Mechanism ethanol dehydration

Microbial Ethanol, Its Polymer Polyethylene, and Applications 3.2.2 Mechanism for Microbial Ethanol Dehydration... 

Under different reaction conditions, the main products of the ethanol dehydration reaction are ethylene and ether. Although many researchers have made great efforts to reveal the mechanism for ethanol dehydration, disputes exist when the mechanisms are apphed to a particular reaction process. Some researchers think that the dehydration of ethanol to ethylene is a parallel reaction process, whereas others think that it is a parallel continuous reaction process, namely, dehydration of ethanol to ether and further dehydration to ethylene. 

Many of the trial results have indicated that the coexistence of an acid and alkaline center is conducive to dehydration of ethanol to ethylene. Hassan suggested the catalytic mechanism of ethanol dehydration catalyzed by solid acid or alkaline catalysts in 1982 (Abd El-Salaam and Hassan 1982). He claimed the acid and alkaline center of the catalyst cooperated in the process of ethanol dehydration. Although the reaction was mainly catalyzed by the acid center, the existence of a modest alkaline center could promote this reaction. Hassan considered ethanol was absorbed in the acid and alkaline center on the surface of the catalyst and then formed adsorption-state compounds, which could further dehydrate to ethylene and release the acid and alkaline center. The mechanism of dehydration of ethanol to ethylene catalyzed by activated alumina suggested by Cosimo et al. (1998) is shown Fig. 4. 

Recently, Kondo et al. (2005) studied the mechanism of ethanol dehydration in acid solutions and on the surface of zeolites respectively. Acid-catalyzed ethanol dehydration on zeolites was shown to proceed via a covalent ethoxy group (C H O) as a stable intermediate, and this was directly observed by infrared spectroscopy (Fig. 5). 

Fig. 5 Comparison of mechanisms of ethanol dehydration (a) in acidic solutions and (b) on zeolites...

Diethyl ether is prepared industrially via the acid-catalyzed dehydration of ethanol. The mechanism of this process is believed to involve an Sivf2 process. 

Building on the proposed mechanism hy Hauffe, ° for metal oxide-catalysed dehydration of alcohols to form olefins, ethers and water which heavily relies on the assumption that the catalyst surface acts as semiconductor, Hasssan et al. purposively attempted to obtain a further insight into the mechanism of alcohol dehydration on pure cadmium oxide. On the basis of the experimental data involving kinetics of the dehydration reaction and the effect of pretreatment of the catalysts along with studies on lattice structure and specific surface areas, a mechanism for ethanol dehydration was put forward (Scheme 17.14). The proposed mechanism entirely depends... 

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). 

Co-feeding of alcohols effects an increased rate of hydrocarbon formation, as shown in early experiments of Emmett and coworkers1"1 using 14C-labeled alcohols. These experiments were carried out in order to support the hydroxyl-carbene mechanism favored at that time. Their experiments were confirmed by Shi and Davis23 for Co catalysts and co-feeding of ethanol. Furthermore, in their study, the argument that ethanol may be dehydrated to ethene, incorporated, and followed by subsequent chain growth via CH2 insertion could be excluded, as co-fed ethanol incorporated much faster than ethene. 

Tertiary > Secondary > Primary The mechanism of dehydration of ethanol involves the following steps Mechanism... 

Write the mechanism of acid dehydration of ethanol to yield ethene. 

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. 

Reactions of anthocyanins and flavanols take place much faster in the presence of acetaldehyde that is present in wine as a result of yeast metabolism and can also be produced through ethanol oxidation, especially in the presence of phenolic compounds, or introduced by addition of spirit in Port wine technology. The third mechanism proposed involves nucleophilic addition of the flavanol onto protonated acetaldehyde, followed by protonation and dehydration of the resulting adduct and nucleophilic addition of a second flavonoid onto the carbocation thus formed. The resulting products are anthocyanin flavanol adducts in which the flavonoid units are linked in C6 or C8 position through a methyl-methine bond, often incorrectly called ethyl-link in the literature. 

The reaction chemistry of simple organic molecules in supercritical (SC) water can be described by heterolytic (ionic) mechanisms when the ion product 1 of the SC water exceeds 10" and by homolytic (free radical) mechanisms when <<10 1 . For example, in SC water with Kw>10-11 ethanol undergoes rapid dehydration to ethylene in the presence of dilute Arrhenius acids, such as 0.01M sulfuric acid and 1.0M acetic acid. Similarly, 1,3 dioxolane undergoes very rapid and selective hydration in SC water, producing ethylene glycol and formaldehyde without catalysts. In SC methanol the decomposition of 1,3 dioxolane yields 2 methoxyethanol, il lustrating the role of the solvent medium in the heterolytic reaction mechanism. Under conditions where K klO"11 the dehydration of ethanol to ethylene is not catalyzed by Arrhenius acids. Instead, the decomposition products include a variety of hydrocarbons and carbon oxides. 

The most important contribution in the field of simultaneous dehydrogenation, condensation, and dehydration made by Russian chemists is the synthesis of butadiene from ethanol over a double oxide catalyst by the method of Lebedev. Much has been published on this process. Lebedev s interest in rubber synthesis began with his researches on conversions of dienes in 1908 and his method of synthesis of butadiene was reported in 1927. An experimental synthetic rubber plant was founded for research in the field and the studies on the mechanism of formation of butadiene and of polymerization were continued after Lebedev s death by his students (103,104,105,188,190,378). A survey of the properties and methods of preparation of butadiene was published by Petrov (289). 

Contrast the mechanisms of the two preceding reactions, the dehydration and condensation of ethanol. 

Determine the rate law and mechanism for the vapor phase dehydration of ethanol. [2nd Ed. P6-21]... 

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