The dehydration of alcohols

Ehminations of HX to give double bonds offer considerable scope for selectivity and choice of reaction conditions. The dehydration of alcohols is the most common example of this class and may be achieved directly or through intermediate derivatives. In most cases, such derivatives are transient species formed in situ, but sometimes e.g. sulfonates, certain other esters and halides) they are isolated and characterized. Eliminations from jS-substituted ketones are very facile. The dehydration of jS-hydroxy ketones has been covered in section V. 

The principal complications in the dehydration of simple alcohols arise through the possibilities of rearrangement and alternative directions for elimination. 

In the early days of steroid chemistry, dehydrations were usually carried out under rather drastic conditions, frequently in the presence of strong acids (see, for example, ref. 176, 181, 182). Such conditions have been replaced by milder and more selective methods, except in those instances when either the product is stable e.g. ref. 183, 184) or rearrangement is desired. The importance of stereochemical factors in rearrangements is widely recognized for instance, the Westphalen rearrangement of 5a-alcohols (92) cf. ref. 185, 

A classic diagnostic use of such stereochemical requirements, due to Ruzicka, is the ring contraction induced in natural products containing the 4,4-dimethyl-5a-3 -ol system (94). The epimeric, axial 3a-alcohols (95) dehydrate without ring contraction. Barton suggested that it is necessary for the four reacting centers (hydroxyl, C-3, C-4, C-5) to be coplanar for ring contraction to occur, and this is only the case with the 3)5-alcohol. 

There are many other examples of selective rearrangement/ although such strict steric requirements are not always observed/  

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Dehydrogenation of alkanes is not a practical laboratory synthesis for the vast majority of alkenes The principal methods by which alkenes are prepared m the labo ratory are two other (3 eliminations the dehydration of alcohols and the dehydrohalo genation of alkyl halides A discussion of these two methods makes up the remainder of this chapter... 

In the dehydration of alcohols the H and OH are lost from adjacent carbons An acid catalyst is necessary... 

The dehydration of alcohols resembles the reaction of alcohols with hydrogen halides (Section 4 7) m two important ways... 

The dehydration of alcohols is an important elimination reaction that takes place under acidic rather flian basic conditions. It involves an El mechanism." The function of the acidic reagent is to convert the hydroxyl group to a better leaving group by protonation ... 

Hydrogenation of alkynes to alkenes using the Lindlai catalyst is attractive because it sidesteps the regioselectivity and stereoselectivity issues that accompany the dehydration of alcohols and dehydrohalogenation of alkyl halides. In tenns of regioselectivity, the position of the double bond is never in doubt—it appears in the carbon chain at exactly the sane place where the triple bond was. In tenns of stereoselectivity, only the cis alkene forms. Recall that dehydration and dehydrohalogenation normally give a cis-trans mixture in which the cis isomer is the minor product. 

Wilson, N. G., McCreedy, T., On-chip catalysis using a lithographically fabricated glass microreactor - the dehydration of alcohols using sulfated zirconia,... 

During the dehydration of alcohols, cleavage of both the C-O and the C-H bonds occurs (Scheme 5.2). 

Many reactions are catalyzed by aluminum oxide, A1203, which is also known as alumina. In the solid, there are sites on the surface where a strongly acidic aluminum ion is available to bond to an electron pair donor. One such reaction involves the dehydration of alcohols to produce alkenes. This process can be represented as follows ... 

In this process the elimination of a proton results in the formation of an alkene. Thus the dehydration of alcohols gives rise to alkene in presence of cone. H2S04... 

Whitmore (16), when developing the idea of carbonium ions, included reactions over dehydrating catalysts. The application of carbonium ion mechanism to the dehydration of alcohols over alumina has found several supporters (17, 18). 

In spite of the fact that alumina is an excellent and widely used catalyst for the dehydration of alcohols, there is no agreement in the literature with regard to the mechanism of this reaction or the nature of the olefinic products. For example, 1-alkenes have been obtained from primary alcohols such as 1-butanol (19-22), 1-pentanol (23), 1-hexanol (24-26), 1-heptano (27), and 1-octanol (25) but mixtures of olefins differing in the position of the double bond (13, 26, 28) or even in the carbon skeleton (29) have been reported from other primary alcohols. 

The dehydration of alcohols is mostly an acid-catalyzed reaction and much work has been done by Taft and co-workers to elucidate the mechanism (7 5-7 7). These investigators proved that the intermediate in the dehydration of tertiary alcohols or hydration of branched olefins in dilute acid solutions resembles the conjugate acid of the olefin and it is... 

Pure alumina catalyst prepared either by hydrolysis of aluminum isopropoxide or by precipitation of aluminum nitrate with ammonia, and calcined at 600-800°, contains intrinsic acidic and basic sites, which participate in the dehydration of alcohols. The acidic sites are not of equal strength and the relatively strong sites can be neutralized by incorporating as little as 0.1 % by weight of sodium or potassium ions or by passing ammonia or organic bases, such as pyridine or piperidine, over the alumina. 

There is a strong parallel between elimination reactions in solution and the dehydration of alcohols over alumina. The trans elimination reactions and the anchimeric assistance of alcohols over aluminas suggest that the dehydration must occur within either the submicroscopical pores, or crevices, or channels of the aluminas. The aluminas therefore must surround the alcohol molecules providing acid sites to act as proton donors or electron acceptors and basic sites to act as proton acceptors or electron donors. For that reason the aluminas seem to act as solvating agents and therefore may be considered as a pseudosolvents for dehydration reactions. 

Silica-alumina mixtures are of great technological importance in the oil industry as catalysts for petroleum processing. The cracking activity is closely linked to surface acidity. Other typical reactions catalyzed by silica-alumina are the dehydration of alcohols and the polymerization of olefins. 

The steric effects may be more pronounced in heterogeneous catalysts than in homogeneous reactions in solution. The rigid, solid surface restricts the approach of the reactants to the active centers and interaction between the reactants. The steric requirements are quite stringent when a two-point adsorption is necessary and when, in consequence, the internal motion of the adsorbed molecules is limited. In this way, the stereoselectivity of some heterogeneous catalytic reactions, for example, the hydrogenation of alkenes on metals (5) or the dehydration of alcohols on alumina and thoria (9), have been explained. 

The dehydration of alcohols on stoichiometric and nonstoichiometric (calcium-deficient) hydroxyapatite (series 8 and 9 in Table II) gave results consistent with the above findings. Although there is a difference in the reaction temperature, it is evident that with the nonstoichiometric catalyst, which must be more acidic, the slope found is more negative than that with the stoichiometric calcium phosphate. 

The dehydrogenation and the dehydration of alcohol are examples of reactions of two opposite classes. One of these classes includes reactions which proceed the faster the higher the Fermi level (other conditions being equal). These are reactions accelerated by electrons. We shall call them acceptor, or n-type, reactions. The other class includes reactions whose rate, on the contrary, is the greater the lower the Fermi level. We shall call them donor, or p-type, reactions. These are reactions accelerated by holes. 

The dehydrogenation of alcohol, as we have seen, is an acceptor reaction, while the dehydration of alcohol, on the contrary, is a donor reaction. This result is in agreement with Garner s opinion (30). 

We have also investigated the properties of several of our nanostructured catalysts as solid acids in reactions such as the dehydration of alcohols and transesterification reactions [99]. One of the best examples of atomically dispersed solid acid catalysts is aluminosilicates [100]. When aluminium is substituted into silicate frameworks and remains isolated from other A1 centers it can behave as a strong acid site [101]. 

Although there are some important differences between what we describe as 3-connected aluminium sites in our bb-matrices and what the active sites are thought to be in zeolites, we have begun a preliminary study of the activities of the Al, Ti and V-containing bb-catalysts as solid acid catalysts in the dehydration of alcohols. For this type of bench marking reaction, there are two parameters that can be used as preliminary indicators of catalytic activity lightoff temperatures and product selectivity. A plot of conversion versus temperature produces what is known as a lightoff curve. The temperature at which 50% of the maximum... 

The complications begin when one of the products (the HX molecules particularly) can influence the reaction rate, not only by adsorption on active sites blocking a fraction of them but by forming new active centres of a different nature. Then the parameter k is no longer a constant, but changes with the composition of the reaction mixture. This possibility has received only limited attention until now, but could explain some unusual empirical rate equations which have been found for the dehydration of alcohols on oxide catalysts [8,69]. As has been outlined in... 

The dehydration of alcohols on solid catalysts is one of the first catalytic reactions discovered and has been studied intensively for many decades. During years of experimental work, a general parallel—consecutive reaction scheme (Scheme 2 below) has been developed by gradual addition of... 

The dehydration of alcohols over solids has been the subject of several excellent reviews which summarise most of the vast literature [7,69,85— 87]. Therefore in this chapter, reference will be made only to the papers which are most significant, those that are newer or which have not obtained adequate attention in preceding reviews. 

The complicated reaction scheme for the dehydration of alcohols (Scheme 2) makes kinetic analysis rather difficult. However, initial reaction rates have been measured, without special problems, for secondary... 

Linear free energy relationships for the dehydration of alcohols over solid catalysts... 

Supercritical hydrogenation is just one example of continuous reactions which can be carried out in SCCO2 solution. Other reactions which have been carried out successfully include Friedel-Crafts alkylation of aromatics by alcohols [64], the dehydration of alcohols to form ethers [65] (using acid catalysts), and the hydroformylation of alkenes [52] (using rhodium catalysts immobilized on Si02). In each of these reactions, it is possible to obtain a selectivity which is at least as good, and often better, than with conventional solvents. However, the precise role of the scCC>2 in these reactions is not as obvious as in supercritical hydrogenation. 

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