Acid hydrolysis mechanism

Sucrose can, however, degrade to D-glucose and D-fructose in slightly alkaline solution at pH up to 8.3 (sucrose is most stable611 at pH 8.3-8.5, although the reason for this requires some elucidation), but this degradation proceeds by the normal acid-hydrolysis mechanism. In sucrose manufacture, therefore, the main reaction causing sucrose loss, between pH 7 and about 8.3, is the same acid hydrolysis that occurs at lower (acid) pH. 

Because alkali degradation of sucrose does not result in inversion products, in slightly alkaline solution (pH < 8.5), the loss of sucrose to invert sugar (glucose + fructose) is a consequence of the acid hydrolysis mechanism, which provides D-glucose and D-fructose for further alkaline degradation. 

The reaction mechanism of a-amylases is referred to as retaining, which means that the stereochemistry at the cleaved bond of the carbohydrate is retained. Hydrolysis of the glycosidic bond is mediated by an acid hydrolysis mechanism, which is in turn mediated by Aspl97 and Glu233 in pig pancreatic amylase. These interactions have been identified from X-ray crystallography. The aspartate residue has been shown to form a covalent bond with the Cl position of the substrate in X-ray structure of a complex formed by a structurally related glucosyltransferase. " The glutamate residue is located in vicinity to the chloride ion and acts as the acidic catalyst in the reaction. The catalytic site of a-amylases is located in a V-shaped depression on the surface of the enzyme. 

Sugar is destroyed by pH extremes, and inadequate pH control can cause significant sucrose losses in sugar mills. Sucrose is one of the most acid-labile disaccharides known (27), and its hydrolysis to invert is readily catalyzed by heat and low pH prolonged exposure converts the monosaccharides to hydroxymethyl furfural, which has appHcations for synthesis of glycols, ethers, polymers, and pharmaceuticals (16,30). The molecular mechanism that occurs during acid hydrolysis operates, albeit slowly, as high as pH 8.5 (18). 

A number of disinfectants apparentiy owe their activity to formaldehyde, although there is argument on whether some of them function by other mechanisms. In this category, the dmg with the longest history is hexamethylenetetramine (hexamine, urotropin) [100-97-0] which is a condensation product of formaldehyde and ammonia that breaks down by acid hydrolysis to produce formaldehyde. Hexamine was first used for urinary tract antisepsis. Other antimicrobials that are adducts of formaldehyde and amines have been made others are based on methylolate derivations of nitroalkanes. The apphcations of these compounds are widespread, including inactivation of bacterial endotoxin preservation of cosmetics, metal working fluids, and latex paint and use in spin finishes, textile impregnation, and secondary oil recovery (117). 

In this expression, a depends on those rate coefficients ia the above mechanism whose values are assumed to be high. Other mechanisms for the acid hydrolysis and esterification differ mainly with respect to the number of participating water molecules and possible iatermediates (21—23). 

The usual hydrolysis mechanism in strongly acidic solution involves addition of water to the O-protonated amide, followed by breakdown of the tetrahedral intermediate ... 

Problem 17.8 asked you to write details of the mechanism describing formation of benzaldehyde diethyl acetal from benzaldehyde and ethanol. Write a stepwise mechanism for the acid hydrolysis of this acetal. 

Alkoxysilanes undergo hydrolysis, condensation (catalysts for alkoxysilane hydrolysis are usually catalysts for condensation), and a bond formation stage under base as well as under acid catalyzed mechanisms. In addition to this reaction of silanols with hydroxyls of the fiber surface, the formation of polysiloxane structures also can take place. 

Conversion of Amides into Carboxylic Acids Hydrolysis Amides undergo hydrolysis to yield carboxylic acids plus ammonia or an amine on heating in either aqueous acid or aqueous base. The conditions required for amide hydrolysis are more severe than those required for the hydrolysis of add chlorides or esters but the mechanisms are similar. Acidic hydrolysis reaction occurs by nucleophilic addition of water to the protonated amide, followed by transfer of a proton from oxygen to nitrogen to make the nitrogen a better leaving group and subsequent elimination. The steps are reversible, with the equilibrium shifted toward product by protonation of NH3 in the final step. 

Taft, following Ingold," assumed that for the hydrolysis of carboxylic esters, steric, and resonance effects will be the same whether the hydrolysis is catalyzed by acid or base (see the discussion of ester-hydrolysis mechanisms. Reaction 10-10). Rate differences would therefore be caused only by the field effects of R and R in RCOOR. This is presumably a good system to use for this purpose because the transition state for acid-catalyzed hydrolysis (7) has a greater positive charge (and is hence destabilized by —I and stabilized by +1 substituents) than the starting ester. 

Bank S, RJ Tyrrrell (1984) Kinetics and mechanism of alkaline and acidic hydrolysis of aldicarb. JAgric Food Chem 32 1223-1232. 

Acid hydrolysis of cyanates is still commonly used, following a first-stage cyanide oxidation process. At pH 2 the reaction proceeds rapidly, while at pH 7 cyanate may remain stable for weeks.24 This treatment process requires specially designed reactors to assure that HCN is properly vented and controlled. The hydrolysis mechanisms are as follows22 ... 

One exception to the A2 hydrolysis mechanism given above is the case of / -lactams, which have an A1 hydrolysis mechanism in aqueous sulfuric acid,273 presumably proceeding from the A-protonated form for this substrate the excess acidity plot of log k - log Ch+ against X is linear.268... 

At lower acidities the iV-nitrobenzamides and /V-methyl-.V-nitrobenzamides have a hydrolysis mechanism that is not acid-catalyzed for these cases plots of log kv - log h2o are linear, as for the acyhmidazoles discussed above. N-Nitroacetamide also hydrolyzes in this way.291 The proposed mechanism is given in Scheme 17, written for TV-nitroacetamide if the hydration shown is a pre-equilibrium (this is a carbonyl compound with a strong electron-withdrawing group attached, so this is likely), only one water molecule will appear in the rate expression (the difference between 3 and 2), as observed.287 Some evidence for hydroxide-catalyzed processes at the very lowest acidities was also found for some of these compounds.287... 

Scheme 6 depicts three possible hydrolysis mechanisms.140 The first (pathway (i)) is normal acid-catalysed ester hydrolysis in which attack of solvent (H20) upon the protonated intermediate is rate determining. The second (pathway (ii)) is the... 

This problem has been confirmed recently in a study of the mechanism of covalent reaction between nylon 6.6 and the sulpha toe thylsulphone dye Cl Reactive Blue 19 (7.37). Acid hydrolysis of the dyed fibre and HPLC analysis of the hydrolysate yielded the 6-aminohexylaminoethylsulphonyl derivative of Blue 19. Even when the dyeing procedure was optimised to achieve maximal exhaustion and fixation to the fibre [128], only about 30% of the N-terminal amino groups in the nylon 6.6 were accessible because of mutual blocking effects between these bulky anionic dye molecules. 

The mechanism of acid hydrolysis is also different in acyclic amides and /1-lactams acid catalysis of acyclic amides proceeds via O-protonation (see Chapt. 4), whereas that of /1-lactams appears to be a unimolecular A1 type process, involving V-protonation (Fig. 5.6,b) [76], A-Protonation is not the result of reduced amide resonance but an intrinsic property of the /1-lactam structure, since bicyclic /1-lactams and monocyclic /1-lactams exhibit similar reactivity and behavior [76],... 

Fig. 9.2. Simplified reaction mechanisms in the hydrolytic decomposition of organic nitrites. Pathway a Base-catalyzed hydrolysis with liberation of nitrite. Pathway b Reversible nitro-syl exchange between organic nitrites and alcohols. Pathway c General acid catalysis with concerted mechanism in the acid hydrolysis of organic nitrites.

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