Rate constant, base hydrolysis pseudo first-order

Figure 1. Hydrolysis pH-rate profiles of phenyl acetate (lower) and a substituted 2-phenyl-l,3-dioxane (HND). Phenyl acetate profile constructed from data of Mabey and Mill (32), HND profile from data of Bender and Silver (33). Phenyl acetate reacts via specific-acid catalyzed, neutral, and base-catalyzed transformation pathways. The pseudo-first-order rate constant is given by Kobs = K(h+) [H+] + Kn + K(qh-) [0H—]. HND hydrolyzes only via an acid-catalyzed pathway the phenolate anion is some 867 times more reactive than its conjugate acid.

Neutral hydrolysis is dependent only on water which is present in excess, so kn is a simple pseudo-first-order rate constant (with units t-1). The acid- and base-catalyzed... 

Hydrolysis base rate constant k < 0.9 M 1 h 1 at 25°C with a calculated t/2 > 900 yr at pH 7 (Ellington et al. 1988) Biodegradation dechlorination pseudo-first order rate constant k = 0.387 d 1 with t/2 = 1.8 d using a mixed enrichment culture derived from a contaminated estuarine sediment (Pavlostathis Prytula 2000). Biotransformation ... 

Each compound (i.e., unprotonated and protonated species) in an aqueous solution undergoes acid-base catalyzed degradation (i.e., hydrolysis) as predicted in Equation (5.156a), Equation (5.156b), and Equation (5.156c). The pseudo-first-order degradation rate constant is ... 

Acetylsalicylic acid (aspirin) (pKa= 3.62) undergoes acid-base catalyzed hydrolysis. The pseudo-first-order rate constants were determined at four different temperatures (25, 35, 50.3, and 60.3°C) as shown in Figure 5.41. The hydrolysis of aspirin follows Equation (5.167) with k0o = 0 and k0 = 0. Each rate constant is given by ... 

The influence of sodium dodec3d sulfate, hexadecyltrimethyl-ammonium bromide and a polyoxyethylene(18) dodecylphenol on the hydrolysis of the Schiff bases p-chlorobenzylidene-l,l-dimethylethyl-amine, p-methoxybenzylidene-l,l-dimethylethylamme, and benz-hydrilidenedimethylammonium iodide (iV,iV-dimethylbenzophenone-imonium iodide) have been investigated (Behme and Cordes, 1965). The observed pseudo-first order rate constant for the hydrolysis ofp-chloro-... 

The aminolysis and methanolysis of ionized phenyl salicylate (189) have been examined under micellar conditions. The effect of CTABr on the rates of aminolysis of (189) by -butylamine, piperidine, and pyrrolidine is to bring about a rate decrease (up to 17-fold with pyrrolidine). The results are interpreted in terms of binding constants for the amines with CTABr and the pseudo-phase model.The effects of mixed surfactants SDS and CTABr on the methanolysis of (189) and the alkaline hydrolysis of phenyl benzoate suggest that micellar aggregates are involved in the processes.The effects of NaOH and KBr on the intramolecular general base-catalysed methanolysis of (189) in the presence of CTABr has been investigated. Pseudo-first-order rate constants were not affected by either additive but other changes were noted. The effect of mixed MeCN-water solvents on the same reaction has also been probed. 

Many hydrolysis reactions are pH dependent with base catalysis predominant. The acid-base catalysis is especially significant with carboxylic acid esters. The pseudo-first-order hydrolysis rate constant it/,(time" ) can be expressed as... 

It can be seen from Eqs. [2-82] and [2-83] that pH exerts a strong effect on base-catalyzed hydrolysis. For example, hydrolysis would be 1 million times faster at a pH of 10 than at a pH of 4. As long as pH is known and constant, however, the rate constant kh can be combined with the concentration of hydroxide ions, resulting in a pseudo-first-order conditional rate constant h h... 

In some second-order reactions the concentration of one of the reactants is many times more than the concentration of the other, so large in fact as to be considered constant throughout the reaction. In these cases, the reaction appears to follow first-order kinetics, even though, strictly speaking, it is still a second-order process. Reactions such as these are termed pseudo first-order reactions. A good example is the acid- or base-catalysed hydrolysis of an ester, in which the concentration of water is so large compared to the concentration of ester as to be considered constant. The rate of the hydrolysis appears to vary only with the concentration of the ester. 

Binuclear zinc complexes of pyrazolate-based chelate ligands (Fig. 61) exhibit differing bis(4-nitrophenyl) phosphate hydrolysis reactivity depending on the nature of the supporting chelate ligand.153 Plots of the pseudo first-order rate constant... 

Two different binuclear copperdi) complexes have been prepared recently, one with a bridging phenoxy ligand having two bis-benzi-midazole arms (12, Fig. 14), and the second having a bis-cyclen-naphthalene ligand (13, Fig. 15) (352, 353). Both of them show bimetallic cooperativity for the hydrolysis of phosphate diesters, contrary to studies with the dinuclear cobalt complex (354). The pseudo-first-order rate constants for hydrolysis of the para-nitrophenylphosphate ester of propylene glycol by bis-benzimidazole-based copper complexes... 

High performance thin layer chromatography (HPTLC), a useful technique for the study of the mechanisms and kinetics of organic reactions, has been applied to the base hydrolysis of a series of 4,5,6,7-tetrahydro-l,3-diazepines when pseudo first-order rate constants were observed <95MI 905-01 >. 

Stopped-flow studies of the base hydrolysis of aminocarboxylato-complexes of (as well as Mo and have been reported. The 1 1 complexes are involved with edda , nta , and edta, and a 1 2 (V ligand) complex with ida . The observed pseudo-first-order rate constant varies with OH according to the equation /robs=(A i+A 2J os[OH"] )/(l-l-Aos[OH ]") where =2 for and = 1 for and Koa is the outer-sphere formation constant between complex and OH ion, and and are rate constants for attack by H2O and OH ion respectively. The non-bonded carboxylate arms of the ligands increase values of k and k by factors of 10 —10 . 

The pseudo-first-order rate constant (/robs) for the base hydrolysis of amino-carboxylato-complexes of Mo, and vary with [OH ] according to the general equation /robs=(/ri-l-/r2i ro[OH-]")/(l-l-J5 o[OH ] ) with n=2 for and n=l for Mo i and W i, where Ko is the outer-sphere association constant and k and k are the rate constants for reaction of the rapidly formed ion-pair with HgO and OH ion respectively. Complexes with ida , edda , nta , and edta ions were investigated, and non-bonded carboxylate arms were found to increase values of k and k by factors of 10 —10 . An analytic procedure has been proposed, based on these reactions, which allows concentrations of Mo, and to be estimated to an accuracy of ca. 5 % at molarities in the region of 10 M. ... 

These CD-based polymers were engaged in hydrolysis reaction of various p-nitrophenol esters derivatives. The reaction was monitored by UV at 400 mn. Among the CD-based polymers, only poly- 3-CD-A showed a high hydrolysis activity. The pseudo first-order rate constant k measured for the ester hydrolysis catalyzed by poly- 3-CD-A is compared to the rate constant measured using the native 3-CD. The rate enhancements resulting from the use of poly- 3-CD-A are presented in Table 2.1. 

Two reports have appeared of kinetic studies of the base hydrolysis of complex ions of the type [Co(NH3)5(OCOR)] +. Activation parameters are reported from rate constants extrapolated to zero ionic strength for R = CM3 CI (/i = 1,2, or 3). When RCO2" = salicylate ion a simple second-order rate law is not observed owing to the importance of the deprotonation of the phenolic group. " If K is the equilibrium constant for this deprotonation reaction, the pseudo-first-order rate constant (k) in the presence of an excess of OH" ion is given by k=(A iis [OH-] k2K[OH-] )IH J5 [OH"])... 

The use of base-catalysed reactions for the template synthesis of co-ordinated, often macrocyclic ligands was discussed in the introduction to this chapter. " Chromium(m) Complexes.—Studies of the base hydrolysis of chromium(ra) complexes at high pH are relatively rare, probably because of the ease with which polymeric hydroxy-complexes can be precipitated. Studies of aqua-chromium(m) complexes even at low pH invariably show that conjugate-base formation is important owing to the acidity of the co-ordinated water molecules. Conjugate-base formation is apparent when the observed pseudo-first-order rate constant, k, varies with acidity according to the equation A =A o+ -x/[H+]. Recent examples include studies of the [Cr(HaO)6(NHs) + and [Cr(ox)2(N3)(H20)]2- ions." ... 

The overall pseudo first order rate constant for the Schiff base hydrolysis calculated from the initial slope was 9x10 sec". ... 

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