Hydrolysis evidence

This model has been reinforced by kinetic studies. The five-carbon sugar xylose, stereochemically similar to glucose but one carbon shorter, binds to hexokinase but in a position where it cannot be phosphorylated. Nevertheless, addition of xylose to the reaction mixture increases the rate of ATP hydrolysis. Evidently, the binding of xylose is sufficient to induce a change in... 

Rha), arabinose (Ara), galactose (Gal), xylose (Xyl), and 4-O-methyl-glucuronic acid (M-GluU) are the same or larger than after intensive hydrolysis, evidence for a more moderate action of the acid. But for a complete hydrolysis of cellulose and glucomannan, the moderate method is not sufficient therefore, the peaks for mannose (Man) and glucose (Glu) are much smaller after this treatment. 

The effect of substituents at C-5 of pyranosides is treated separately, because there has been much interest in, investigation of, and speculation as to the reason that aldobiouronic acids are stable to acid hydrolysis. Evidence is accumulating that this stability may, for the most part, be attributed to steric factors. 

Rearrangement to an open chain imine (165) provides an intermediate whose acidity toward lithiomethylthiazole (162) is rather pronounced. Proton abstraction by 162 gives the dilithio intermediate (166) and regenerates 2-methylthiazole for further reaction. During the final hydrolysis, 166 affords the dimer (167) that could be isolated by molecular distillation (433). A proof in favor of this mechanism is that when a large excess of butyllithium is added to (161) at -78°C and the solution is allowed to warm to room temperature, the deuterolysis affords only dideuterated thiazole (170), with no evidence of any dimeric product. Under these conditions almost complete dianion formation results (169), and the concentration of nonmetalated thiazole is nil. (Scheme 79). This dimerization bears some similitude with the formation of 2-methylthia-zolium anhydrobase dealt with in Chapter DC. Meyers could confirm the independence of the formation of the benzyl-type (172) and the aryl-type... 

Additional evidence for carbocation intermediates in certain nucleophilic substitutions comes from observing rearrangements of the kind normally associated with such species For example hydrolysis of the secondary alkyl bromide 2 bromo 3 methylbutane yields the rearranged tertiary alcohol 2 methyl 2 butanol as the only substitution product... 

The most important species m the mechanism for ester hydrolysis is the tetrahe dral intermediate Evidence m support of the existence of the tetrahedral intermediate... 

Convincing evidence that ester hydrolysis in base proceeds by the second of these two paths namely nucleophilic acyl substitution has been obtained from several sources In one experiment ethyl propanoate labeled with 0 m the ethoxy group was hydrolyzed On isolating the products all the 0 was found m the ethyl alcohol there was no 0 enrichment m the sodium propanoate... 

Ammonium fluorosulfate is produced from ammonium fluoride by reaction with sulfur trioxide, oleum, or potassium pyrosulfate, 1 2820 (48). Solutions of ammonium fluorosulfate show Htfle evidence of hydrolysis and the salt may be recrystallized from hot water. Ammonium fluorosulfate absorbs anhydrous ammonia to form a series of Hquid amines that contain 2.5—6 moles of ammonia per mole of salt (77). 

Hydroxyall l Hydroperoxyall l Peroxides. There is evidence that hydroxyalkyl hydroperoxyalkyl peroxides (2, X = OH, Y = OOH) exist in equihbrium with their corresponding carbonyl compounds and other a-oxygen-substituted peroxides. For example, reaction with acyl haUdes yields diperoxyesters. Dilute acid hydrolysis yields the corresponding ketone (44). Reduction with phosphines yields di(hydroxyalkyl) peroxides and dehydration results in formation of cycHc diperoxides (4). 

There is no evidence for the existence of thallic hydroxide addition of hydroxide to an aqueous solution of a T1(III) salt gives TI2O2 instead. ThaHous hydroxide can be isolated as yellow needles by the hydrolysis of thaHous ethoxide [20398-06-5] which is conveniendy prepared as a heavy oH by the oxygen oxidation of thallium metal in ethanol vapor. ThaHous hydroxide darkens at room temperature and decomposes to TI2O and H2O on warming. 

High quahty SAMs of alkyltrichlorosilane derivatives are not simple to produce, mainly because of the need to carefully control the amount of water in solution (126,143,144). Whereas incomplete monolayers are formed in the absence of water (127,128), excess water results in facile polymerization in solution and polysiloxane deposition of the surface (133). Extraction of surface moisture, followed by OTS hydrolysis and subsequent surface adsorption, may be the mechanism of SAM formation (145). A moisture quantity of 0.15 mg/100 mL solvent has been suggested as the optimum condition for the formation of closely packed monolayers. X-ray photoelectron spectroscopy (xps) studies confirm the complete surface reaction of the —SiCl groups, upon the formation of a complete SAM (146). Infrared spectroscopy has been used to provide direct evidence for the hiU hydrolysis of methylchlorosilanes to methylsdanoles at the soHd/gas interface, by surface water on a hydrated siUca (147). 

Hydroxides. The hydrolysis of uranium has been recendy reviewed (154,165,166), yet as noted in these compilations, studies are ongoing to continue identifying all of the numerous solution species and soHd phases. The very hard uranium(IV) ion hydrolyzes even in fairly strong acid (- 0.1 Af) and the hydrolysis is compHcated by the precipitation of insoluble hydroxides or oxides. There is reasonably good experimental evidence for the formation of the initial hydrolysis product, U(OH) " however, there is no direct evidence for other hydrolysis products such as U(OH) " 2> U(OH)" 2> U(OH)4 (or UO2 2H20). There are substantial amounts of data, particulady from solubiUty experiments, which are consistent with the neutral species U(OH)4 (154,167). It is unknown whether this species is monomeric or polymeric. A new study under reducing conditions in NaCl solution confirms its importance and reports that it is monomeric (168). 8olubihty studies indicate that the anionic species U(OH) , if it exists, is only of minor importance (169). There is limited evidence for polymeric species such as Ug(OH) " 25 (1 4). 

Syn- and anti-orientations are possible and there is evidence that the anti-orientation does not favor orbital overlap such an orientation is favored with larger branched-chain substituents. A C-nmr study found that the TT-electron density on the vinyl P-carbon is lower as the reactivity of the monomer increases (20). Methyl vinyl ether exists almost entirely ia the syn-stmcture, a favorable orbital overlap situation, and MVE for this reason is less reactive to both polymerization and hydrolysis (21). 

Successive introduction of two methyl groups at ring carbon increases the hydrolysis rate by a factor of 10 in each step, indicating cation formation in the transition state as in acetal hydrolysis. Equilibrium protonation before hydrolysis becomes evident from an increasing rate of hydrolysis with a decreasing pH value (Table 3). Below pH 3 no further increase of rate is observed, so that protonation is assumed to be complete. 

One of the major differences between penicillins and cephalosporins is the possibility for a concerted elimination of the C-3 substituent in the case of cephalosporins (6->7). There is now considerable evidence to support the idea that an increase in the ability of the C-3 substituent to act as a leaving group results in an increased reactivity of the 8-lactam carbonyl (75JMC408). Thus, both the hydrolysis rate of the 8-lactam and antibacterial activity... 

Na/NH3, -30°, 3 min, 1(X)% yield. This protective group is stable to acidic hydrolysis (4.5 N HBr/HOAc 1 N HCV, CF3CO2H, reflux). There is no evidence of S N acyl migration in 5-(A-ethylcarbamates) (RS = cysteinyl). Oxidation of 5-(A-ethylcarbamoyl)cysteine with performic acid yields cysteic acid. ... 

This variation from the ester hydrolysis mechanism also reflects the poorer leaving ability of amide ions as compared to alkoxide ions. The evidence for the involvement of the dianion comes from kinetic studies and from solvent isotope effects, which suggest that a rate-limiting proton transfer is involved. The reaction is also higher than first-order in hydroxide ion under these circumstances, which is consistent with the dianion mechanism. 

Hydrolysis of aspirin in H2 0 leads to no incorporation of into the product salicylic acid, ruling out the anhydride as an intermediate and thereby excluding mechanism 1. The general acid catalysis of mechanism III can be ruled out on the basis of failure of other nucleophiles to show evidence for general acid catalysis by the neighboring carboxylic acid group. Because there is no reason to believe hydroxide should be special in this way, mechanism III is eliminated. Thus, mechanism II, general base catalysis of hydroxide-ion attack, is believed to be the correct description of the hydrolysis of aspirin. 

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