Mechanism of dehydration

Activated alumina and phosphoric acid on a suitable support have become the choices for an iadustrial process. Ziac oxide with alumina has also been claimed to be a good catalyst. The actual mechanism of dehydration is not known. In iadustrial production, the ethylene yield is 94 to 99% of the theoretical value depending on the processiag scheme. Traces of aldehyde, acids, higher hydrocarbons, and carbon oxides, as well as water, have to be removed. Fixed-bed processes developed at the beginning of this century have been commercialized in many countries, and small-scale industries are still in operation in Brazil and India. New fluid-bed processes have been developed to reduce the plant investment and operating costs (102,103). Commercially available processes include the Lummus processes (fixed and fluidized-bed processes), Halcon/Scientific Design process, NIKK/JGC process, and the Petrobras process. In all these processes, typical ethylene yield is between 94 and 99%. 

The Mechanism of Dehydration of Alcohols over Alumina Catalysts Herman Pines and Joost Manassen Complex Adsorption in Hydrogen Exchange on Group VIII Transition Metal Catalysts... 

The kinetics and mechanisms of dehydration of crystalline hydrates are considered in Chaps. 4 and 5. It is, however, relevant to mention here that the conditions during dehydration can influence the reactivity of the anhydrous salt (or lower hydrate) formed. Thus, anhydrous solids produced under high vacuum are often amorphous, while those prepared in the presence of appreciable amounts of water vapour may be crystalline [281-283,374],... 

The Mechanism of Dehydration of Alcohols over Alumina Catalysts... 

One of the most fruitful approaches to the elucidation of reaction mechanisms in organic chemistry is the study of the effect of structure on the reactivity and the course of the reaction. This approach is used extensively in homogeneous reactions and found to be equally rewarding in the study of the mechanism of dehydration of alcohols over alumina catalysts. Much information was obtained by changing the configuration of the alcohols. 

Before discussing the mechanism of dehydration of primary alcohols, it might be worthwhile to consider some of the published results on the dehydration of ethyl alcohol. Chiefly, two products result ethyl ether and ethylene. Most of the discussions over the years have centered around the problem whether ether is formed simultaneously, in-... 

Before discussing the problem of dehydration of primary alcohols over aluminas it is helpful to review what is known about the mechanism of dehydration of alcohols in solution. 

In order to shed more light on the mechanism of dehydration of -substituted propanols Pines and Herling (83) studied the dehydration... 

The mechanism of dehydration of alcohols over acidic and non-acidic alumina is the same. In the presence of the acidic alumina, however, readsorption of the dehydrated product can occur, leading to either double bond migration or skeletal isomerization, depending on the strength of the acid sites, the structure of the olefins produced, and the experimental conditions. 

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

Kurata and Sakurai124 have investigated the mechanism of dehydration of L-ascorbic acid by examining the products from it and from L-xyZo-hexulosonic acid after treatment for 1 hour at 100° at pH 2.2, or with 5% sulfuric acid at 100°. In both experiments, 2-furaldehyde and 3-deoxy-L-threo-pentosulose (isolated as the phenylhydrazone) were the major products no pentose was detected. 

Figure 13.10. Mechanism of dehydration of asparagine during activation [9].

This is the reverse of acid-catalyzed hydration of alkenes discussed previously (Section 10-3E) and goes to completion if the alkene is allowed to distill out of the reaction mixture as it is formed. One mechanism of dehydration involves proton transfer from sulfuric acid to the alcohol, followed by an E2 reaction of hydrogen sulfate ion or water with the oxonium salt of the alcohol ... 

Walker, G.F., 1956. The mechanism of dehydration of Mg-vermiculite. Proc. Natl. Conf. Clays Clay Miner. 4th-Natl. Acad. ScL Natl. Res. Counc., Publ, 456 101-115. 

The mechanism of dehydration is shown below (Fig.L). First of all, the acidic proton is removed and a new enolate ion is formed. The electrons in the enolate ion can then move in such a fashion that the hydroxyl group is expelled to give the final product, i.e. an a, p-unsaturated aldehyde. In this example, it is possible to change the conditions such that one gets the Aldol reaction product or the a, P-unsaturated aldehyde, but in some cases only the a, p-unsaturated carbonyl product is obtained, particularly when extended conjugation is possible. 

The mechanism of dehydration depends on the structure of the alcohol 2° and 3° alcohols react by an El mechanism, whereas 1° alcohols react by an E2 mechanism. Regardless of the type of alcohol, however, strong acid is always needed to protonate the O atom to form a good leaving group. 

The mechanism of dehydration consists of two steps deprotonation followed by loss of OH, as shown in Mechanism 24.2. 

Use of Model Alcohols in Mechanistic Studies. - Much use has been made of model alcohols of various types in order to elucidate the detailed mechanism of dehydration, and in so doing, most catalysts have been compared with either alumina or thoria representing respectively E1/E2 and ElcB mechanisms. 

The identification of similarities and of differences of behaviour of reactant hydrates may provide insights into not only the mechanisms of dehydration, but also a wider range of crystolysis reactions. Inconsistencies of behaviour may be recognized and directions for future research identified. The progress achieved towards a general classification of solid state decompositions is discussed in Chapter 18,... 

Mechanisms of dehydrations of crystalline hydrates (proposed basis for classification of these reactions)... 

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