Types of dehydration reactions

In general, dehydration means loss of water molecules from chemical substances, irrespective of their structure. Even if all cases where water is bonded in hydrate form are excluded, a number of reactions remain which also include formation of nitriles from amides, lactones from hydroxy acids etc. However, the present treatment will concentrate on the heterogeneous catalytic decomposition of alcohols in the vapour phase, which can be either olefin-forming or ether-forming reactions, and on the related dehydration of ethers to olefins. 

Other factors influencing the olefin/ether ratio are the partial pressure of the starting alcohol because olefin-forming and ether-forming reactions obey different kinetics (see Sect. 2.2.3) and the nature of the catalyst. 

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. 

Hundreds of substances of many types have been tested as dehydration catalysts and found active. Lists can be found in the literature [69,76,85] and we need to name here only such catalysts which show high activity and selectivity. The latter parameter is more important because a number of solids, especially oxides, can catalyse both the dehydration and the dehydrogenation of alcohols. The formation of aldehydes or ketones is then a parallel reaction to the dehydration, and the ratio of the rates depends on the nature of the catalyst. Only few oxides are clean dehydration or dehydrogeneration catalysts, but the selectivity may be shifted to some extent in either direction by the method of catalyst preparation. 

The important groups of dehydration catalysts are oxides, aluminosilicates (both amorphous and zeolitic), metal salts and cation exchange resins. Most work on mechanisms has been done with alumina. 

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The simplest type of dehydration reaction is involved in the reduction of aliphatic aldehydes (76) of the type R—CHO, where R is an alkyl group (but not hydrogen). Dehydration follows the scheme (29), where B is a base ... 

Fatty acids can be covalently attached to another molecule by a type of dehydration reaction called esterification, In which the OH from the carboxyl group of the fatty acid and a H from a hydroxyl group on the other molecule are lost. In the combined molecule formed by this reaction, the portion derived from the fatty acid is called an acyl group, or fatty acyl group. This is illustrated by triacylglycerols, which contain three acyl groups esterfied to glycerol ... 

Polymers usually are prepared by two different types of polymerization reactions — addition and condensation. In addition polymerization all of the atoms of the monomer molecules become part of the polymer in condensation polymerization some of the atoms of the monomer are split off in the reaction as water, alcohol, ammonia, or carbon dioxide, and so on. Some polymers can be formed either by addition or condensation reactions. An example is polyethylene glycol, which, in principle, can form either by dehydration of 1,2-ethanediol (ethylene glycol), which is condensation, or by addition polymerization of oxacyclopropane (ethylene oxide) 1... 

Another type of photochemical reaction involving a pyrimidine base is the addition of a molecule of water across the 5,6 double bond of C to yield a 5,6-dihydro-6-hydroxy derivative called the cytosine hydrate. The quantum yield for the formation of cytosine hydrates in UV-irradiated DNA is greater in single-stranded than in duplex-DNA (45). Hydrates of cytosine, deoxycytidine, CMP, or dCMP are unstable, readily reverting to the parent form by rehydration (45). However, their half-life is dramatically increased in DNA, and cytosine hydrate may be the major nondimer C photoproduct. Cytosine hydrate can undergo deamination and dehydration to yield uracil (1). The hydrate of 5-methylcytosine may undergo deamination to yield 5-thymine hydrate, which can convert to thymine upon dehydration (1). 

Under the conditions used in peptide synthesis, unprotected aliphatic hydroxy groups can undergo two types of side reactions they can be acylated or dehydrated, the latter leading to dehydroamino acids. The hydroxy group of serine is a primary alcoholic function and therefore exhibits the highest reactivity. The secondary alcoholic functions of threonine, hydroxyproline, (3-phenylserine, hydroxynorvaline, and hydroxynorleucine, as well as of other noncoded amino acids, are less reactive and thus more suited for use in the unprotected form. The aromatic hydroxy group of tyrosine is more acidic than the ahphatic hydroxy groups nevertheless, it can be acylated to form esters. These are active esters which in turn can react with primary amines to form amide bonds. 

The generic equations representing the different types of organic reactions you have learned—substitution, elimination, addition, oxidation-reduction, and condensation—can be used to predict the products of other organic reactions of the same types. For example, suppose you were asked to predict the product of an elimination reaction in which 1-butanol is a reactant. You know that a common elimination reaction involving an alcohol is a dehydration reaction. 

Dehydration reactions are another common degradation pathway. Ring closures are a fairly common type of dehydration, as is seen for both lactose18,19 and glucose.20 22 Both of these compounds dehydrate to form 5-(hydroxymethyl)-2-fur-fural. Batanopride is another example of a compound which can undergo a dehydration reaction.23... 

Dehydration of Lithium Sulfate Monohydrate In a discussion of the dehydration of Li2S04 H20, another dehydration for which rate characteristics vary with reaction conditions (79), L vov et al. (28) resolve the kinetic inconsistencies and distinguish two types of behavior. Reactions in the equimolar mode proceed in the absence of volatile products, whereas in the isobaric mode the (constant) partial pressure of a volatile reaction product significantly exceeds its equilibrium dissociation pressure. This accounts quantitatively for the influence of water vapor in diminishing this dehydration rate. 

This is about the most difficult type of aldol reaction two shghtly different aldehydes, both enolizable, both capable of self-condensation. The only solution is to couple the silyl enol ether of one aldehyde with the other aldehyde using a Lewis acid as catalyst. This gives the aldol itself that can be dehydrated to the enal. 

Extensive discussion of these and many other types of elimination reactions can be found in several reviews.We will focus here on 1,2-eliminations, especially dehydrohalogenation, dehydration, dehalogenation, deamination reactions, and pyrolytic eliminations. 

Dermatitis Inflammation of the skin from any cause. There are two general types of skin reaction primary irritation dermatitis and sensitization dermatitis. Desiccant A chemical that is capable of absorbing or removing moisture. The term desiccation refers to dehydration (removal of tissue moisture) by chemical or physical action. 

The typical acid catalysts used for novolak resins are sulfuric acid, sulfonic acid, oxaUc acid, or occasionally phosphoric acid. Hydrochloric acid, although once widely used, has been abandoned because of the possible formation of toxic chloromethyl ether by-products. The type of acid catalyst used and reaction conditions affect resin stmcture and properties. For example, oxaUc acid, used for resins chosen for electrical appHcations, decomposes into volatile by-products at elevated processing temperatures. OxaUc acid-cataly2ed novolaks contain small amounts (1—2% of the original formaldehyde) of ben2odioxanes formed by the cycli2ation and dehydration of the ben2yl alcohol hemiformal intermediates. 

Mineral acids are used as catalysts, usually in a concentration of 20— 40 wt % and temperatures of 30—60°C. An efficient surfactant, preferably one that is soluble in the acid-phase upon completion of the reaction, is needed to emulsify the a-pinene and acid. The surfactant can then be recycled with the acid. Phosphoric acid is the acid commonly used in the pine oil process. Its mild corrosion characteristics and its moderate strength make it more manageable, especially because the acid concentration is constandy changing in the process by the consumption of water. Phosphoric acid is also mild enough to prevent any significant dehydration of the alcohols formed in the process. Optimization of a process usually involves considerations of acid type and concentration, temperature, surfactant type and amount, and reaction time. The optimum process usually gives a maximum of alcohols with the minimum amount of hydrocarbons and cineoles. 

In recent years, the rate of information available on the use of ion-exchange resins as reaction catalysts has increased, and the practical application of ion-exchanger catalysis in the field of chemistry has been widely developed. Ion-exchangers are already used in more than twenty types of different chemical reactions. Some of the significant examples of the applications of ion-exchange catalysis are in hydration [1,2], dehydration [3,4], esterification [5,6], alkylation [7], condensation [8-11], and polymerization, and isomerization reactions [12-14]. Cationic resins in form, also used as catalysts in the hydrolysis reactions, and the literature on hydrolysis itself is quite extensive [15-28], Several types of ion exchange catalysts have been used in the hydrolysis of different compounds. Some of these are given in Table 1. 

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