3.3- Dimethyl-2-butanol dehydration

A survey of catalysis literature reveals that for the majority of transition metal oxide catalyzed reactions, maximum TOFs most frequently correlate with maxima in polymeric surface species content. A limited subset of examples includes WO /ZrOj for o-xylene isomerization [33], n-pentane isomerization [29], and 2-butanol dehydration [34] MoO /AljOj and VO /AljOj for dimethyl ether oxidation [35] MoO /AljOj for propane oxidative dehydrogenation [36] andMoO, supported on TiOj, ZrOj, AljOj, and NbjOs for methanol oxidation [37]. 

Evidence for the intermediate carhocations in the acid-catalvzed dehydration of alcohols comes from the observation that rearrangements sometimes occur. Propose a mechanism to account for the formation of 2,3-dimethyl-2-butene from 3,3-dimethyl-2-butanol. ... 

From 3,3-dimethyl-2-butanol, the major product of rearrangement is 2,3-dimethyl-1-butene. The distribution of the primary dehydration products is far from equilibrium. The maximum ratio of 2,3-dimethyl-1-butene to 2,3-dimethyl-2-butene obtained from 2,3-dimethyl-2-butanol is about 10. This is higher than that to be expected if a proton is removed from the l,l,2-trimethyl-2-propyl carbonium ion in a statistical manner. The maximum ratio of the two olefins obtained from 2,3-dimethyl-2-butanol is also about 10. Hence it can be argued that the high yield of 2,3-dimethyl-1-butene from 3,3-dimethyl-2-butanol does not necessarily rule out a classical carbonium ion mechanism. It is very unlikely, however, that the same intermediate is involved from both alcohols. If such were the case the product of dehydration of 2,3-dimethyl-2-butanol would contain appreciable amounts of 3,3-dimethyl-l-butene. 

The products from the dehydration of 3,3-dimethyl-2-butanol can be explained by anchimeric assistance of the methyl group and the removal of the proton from the y-carbon atom ... 

The dehydration of the two alcohols over alumina catalyst in the presence of piperidine was studied by Pillai and Pines [84). The experimental results which are given in Table X indicate that, although carbonium ion mechanism can interpret the products obtained from the tertiary alcohols, another mechanistic path has to prevail in order to account for the formation of the various dehydration products from 3,3-dimethyl-2-pentanol. The mechanism, as proposed above for the dehydration of 3,3-dimethyl-2-butanol, would also explain the hydrocarbons formed from the dehydration of 3,3-dimethyl-2-pentanol. 

Problem 6.55 Dehydration of 3,3-dimethyl-2-butanol, (CH JjCCHOHCH, yields two alkenes, neither of which is (CHj),CCH==CH2. What are their structures <... 

SAMPLE SOLUTION (a) Dehydration of 2,3-dimethyl-2-butanol can lead to either 2,3-dimethyl-1-butene by removal of a C-1 hydrogen or to 2,3-dimethy 1-2-butene by removal of a C-3 hydrogen. 

In the dehydration of 3,3-dimethyl-2-butanol, a secondary carbocation is formed initially, which rearranges to a tertiary carbocation when a neighboring methyl group with its bonding electron pair migrates to the positive carbon. The charge is thereby transferred to the tertiary carbon ... 

Sometimes unexpected products are formed in dehydration that is, the carbon skeletons of the starting material and product might be different, or the double bond might be in an unexpected location. For example, the dehydration of 3,3-dimethyl-2-butanol yields two alkenes, whose carbon skeletons do not match the carbon framework of the starting material. 

The dehydration of 3,3-dimethyl-2-butanol illustrates the rearrangement of a 2° to a 3° carbocation by a 1,2-methyl shift, as shown in Mechanism 9.3. The carbocation rearrangement occurs in Step [3] of the four-step mechanism. 

Steps [1], [2], and [4] in the mechanism for the dehydration of 3,3-dimethyl-2-butanol are exactly the same steps previously seen in dehydration protonation, loss of H2O, and loss of a proton. Only Step [3], rearrangement of the less stable 2° carbocation to the more stable 3° carbocation, is new. 

It can be considered that these pillared products will be intercalated by accompanying with proton to produce a solid acid catalyst, because they exhibited acidity as shown in Table 1. To examine the acidic property of the catalysts dehydrations of methanol and 1-butanol were attempted by a flow reactor. The dehydration products of methanol were dimethyl ether and water, and those of 1-butanol were 1-, cis-2-, and trans-2-butenes and water. At relatively low temperature (250°C to 300°C) in hydration of 1-butanol a... 

Since a 1,2-shift of a methyl group can convert the initially formed secondary cation into the more stable tertiary cation, such a rearrangement does occur, and much of the product is derived from this new ion. (If we compare this change in carbon skeleton with the one accompanying dehydration of 3,3-dimethyl-2-butanol (p. 171), we can begin to see how the idea arose that these apparently unrelated reactions proceed through the same intermediate.)... 

Water can be removed from methanol by a membrane of polyvinyl alcohol cross-linked with polyacrylic acid, with a separation factor of 465.204 A polymeric hydrazone of 2,6-pyridinedialdehyde has been used to dehydrate azeotropes of water with n- and /-propyl alcohol, s- and tort butyl alcohol, and tetrahydrofuran.205 The Clostridium acetobutylicum which is used to produce 1-butanol, is inhibited by it. Pervaporation through a poly(dimethyl-siloxane) membrane filled with cyclodextrins, zeolites, or oleyl alcohol kept the concentration in the broth lower than 1% and removed the inhibition.206 Acetic acid can be dehydrated with separation factors of 807 for poly(4-methyl-l-pentene) grafted with 4-vinylpyridine,207 150 for polyvinyl alcohol cross-linked with glutaraldehyde,208 more than 1300 for a doped polyaniline film (4.1 g/m2h),209 125 for a nylon-polyacrylic acid membrane (5400 g/m2h), and 72 for a polysulfone.210 Pyridine can be dehydrated with a membrane of a copolymer of acrylonitrile and 4-styrenesulfonic acid to give more than 99% pyridine.211 A hydrophobic silicone rubber membrane removes acetone selectively from water. A hydrophilic cross-linked polyvinyl alcohol membrane removes water selectively from acetone. Both are more selective than distillation.212... 

Boric acid is a mild dehydrating agent suitable for removal of water from some primary, secondary, or tertiary alcohols. Since the acid and the alcohol form first a trimeric metaboric ester, which then regenerates the boric acid when it decomposes to the olefin,36 the reaction is somewhat similar to pyrolysis of carboxylic esters but the boric acid dehydration occurs at appreciably lower temperatures (250-300°). Olefins are readily obtained by heating approximately molar equivalents of boric acid and 1-octanol, 1-heptanol, 1-hexanol, (—)-menthol, cyclohexanol, or 5cyclohexane-methanol, and cyclobutanemethanol.38... 

When heated with H2SO4, both 3,3-dimethyl-2-butanol and 2,3-dimethyl-2-butanol are dehydrated to form 2,3-dimethyl-2-butene. Which alcohol dehydrates more rapidly ... 

Acid-catalyzed dehydration of 2,3-dimethyl-2-butanol can proceed in either of two directions. 

Under conditions of kinetic control, the dehydration of 2-butanol follows the Saytzeff rule, but a greater yield of m-2-butene than trans-l-batenc is obtained. These observations have no parallel in acid- or base-catalysed or pyrolytic eliminations. However, the dehydration of 2,3-dimethyl-2-butanol gives 2,3-dimethyl-l-butene (88.4%) and 2,3-dimethyl-2-butene (9.9%) and is thus oriented towards the Hofmann rule despite being more probably a carbonium ion process. Under similar reaction conditions the quite distinctly different products arising from the secondary alcohol, 3,3-dimethyl-2-butanol [3,3-dimethyl-1-butene (70%), 2,3-dimethyl-l-butene (23.5%), 2,3-dimethyl-2-butene (3.9%), l,l-dimethyI-2-methylcyclopropane (2.1%)] are accommodated in terms of concerted rather than a carbonium-ion mechanism. 

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... 

Organic chemists have conducted experiments to understand the formation of multiple products in acid-catalyzed reactions such as that of 3,3-dimethyl-2-butanol. Let s consider the acid-catalyzed dehydration reaction of 3,3-dimethyl-2-butanol and explore the possibility of rearrangement. 

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