Hydration and Dehydration Reactions

Ans. Both are correct. There is an equilibrium between the dehydration and hydration reactions. Reaction conditions determine whether the equilibrium lies on the hydration or dehydration side. If hydration of an alkene is the desired reaction, we add a large amount of water to the alkene in order to push the equilibrium toward alcohol. If dehydration of an alcohol is the desired reaction, we allow the alkene to distill out of the reaction vessel as it is formed (since the alkene has a lower boiling point than the alcohol). This pushes the equilibrium toward alkene. 

FIGURE 3. Difference IR spectra between dehydrated and hydrated reaction centers. 

In particular, the dehydration/hydration catalysis in intermediary metabolism, by proteins that contain an asymmetric cubane, is well established. The present picture is predominantly based on the extensive studies with aconitase for which a detailed reaction mechanism has been proposed. The dehydration and hydration steps in this mechanism are well understood however, the details of the flip of the intermediate are still to be determined. 

Prussian blue, Fe4[Fe(CN)6]3xH20, consists of a cubic lattice of alternating Fe2+ and Fe3+ ions connected with cyanide bridges (Figure 8.3). The [Fe(CN)6]4 sites are 75 % occupied and the exposed Fe3+ ions are coordinated with H2O. Dehydration and hydration by moisture is reversible hence, Prussian blue is a porous solid albeit one with very small pores. To increase pore size, the [Fe(CN)6]4 complexes were replaced with octahedral [Rer TeACNAJ4 clusters (Figure 8.4). This cluster displays an octahedral array of N donors on a sphere about 4 A larger in diameter than the mononuclear complex. Reaction of Fe3+ with the cluster in... 

Hydrolysis of monoorganotin compounds can give rise to a wider variety of hydroxides and oxides through a combination of hydrolysis, dehydration, and aggregation reactions,116 but less work has been done in this field. In moist air, ethyl-, butyl- and octyl-tin trichloride are hydrolysed as far as the hydrated hydroxide dichlorides, RSnCl2(OH)... 

The current standard process route to melamine uses urea as a starting material. Urea is heated in the presence of ammonia at 250—300°C and 4—20 MPa. The reaction probably involves the simultaneous dehydration and hydration of urea to form cyanamide and ammonium carbamate trimerization of the cyanamide then leads to melamine [10]. 

Acid catalysts are mainly used in alkylation processes, but also for hydration, dehydration, and condensation reactions. In olefin reactions, heterogeneous catalysts are mainly employed for metathesis reactions and the production of polymers. 

One of the most common types of elimination is the dehydration of an alcohol. Formally, these reactions are the microscopic reverse of the hydration of an alkene, and therefore we have already covered these reactions (see Section 10.2). However, several points should be stressed here, because dehydrations are commonly used in synthesis. Furthermore, dehydrations and hydrations are important biosynthetic reactions, as many natural products possess alkenes and alcohols. The Connections highlight at the end of this section discusses how enzymes catalyze these reactions. 

Dehydration and Hydration—Addition or removal of water from a compound may be accomplished by a catalyst. Dehydration may occur as an independent reaction or as a heterogeneous reaction such as the condensation of an alcohol with ammonia, or the formation of esters fi m alcohols and acids. Alcohols can be dehydrated to form olefins or ethers. Other dehydration reactions are glycerine to acrolein and acetic acid to acid anhydride. Olefins can be condensed with water to form ethers, aldehydes, ketones and alcohols. The hydration reactions are governed by pressure, temperature, and the mole ratio of reactants. High pressures are occasionally necessary to counteract dehydration tendency and promote hydration. 

Iron(II) bromide [7789-46-0] FeBr2, can be prepared by reaction of iron and bromine ia a flow system at 200°C and purified by sublimation ia oitrogea or uader vacuum. Other preparative routes iaclude the reactioa of Fe202 with HBr ia a flow system at 200—350°C, reactioa of iroa with HBr ia methanol, and dehydration of hydrated forms. FeBr2 crystallizes ia a layered lattice of the Cdfy type and has a magnetic moment of... 

Acid—Base Catalysis. Inexpensive mineral acids, eg, H2SO4, and bases, eg, KOH, in aqueous solution are widely appHed as catalysts in industrial organic synthesis. Catalytic reactions include esterifications, hydrations, dehydrations, and condensations. Much of the technology is old and well estabhshed, and the chemistry is well understood. Reactions that are cataly2ed by acids are also typically cataly2ed by bases. In some instances, the kinetics of the reaction has a form such as the following (9) ... 

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. 

The reaction occurs in the liquid phase at relatively low temperatures (about 50°C) in the presence of a solid acid catalyst. Few side reactions occur such as the hydration of isohutene to tertiary hutyl alcohol, and methanol dehydration and formation of dimethyl ether and water. However, only small amounts of these compounds are produced. Figure 5-8 is a simplified flow diagram of the BP Etherol process. 

The amino acid leucine is biosynthesized from n-ketoisocaproate, which is itself prepared from -ketoisovalerate by a multistep route that involves (1) reaction with acetyl CoA, (2) hydrolysis, (3) dehydration, (4) hydration. (5) oxidation, and (6) decarboxylation. Show lhe steps in the transformation, and propose a mechanism for each. 

There have been many instances of examination of the effect of additive product on the initiation of nucleation and growth processes. In early work on the dehydration of crystalline hydrates, reaction was initiated on all surfaces by rubbing with the anhydrous material [400]. An interesting application of the opposite effect was used by Franklin and Flanagan [62] to inhibit reaction at selected crystal faces of uranyl nitrate hexa-hydrate by coating with an impermeable material. In other reactions, the product does not so readily interact with reactant surfaces, e.g. nickel metal (having oxidized boundaries) does not detectably catalyze the decomposition of nickel formate [222],... 

In a discussion of these results, Bertrand et al. [596,1258] point out that S—T behaviour is not a specific feature of any restricted group of hydrates and is not determined by the nature of the residual phase, since it occurs in dehydrations which yield products that are amorphous or crystalline and anhydrous or lower hydrates. Reactions may be controlled by interface or diffusion processes. The magnitudes of S—T effects observed in different systems are not markedly different, which indicates that the controlling factor is relatively insensitive to the chemical properties of the reactant. From these observations, it is concluded that S—T behaviour is determined by heat and gas diffusion at the microdomain level, the highly localized departures from equilibrium are not, however, readily investigated experimentally. 

Non-isothermal measurements of the temperatures of dehydrations and decompositions of some 25 oxalates in oxygen or in nitrogen atmospheres have been reported by Dollimore and Griffiths [39]. Shkarin et al. [606] conclude, from the similarities they found in the kinetics of dehydration of Ni, Mn, Co, Fe, Mg, Ca and Th hydrated oxalates (first-order reactions and all values of E 100 kJ mole-1), that the mechanisms of reactions of the seven salts are probably identical. We believe, however, that this conclusion is premature when considered with reference to more recent observations for NiC204 2 H20 (see below [129]) where kinetic characteristics are shown to be sensitive to prevailing conditions. The dehydration of MnC204 2 H20 [607] has been found to obey the contracting volume... 

Thus removal of water from classical rather inactive fluoride reagents such as tetrabutylammonium fluoride di- or trihydrate by silylation, e.g. in THF, is a prerequisite to the generation of such reactive benzyl, allyl, or trimethylsilyl anions. The complete or partial dehydration of tetrabutylammonium fluoride di- or trihydrate is especially simple in silylation-amination, silylation-cyanation, or analogous reactions in the presence of HMDS 2 or trimethylsilyl cyanide 18, which effect the simultaneous dehydration and activation of the employed hydrated fluoride reagent (cf, also, discussion of the dehydration of such fluoride salts in Section 13.1). For discussion and preparative applications of these and other anhydrous fluoride reagents, for example tetrabutylammonium triphenyldifluorosilicate or Zn(Bp4)2, see Section 12.4. Finally, the volatile trimethylsilyl fluoride 71 (b.p. 17 °C) will react with nucleophiles such as aqueous alkali to give trimethylsilanol 4, HMDSO 7, and alkali fluoride or with alkaline methanol to afford methoxytri-methylsilane 13 a and alkali fluoride. 

CO3 species was formed and the X-ray structure solved. It is thought that the carbonate species forms on reaction with water, which was problematic in the selected strategy, as water was produced in the formation of the dialkyl carbonates. Other problems included compound solubility and the stability of the monoalkyl carbonate complex. Van Eldik and co-workers also carried out a detailed kinetic study of the hydration of carbon dioxide and the dehydration of bicarbonate both in the presence and absence of the zinc complex of 1,5,9-triazacyclododecane (12[ane]N3). The zinc hydroxo form is shown to catalyze the hydration reaction and only the aquo complex catalyzes the dehydration of bicarbonate. Kinetic data including second order rate constants were discussed in reference to other model systems and the enzyme carbonic anhy-drase.459 The zinc complex of the tetraamine 1,4,7,10-tetraazacyclododecane (cyclen) was also studied as a catalyst for these reactions in aqueous solution and comparison of activity suggests formation of a bidentate bicarbonate intermediate inhibits the catalytic activity. Van Eldik concludes that a unidentate bicarbonate intermediate is most likely to the active species in the enzyme carbonic anhydrase.460... 

The 8,9- and 10,11-dihydrodiols formed in the metabolism of BA and DMBA respectively are all highly enriched (>90%) in R,R enantiomers (Table III). Labeling experiments using molecular oxygen-18 in the in vitro metabolism of the respective parent compounds and subsequent mass spectral analyses of dihydrodiol metabolites and their acid-catalyzed dehydration products indicated that microsomal epoxide hydrolase-catalyzed hydration reactions occurred exclusively at the nonbenzylic carbons of the metabolically formed epoxide intermediates (unpublished results). These findings indicate that the 8,9- and 10,11-epoxide intermediates, formed in the metabolism of BA and DMBA respectively, contain predominantly the 8R,9S and 10S,11R enantiomer, respectively. These stereoselective epoxidation reactions are relatively insensitive to the cytochrome P-450 isozyme contents of different rat liver microsomal preparations (Table III). 

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