Acid catalysis buffer ratio

The experimentally observed pseudo-first order rate constant k is increased in the presence of DNA (18,19). This enhanced reactivity is a result of the formation of physical BaPDE-DNA complexes the dependence of k on DNA concentration coincides with the binding isotherm for the formation of site I physical intercalative complexes (20). Typically, over 90% of the BaPDE molecules are converted to tetraols, while only a minor fraction bind covalently to the DNA bases (18,21-23). The dependence of k on temperature (21,24), pH (21,23-25), salt concentration (16,20,21,25), and concentration of different buffers (23) has been investigated. In 5 mM sodium cacodylate buffer solutions the formation of tetraols and covalent adducts appear to be parallel pseudo-first order reactions characterized by the same rate constant k, but different ratios of products (21,24). Similar results are obtained with other buffers (23). The formation of carbonium ions by specific and general acid catalysis has been assumed to be the rate-determining step for both tetraol and covalent adduct formation (21,24). 

The addition of buffers is required to maintain constant pH during the reaction when experiments are to be carried out in the range 3 < PH <11. However, keto-enol tautomerization reactions usually exhibit so-called general acid and base catalysis. 26 The observed rate acceleration with increasing buffer concentration implies that the components of the buffer participate in some rate-determining step of the reaction. In most cases, the rate of reaction increases linearly with increasing buffer concentration at constant buffer ratio, chb/cb3 = const (Fig. 4a). 

In order to detect a general acid or general base catalysis, it is necessary to do a series of kinetic experiments in which [H+] or [OH-] and the ionic strength are kept constant while [HA] or [B] are varied in such a way that the buffer ratio remains unchanged within the same series. Details of studies of this type have been adequately described in the literature [1, 3, 4]. 

With these provisos, buffer catalysis is generally measured from gradients of plots of observed first-order rate constants against buffer concentration the separation of the second-order buffer catalysis constant into catalytic constants for acid and basic components of the buffer is achieved by secondary plots of these second-order constants against buffer ratio. This is illustrated in Figure 1.17. 

General acid-base catalysis is defined experimentally by the appearance in the rate law of acids and/or bases other than lyonium or lyate ions. For example, the hydrolysis of enol ethers 1.2 (Scheme 2.2) is general acid-catalyzed. In strong acid the rate expression will be the same as in Scheme 2.1, but near neutral pH the rate is found to depend also on the concentration of the buffer (HA + A ) used to maintain the pH. Measurements at different buffer ratios show that the catalytic species is the acid HA. (If more than one acid is present there will be an additional term kHAi[HA ][1.2] for each.)... 

When the carbanion decomposes more readily than it reprotonates, kinetic behaviour intermediate between that of the carbanion and bimolecular mechanism is predicted. For only a small extent of substrate ionisation in low conjugate acid concentration (k-i s>, [6h]), general base catalysis is observed. At constant buffer ratio, an increase in base concentration causes a linear increase in observed rate coefficient until / [6h] approaches/ .2. Under this condition the rate coefficient attains a maximum with further increase in base concentration, the kinetics parallel the carbanion mechanism and specific base catalysis is observed . ... 

A plot of k versus [AH] should generate a linear plot with slope ( ah + k /r), and analysis of several studies, each with a different value of r, can yield the values of /cah and k, separately. If it can be demonstrated that only general acid catalysis is significant, then keeping a 1 1 buffer ratio r — 1) allows kAH fo be determined directly from a plot of k versus... 

Polymerization Chemistry. The amine groups of urea react very readily with formaldehyde, forming methylol nreas (Fig. 3.9). The A-stage reaction is controlled by the urea/formaldehyde ratio (1/1.3 to 1/2.2), an alkalme buffer at pH 7.5-8.0, and refluxing up to 8 hr, to produce a mixture of mono-, di-, and trimethylol ureas. These condense to form oligomers and Anally, with acid catalysis and heat, highly cross-linked thermoset polymers. 

The operational characteristic of general acid or base catalysis in homogeneous solution is the dependence of reaction velocity on total acid (or base) concentration when the concentration of hydrogen ion is being kept constant. Such variations in composition can be achieved by the use of buffer solutions. A constant buffer ratio (at constant and low ionic strength) ensures constancy of hydrogen ion... 

The procedure cannot immediately be transferred to the study of catalysis by solid macromolecular acids since the hydrogen ion concentration in the resin phase is not so easily regulated. Because of the Donnan equilibrium between internal and external phases, the internal and external hydrogen ion concentrations can differ by several powers of ten [48], and there is no straightforward method of relating buffer ratio and external or internal pH. 

In reactions analogous to those of the lower homologues, bicyclo [4,1,0]-heptan-2-yl compounds isomerize to cyclohept-3-enyl derivatives under acid catalysis. " It was shown, however, that earlier results of buffered solvolyses, which give exo- and endo-bicyclo[4,l,0]heptan-2-yl products in the ratio ca. 

One can test for general acid-base catalysis by varying [BH+] and [B] at constant pH. An easy test is to dilute the buffer progressively at a constant ratio of [BH+]/[B], making up any ionic strength change so as not to introduce a salt effect. If the rate is invariant with this procedure, then general acid-base catalysis is absent under the circumstances chosen. 

The usual means of finding general catalysis is to measure reaction rate with various concentrations of the general acids or bases but a constant concentration of H30 +. Since the pH depends only on the ratio of [HA] to [A-] and not on the absolute concentrations, this requirement may be satisfied by the use of buffers. Catalytic rate constants have been measured for a number of acids and bases in aldehyde hydration-dehydration, notably by Bell and co-workers.10 For formaldehyde, a = 0.24, /3 = 0.40 earlier work11 gave for acetaldehyde a = 0.54, /3 = 0.45 and for symmetrical dichloroacetone a = 0.27, /3 = 0.50. 

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