Hydroxy-, rate of hydration

ScholesG, Willson RL, Ebert M (1969) Pulse radiolysis of agueous solutions of deoxyribonudeotides and of DNA reaction with hydroxy-radicals. Chem Commun 17-18 Schuchmann MN, von Sonntag C (1982) Flydroxyl radical induced oxidation of diethyl ether in oxygenated aqueous solution. A product and pulse radiolysis study. J PhysChem 86 1995-2000 Shragge PC, Michaels FIB, Flunt JW (1971) Factors affecting the rate of hydrated electron attack on polynucleotides. Radiat Res 47 598-611... 

In the pH range that fumarase exhibits activity, phosphate exists as the monobasic and dibasic salts. Since the two forms may influence the enzyme differently, pH curves were established with tris(hydroxy-methyl)aminomethane (Tris) buffer. The most striking result of these studies is that the curves obtained with malate and fumarate as substrates are quite different. The maximum rate of hydration of fumarate occurs at pH 6.5, whereas the dehydration of malate is most rapid near pH 8 (Fig. 18). 

The introduction of chloro-substituents into the methylene group of 2-methylene-1,3-dioxolan causes a decrease in the rate of the hydronium-catalysed hydration and also in the rate of the hydronium ion-catalysed breakdown of the resulting hemiorthoester. Since both rate decreases are similar, 2-hydroxy-2-chloromethyl- 1,3-dioxolan [96] and 2-hydroxy-2-dichloromethyl- 1,3-dioxolan [98] can be detected in hydration of 2-chloro-methylene-1,3-dioxolan [95] (Scheme 10) and 2-dichloromethyl-1,3-dioxolan [97] (Scheme 11) (Capon and Dosunmu, 1984). 

The oxidation may proceed through the hydrated form of the carbonyl group > CH(OH)2. The rate of oxidation is 1,2-glycols > a-hydroxy aldehydes > a-hydroxy ketones > a-hydroxy acids. 

Similar studies were carried out using photoionization of benzidine derivatives (Narayanaetal., 1982), phenothiazine(Alkaitisetal., 1975), or pyrene (Gratzel and Thomas, 1974) dissolved in the hydrophobic core of charged micelles. A laser pulse ejects an electron which is rapidly hydrated and then decays into hydroxy radicals, or reacts with the excited cation. The charge of the micelle has a dramatic effect on the rate of the latter reaction but the presence of the former reaction forbade a precise kinetic analysis of the overall reaction. 

Barium iodate 1-hydrate in granular form is obtained by the reaction in solution of barium ion with iodate ion formed by the slow reduction of periodic acid by lactic acid at room temperature. Periodic acid is a specific oxidant for glycols and a-hydroxy aldehydes, ketones, and acids, but the rate of oxidation is very slow for a-hy-droxy acids. Thus the barium iodate 1-hydrate crystals are slowly formed from a dilute solution of iodate ion which is being formed as a reaction product. 

An issue with the ferric peroxide mechanism is that the substrate is in the (hydrated) gew-diol form following the second reaction (Fig. 9.28). However, the proposed ferric peroxide mechanism involves a nucleophilic attack (Fe " 02 ) on the carbonyl (aldehyde). Thus, the gew-diol must be dehydrated before this step can run. The rate of dehydration has been estimated at > 0.5 s (in the absence of P450 19A1 using 0 exchange methods [2295], which is faster than the (8 min ) for going from 19-hydroxy androstenedione to estrone [220]. The reaction could occur with the aldehyde or the ge/w-diol, the latter of... 

Lewis acidic boron in much the same way that hydrated Lewis acidic metal ions exchange bound water. A pertinent comparison can be found with the ionisation of Zn in water, the reaction Zn OH2 - Zn-OH + having a p.Ka of 8.8. Boronic acids have been reported to rapidly and reversibly " interact with dicarboxylic acids,a-hydroxy carboxylic acids and diols to form esters in aqueous media.The most common interaction is with 1,2 and 1,3 diols to form five- and six-membered rings, respectively. From experimental observations, it is well known that the kinetics of this interconversion are fastest in aqueous basic media where the boron is present in its tetrahedral anionic form. Typically, differences in rate of 10" are observed between boron in its trigonal and tetrahedral forms. ... 

No hydration could be detected in 1,4,5-triazanaphthalene. However, the cation of the 2-hydroxy derivative had been shown to hydrate in the 3,4-position, and the ratio of the hydrated to the anhydrous forms is 16, The hydrate of the neutral species is unstable but has a half-life of 6 min at pH 7, which permits easy measurement of the ionization and rate constants. ... 

The reversible hydration of nitrogen-containing heterocyclic bases and their hydroxy and mercapto derivatives is acid-base catalyzed and, at constant pH, the reactions obey first-order rate equations. 2 26.27-33... 

In contrast, the acid-catalyzed hydration of arylbenzoylacetylenes differs markedly from the hydration of a-unsaturated ketones. Hydration of unsaturated ketones has been shown to proceed via a 1,4-addition mechanism where protonation occurs on oxygen to give an oxonium salt, followed by attack of water at the 0-carbon to give a hydroxy enol. The rate-limiting step has been shown to be the protonation of the hydroxy enol (27) ... 

Addition of hydrogen sulfide and thiols is qualitatively similar to reaction with alcohols in that there are two stages, formation of hemithioacetal (or hemithio-ketal) followed by acid-catalyzed elimination of the hydroxy group and substitution of a second —SR (Equations 8.47 and 8.48). The transformation has been studied less extensively than hydration and acetal formation, and relatively little information on mechanism is available. The initial addition appears to be specific base-catalyzed, an observation that implies that RS is the species that adds. The situation is thus similar to cyanide addition. General acid catalysis has, however, been found at pH 1 to 2 for addition of weakly acidic alkyl thiols, and the reaction rate as a function of pH has a minimum and rises both on the... 

Cyclohexadienol was prepared by Rickborn in 1970 from reaction of the epoxide of 1,4-cyclohexadiene with methyl lithium.100 A hydrate of naphthalene, 1-hydroxy-1,2-dihydro-naphthalene was prepared by Bamberger in 1895 by allylic bromination of O-acylated tetralol (1-hydroxy-l,2,3,4-tetrahydronaphthalene) followed by reaction with base.101 Hydrates of naphthalene and other polycylic aromatics are also available from oxidative fermentation of dihydroaromatic molecules, which occurs particularly efficiently with a mutant strain (UV4) of Pseudomonas putida.102,103 The hydrates are alcohols and they undergo acid-catalyzed dehydration to form the aromatic molecule by the same mechanism as other alcohols, except that the thermodynamic driving force provided by the aromatic product makes deprotonation of the carbocation (arenonium ion) a fast reaction, so that in contrast to simple alcohols, formation of the carbocation is rate-determining (Scheme 6).104,105... 

The acid catalyzed hydration of compounds 18 to give both the expected /l-hydroxy ester 19 and the unsaturated ester 20 proceeds at a rate which is larger by a factor of 50-100 than calculated on the basis of the Taft p o relationship (see 7b and 8 of Table 1) (Hekkert and Drenth, 1961). The rate enhancement has been explained in terms of anchimeric assistance efieets due to the hydroxyl group, and a four membered ring structure of type 21 has been proposed for the cationic intermediate. 

Hydrated ruthenium dioxide will act as a catalyst for the oxidation of primary allylic alcohols (equations 8 and 9) in an oxygen atmosphere (a trace of the antioxidant 2,6-di-r-butyl-4-methylphenol is required to prevent autoxidation of the aldehyde to the acid). The oxidation is not accompanied by any loss in double bond stereochemistry, secondary allylic alcohols are oxidized but at a decreased rate, and saturated alcohols are scarcely oxidized at all. However, a-hydroxy ketones and a-hydroxylactones will oxidize under forcing conditions, so there is clearly likely to some degree of substrate dependence. ... 

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