Enhancer Dehydration reactions

The addition of a Lewis acid, i.e., ZnC significantly decreases the production of tar and enhances the production of char due to the enhanced dehydration reactions. At higher temperatures the glycosyl units and the random condensation products are further degraded to a variety of volatile products, as shown in Table V (9). Comparison of this table with the high temperature pyrolysis products listed for cellulose in Table III shows that the products of both fractions are basically similar. The significant increase in the yields of 2-furaldehyde, water and char and decrease in the yield of tar by the addition of ZnCl verifies the enhanced dehydration and is similar to observed effects in cellulose pyrolysis. 

Aldehydes and ketones can react with primary and secondary amines to form Schiff bases, a dehydration reaction yielding an imine (Reaction 45). However, Schiff base formation is a relatively labile, reversible interaction that is readily cleaved in aqueous solution by hydrolysis. The formation of Schiff bases is enhanced at alkaline pH values, but they are still not stable enough to use for crosslinking applications unless they are reduced by reductive amination (see below). 

MW-expedited dehydration reactions using montmorillonite K 10 clay [70] (Schs. 6.20 and 6.21) or Envirocat reagent, EPZG [71] (Schs. 6.20 and 6.21) have been demonstrated in a facile preparation of imines and enamines via the reactions of primary and secondary amines with aldehydes and ketones, respectively. The generation of polar transition state intermediates in such reactions and their enhanced... 

PEG as a support could enhance the reaction, which could be explained with the proposed mechanism as shown in Scheme 5.14. The reaction involves two steps, that is, formation of the ammonium carbamate and dehydration to the urea. In this reaction, PEG could form [R2NH2PEG]+ [R2NC02], and thus could increase the thermodynamic stability of [R2NH2]+. Increasing the basicity of the base with the help of PEG could also facilitate formation of the ammonium carbamate salt. 

Experiments on reaction mechanisms showed that urea enhanced the dehydration (weight loss) reactions of monosaccharides in nonacidic environments. Addition of acid to a glucose-urea mixture slowed its dehydration at 108 °C. The addition of sufficient urea to an acid-catalyzed polyol solution appears to prevent, or at least delay considerably, the normal acid dehydration reaction of the polyol. 

The difference between two reactions may be attributed to the activity of water present in the reaction system, since the overall reaction is the dehydration reaction (Equation 2.1). However, intentional addition of a small amount of water caused enhancement of a-alumina formation rather than the retardation expected from the equilibrium point of view. Another important factor is the difference in the thermodynamic stabilities of the intermediates between glycothermal and hydrothermal reactions that is, the glycol derivative of boehmite vs. well-crystallized boehmite. The latter compound is fairly stable and therefore conversion of this compound into a-alumina has only a small driving force. On the other hand, the glycol derivative of boehmite has Al-O-C bonds and therefore is more unstable with respect to a-alumina. Thus conversion of this compound into a-alumina has a much larger driving force. The smaller crystallite size of the glycol derivative of boehmite also contributes to the instability of the intermediate. 

Prodan et al. [75] studied the low pressure (lO" Torr), low temperature (fi om 273 to 373 K) dehydration of Na5P30,Q.6H20 in the form of fine crystals. Reaction occurred in two stages (with = 56 and 84 kJ mol ) both of which were diffiision controlled. The activation energy increased with extent of reaction. The rate of reaction of this salt was enhanced [76] by water vapour, attributed to its ability to reorganize the diffusion layer. This effect (Smith-Topley behaviour) has been noted in many dehydration reactions (Chapter 7). 

Dolomite cement components were all sourced from outside the sand body, most probably from local or basinal mudrocks. Stable isotope data indicate a mixed organogenic-marine carbonate source, and precipitation at relatively low temperatures (s70°C, if pore fluids were sourced from clay mineral dehydration reactions during deep burial of Carboniferous mudrocks in the Rathlin basin 55°C if they were locally sourced). Thermobaric mass transfer was enhanced by tectonic pulsing and dolomite precipitation was driven by CO2 degassing. 

Thermolysis reactions have mostly been studied for group IV metal alkoxides, although metal oxide deposits have been obtained for many metals [5]. Oxoalkoxides are often formed as intermediates, but mechanistic studies remain scarce [68]. The decomposition can be enhanced by hydrolysis as a result of either residual water on the surface or dehydration reactions of tertiary alcohols. 

Sn2 nucleophilic substitutions take place in scC02. The presence of silica-supported onium salts 42 as a phase-transfer catalyst enhances the reaction (Scheme 72). Esters formation from carboxylic acids and alcohols via dehydration also proceeds in scC02. The conversion increases as the CO2 pressure increases. A single homogeneous phase is obtained around the critical point, at which the conversion is maximized. 

Linoleic acid A9 hydratase, which is involved in the linoleic acid saturation metabolism of Lactobacillus plantarum AKU 1009a, was cloned as his-tagged recombinant enz une, purified with affinity column, and characterized [30]. The enzyme required FAD as a cofactor for its activity, and the activity was enhanced by NADH. The maximum activities for hydration of linoleic acid and for dehydration of lO-hydroxy-czs-12-octadecenoic acid (HYA) were observed at 37°C, pH5.5, with 0.5M NaCl. C16 and CIS free fatty acids with cis-9 double bond served as substrates for hydration with CIO regiospecificity and (S) stereospecificity (Figure 22.9). 10-Hydroxy fatty acids served as substrates for dehydration reactions. The apparent value for linoleic acid was estimated to be 92 irM with its values at 2.6 x 10 s and Hill factor was 3.3. The apparent K value for HYA was estimated to be 98 iM with its values at 1.2 x 10 s ... 

The above results indicate that dehydration of a-nitro-ketones to nitrile oxides occurs (at least in part) prior to cycloaddition, leading to the corresponding furoxans together with the cycloadducts. It is worth remembering that enhanced dehydration of a-nitroketones to nitrile oxides has been noticed previously in more drastic conditions (heating with PTSA) and related to tautomerization [16] or by treatment with concentrated mineral acids [17]. Both reaction pathways (Scheme 8.2) are possible, depending on relative reaction rates and on the base employed. 

The point was made earlier (Section 5 9) that alcohols require acid catalysis in order to undergo dehydration to alkenes Thus it may seem strange that aldol addition products can be dehydrated in base This is another example of the way in which the enhanced acidity of protons at the a carbon atom affects the reactions of carbonyl com pounds Elimination may take place in a concerted E2 fashion or it may be stepwise and proceed through an enolate ion... 

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