-methyl-2-butene by carbocation rearrangement. 

Acid-catalyzed dehydration of 3-methyl-2-butanol gives three alkenes 2-methyl-2-butene, 3-methyl-l-butene, and 2-methyl-l-butene. Propose a mechanism to account for the formation of each product. 

Write the structures of the products of the dehydration of 3-methyl-2-butanol. 

Except for the biochemical example just cited the stractures of all of the alcohols m Section 5 9 (including those m Problem 5 13) were such that each one could give only a single alkene by p elimination What about ehmmahon m alcohols such as 2 methyl 2 butanol m which dehydration can occur in two different directions to give alkenes that are conshtutional iso mers Here a double bond can be generated between C 1 and C 2 or between C 2 and C 3 Both processes occur but not nearly to the same extent Under the usual reachon con dihons 2 methyl 2 butene is the major product and 2 methyl 1 butene the minor one... 

When an alcohol has two or three different P carbons, dehydration is regioselective and follows the Zaitsev rule. The more substituted alkene is the major product when a mixture of constitutional isomers is possible. For example, elimination of H and OH from 2-methyl-2-butanol yields two constitutional isomers the trisubstituted alkene A as major product and the disubstituted alkene B as minor product. 

Cychdehydration. The sulfonic acid resin Ambcrlite-15 was found to be more satisfactory than Bradsher s reagent (2, 214), sulfuric acid, or formic acid for cyclo-dehydration of 2-methyl-4-phenyl-2-butanol (1) to 1,1-dimethylindane (2). ... 

Similar interpretation can be used in explaining the results of Skell and Maxwell (86) who dehydrated 2-methyl-1-butanol in the presence of bromoform and aqueous potassium hydroxide in solution. 

The ease of dehydration depends largely on the reaction conditions and still more on the structure of the alcohol in question. Increasing substitution by alkyl groups increases the rate of olefin formation. Also, the ease of dehydration increases in the order primary > secondary > tertiary alcohols for the preparation of olefins it often suffices to warm a tertiary alcohol with a trace of iodine or in no more than 15% sulfuric acid.9 For example, 2-methyl-2-hexene is formed merely by distilling 2-methyl-2-hexanol containing a trace of iodine,10 and refluxing 2-methyl-2-butanol with 15% sulfuric acid affords 2-methyl-2-butene with a little 2-methyl-l-butene 11... 

Two reviews have appeared which are concerned particularly with the role of solid catalysts in dehydration reactions . Kieboom and van Bekkum have reported details of the dehydration of 2-aryl-3-methyl-2-butanols by sulphuric acid and by dimethyl sulphoxide. Surprisingly, for both reactions, the kinetic product is the thermodynamically less stable 2-aryl-3-methyl-l-butene, which subsequently isomerises to 2-aryl-3-methyl-2-butene. Steric interactions were proposed to account for the initial formation of the less stable alkene in the reaction of the carbonium ion. Significant proportions of the Hoffmann... 

Methyl vinyl ketone can be produced by the reactions of acetone and formaldehyde to form 4-hydroxy-2-butanone, followed by dehydration to the product (267,268). Methyl vinyl ketone can also be produced by the Mannich reaction of acetone, formaldehyde, and diethylamine (269). Preparation via the oxidation of saturated alcohols or ketones such as 2-butanol and methyl ethyl ketone is also known (270), and older patents report the synthesis of methyl vinyl ketone by the hydration of vinylacetylene (271,272). 

Dehydration of 1-pentanol or 2-pentanol to the corresponding olefins has been accompHshed, in high purity and yields, by vapor-phase heterogeneous catalyzed processes using a variety of catalysts including neutral gamma —Al Og catalyst doped with an alkah metal (23), zinc aluminate (24,25), hthiated clays (26), Ca2(P0 2 montmorillonite clays (28). Dehydration of 2-methyl-1-butanol occurs over zinc aluminate catalyst at... 

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