Dehydration, mechanisms

Rosenow, M. A., Huffman, H. A., Phail, M. E., and Wachter, R. M. (2004). The crystal structure of the Y66L variant of green fluorescent protein supports a cyclization-oxidation-dehydration mechanism for chromophore maturation. Biochemistry 43 4464 1472. 

The aim of this study is to follow in situ the dehydration mechanism of the zeolite ECR1, in its as-synthesized and NH4-cxchangcd form in order to determine its thermal stability and whether the structure undergoes phase transition during dehydration. 

Mild benzhydrylation (diphenylmethylation) of primary and secondary saccharide alcohols, and tritylation of the primary alcohols, were promoted by 4A AW-300 MS at room temperature through a dehydration mechanism in the absence of any strong protic or Lewis acid. While protection of a single primary or secondary hydroxyl... 

The retention of optical activity of camphene rules out methyl migration (Nametkin rearrangement) (71) or a symmetrical intermediate. On the acidic alumina at low contact time the retention of optical activity is high, about 80%. At longer contact time, however, there is essentially complete racemization. Hence, the dehydration mechanism seems to be the same on the acidic and on the base-modified alumina. The acidic alumina, however, causes the readsorption of the dehydration product leading to isomerization and equilibration. 

Hotta,Y Ozeki, S. Suzuki,T. Imal, S. Ka-neko, S. (1991) Surface characterization of titanated a-Fe203. Langmuir 7 2649—2654 Howe, A.T. Gallagher, K.J. (1975) Mossbauer studies in the colloidal system P-FeOOH — P-Fe20j Structures and dehydration mechanism. J. Chem. Soc. Faraday Trans. I. 71 22-34 Hsi, C.D. Langmuir, D. (1985) Adsorption of uranyl onto ferric oxyhydroxides Application of the surface complexation site-binding model. Geochim. Cosmochim. Acta 49 1931-1941... 

Ramaswamy, V., Dehydration Mechanism in the Antarctic Stratosphere during Winter, Geophys. Res. Lett., 15, 863-866 (1988). 

The major products formed from hexoses that react in aqueous acidic solution are 5-(hydroxymethyl)-2-furaldehyde, levulinic acid, and polymeric materials. In addition, many minor dehydration products are found. In a study41 of D-fructose, 2-(2-hydroxyacetyl)furan (13), 2-acetyl-3-hydroxyfuran (isomaltol 16), 2,3-dihydro-3,5-dihydroxy-6-methyl-4H-pyran-4-one, and 3,4,5-trihydroxy-3,5-hexadien-2-one (acetylformoin) were identified. Products not formed solely by dehydration mechanisms include acetone,56 formaldehyde, acetalde-... 

Cu(II) complexes have been synthesized and stndied to mimic the catalytic activity of CA24 247 Mechanistic stndies have assisted to reveal the details of the hydration and dehydration mechanism and the catalytic activity of the model complexes 51... 

Write the steps in the mechanism for the dehydration of a given alcohol. Given alcohols of different classes, tell which dehydration mechanism is most likely, and what the relative dehydration rates will be. 

Although some investigators believe that dihydrofuranone is derived from ribose-5-phosphate through a dephosphorylation-dehydration mechanism, others believe it can be formed by a typical Halliard reaction betveen amines and sugars, in which the Amadori products dehydrate and eliminate to amines. Both routes are likely to occur. 

Fig. 5-20. Proposal for the hydration-dehydration mechanism of carbonic anhydrase. The pK of the zinc bound water is envisaged to be lowered to about 7.0 by the charge distribution on the metal ion and also helped by Glu-196 by the hydrogen bonding through Thr-199.

Keratinized epithelium is dehydrated, mechanically tough and chemically resistant. It is found in areas of the oral cavity subject to mechanical stress such as the mucosa of the gingiva (gums) and hard palate (roof of mouth). Non-keratinized epithelium is relatively flexible and is found in areas such as the soft palate, the floor of the mouth, the lips and the cheeks. Thus the regions of the oral cavity pertinent to drag delivery (i.e. the sublingual and buccal regions) have a non-keratinized epithelium. 

The /3-hydroxy ketone must be dehydrated to give the final product. Under these basic conditions, the usual alcohol dehydration mechanism (protonation of hydroxyl, followed by loss of water) cannot occur. Removal of another proton gives an enolate ion that can lose hydroxide in a strongly exothermic step to give the final product. 

Then, during the acid-catalyzed reaction, the weight equivalent of another 0.75 moles of water per mole of glucose was lost. After this stage, all samples were dark brown. Feather and Harris (11) describe a number of intra- and intermolecular dehydration mechanisms for carbohydrates in acidic solution. In that study, some of those mechanisms were aided by the presence of amines. 

The first step of the dehydration mechanism is protonation of an alcohol. Water is then displaced by another molecule of alcohol to form an ether. If two different alcohols are present, either one can be protonated and either one can displace water, yielding a mixture of products. 

Althougir the dehydration mechanism seems rational for the homologation of alcohols having a hydrogen in the position of the hydroxyl function, difficulties arise with alcohols such as methanol and benzylalcohol. Ziesccke proposed carbene or phenyl-carbcnc as intermediates in these cases [56]. No detailed description of the reaction path was given but a mechanism according to Equation (18) is conceivable. 

Figures 2a and 2b display the acid catalyzed E2 and El mechanisms for the dehydration of 1-propanol and 2-propanol. Note that the El mechanism involves four more rate constants (kinetic parameters) than the related E2 dehydration mechanism. Chemists employ the terminology (1) Adg3 to describe the hydration mechanism which forms 2-propanol from propene in Figure 2a, and Ad 2 to refer to the mechanism which forms 2-propanol from propene in Figure 2b. In this paper we do not distinguish between bare carbocations, Il-complexes, encumbered carbocations and symmetrically solvated carbocations, since these intermediates all manifest themselves similarly in the El kinetic model.

Neither the AdE3 nor the AdE2 mechanism is consistent with available data concerning the acid catalyzed hydration of propene to 2-propanol in supercritical water at 375 C and 34.5 MPa. More data are being accumulated to sustain a rigorous kinetic examination of the hydration/dehydration mechanism. 

Research into alcohol dehydration mechanism has proved to be a very fertile area of investigation and the possible mechanistic pathways have been out-... 

Thomke has used model alcohols very effectively in his studies of dehydration mechanisms. For several oxides including thoria a number of deuteriated butanols were employed including d,l erthro(threo)-[3- Y i -butan-2-ol(a), [l,l,l,2,3,3- H6]butan-2-ol (b), and [2- Hi]butan-2-ol (c). 

Product structure. Dehydration may result in relatively minor changes in structure such as decreases in some or all unit cell dimensions in topotactic processes. Alternatively, reaction may result in fundamental reorganization through recrystallization, or conversion to an amorphous or zeolitic material. Dehydration. Dehydration mechanisms encompass a wide variety of routes, summarized in Figure 7.4. The following features require comment. 

Some dehydration mechanisms may be more complicated than described above, for example stepwise dehydration may occur in a sequence of concurrent/ consecutive rate processes. The structures and textures of the residual solids may vary with reaction conditions (availability of water vapour, rate of heating, etc.). 

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