Rate of the Acid-Base Reaction

Free energies of reactants and products. 0 The products are more stable than the reactants and are favored at equilibrium. The reactants are more stable and are favored. 

As you should recall from general chemistry, a favorable equilibrium constant is not sufficient to ensure that a reaction will occur. In addition, the rate of the reaction must be fast enough that the reaction occurs in a reasonable period of time. The reaction rate depends on a number of factors. First, the reactants, in this case the acid and the base, must collide. In this collision the molecules must be oriented properly so that the orbitals that will form the new bond can begin to overlap. The orientation required for the orbitals of the reactants is called the stereoelectronic requirement of the reaction. (,Stereo means dealing with the three dimensions of space.) In the acid-base reaction, the collision must occur so that the atomic orbital of the base that is occupied by the unshared pair of electrons can begin to overlap with the is orbital of the acidic hydrogen. 

In the case of the reaction of ammonia with acetic acid, the stereoelectronic requirement can be pictured as follows  

instead, the collision were to occur so that the orbital on the nitrogen bumped into some other part of the acetic acid molecule, that collision would not lead to an acid-base reaction. 

Free energy versus reaction progress diagram for the reaction of HBr with H20. 

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CYANIDE REACTIONS The rates of the acid-base reactions (1)... 

In the case of refractory materials made of oxide ingredients, the reactions are mainly an acid-base ones in materials based on SiC and Si3N4, oxidation-reduction reactions also take place. In general, the intensity and rate of the acid-base reactions is highest when one of the substances involved is acidic and the other basic. The rate decreases when the acidity or basicity of the substances is similar. This is illustrated in the example in Table 2.5, in which shown is the dissolution rate of an acidic refractory material in a molten basic FeO. 

Because the potential is more positive than E i, the formation of only one wave is observed. In acidic and alkaline media where the rate of the acid-base catalysed reaction (35 b) is fast and during the reaction all the phenylhydroxylamine derivative is transformed to quinoneimine, the height of the single wave corresponds to a transfer of six electrons [(35 a) plus (35 c)]. Because the life-time of the quinoneimine intermediate is short, its hydrolysis to form quinone does not affect the electrode process. In the medium pH range where the rate of dehydration is slow, the wave-height corresponds to a four-electron process. A theoretical... 

The choice of techique for the measurement of rates of protonation must also be carefully considered. In CV, DCV, and LSV measurement, complications due to reduction of reactants other than the probase are relatively easily detected. In contrast, processes taking place just after the PB reduction may affect the results obtained by methods such as DPSC, which involves stepping beyond the probase reduction peak. An example comes from a study of the reaction between 9-fluorenylidene dianions and phosphonium salts. The product of the acid-base reaction is the ylid, which turns out to be reducible in the region to which the potential is stepped in DPSC experiments [81]. For reaction in DMSO between the dianion of 9-fluorenylidene derivative (7b, Scheme 23) and [Ph3PCH2Ph]" " the rate constant determined [81] by DPSC appears to be 4.5 x 10 M s . In contrast, DCV gives [43] a rate constant of only 12.31 M s . The DPSC method works well for... 

Generally speaking, dehydration reactions, being slower than most of the acid-base reactions, are not so much affected by factors dependent on the surface of the electrode and hence polarographically determined rate constants are more reliable. 

Deviations from the Brdnsted relation may appear when there are specific interactions in the transition state. The ordinary type of steric hindrance has little effect on acid-base equilibria on account of the small size of the proton, but it may of course affect the rate of an acid-base reaction, since the two species and B2 must be close together in the transition state, and this may be hindered by the presence of bulky groups in one or both reactants. There are few clear examples of this effect in... 

We will use the concepts just set forth in many subsequent chapters of this book. Yet, now we can use them to better understand acid-base chemistry. Let s first focus on issues that affect the rate of an acid-base reaction. 

Table 4-1 lists some rate constants for acid-base reactions. A very simple yet powerful generalization can be made For normal acids, proton transfer in the thermodynamically favored direction is diffusion controlled. Normal acids are predominantly oxygen and nitrogen acids carbon acids do not fit this pattern. The thermodynamicEilly favored direction is that in which the conventionally written equilibrium constant is greater than unity this is readily established from the pK of the conjugate acid. Approximate values of rate constants in both directions can thus be estimated by assuming a typical diffusion-limited value in the favored direction (most reasonably by inspection of experimental results for closely related... 

An inflection point in a pH-rate profile suggests a change in the nature of the reaction caused by a change in the pH of the medium. The usual reason for this behavior is an acid-base equilibrium of a reactant. Here we consider the simplest such system, in which the substrate is a monobasic acid (or monoacidic base). It is pertinent to consider the mathematical nature of the acid-base equilibrium. Let HS represent a weak acid. (The charge type is irrelevant.) The acid dissociation constant, = [H ][S ]/[HS], is taken to be appropriate to the conditions (temperature, ionic strength, solvent) of the kinetic experiments. The fractions of solute in the conjugate acid and base forms are given by... 

Analogous, but slightly different, is the treatment of the acid-base equilibria and the basic hydrolysis rates of the phthalimids (19). In both of these cases, the two paths to the reaction site are equivalent, hence pi and po of Eq. (3) are equal, and the equation reduces to ... 

The rate of reaction of a series of nucleophiles with a single substrate is related to the basicity when the nucleophilic atom is the same and the nucleophiles are closely related in chemical type. Thus, although the rates parallel the basicities of anilines (Tables VII and VIII) as a class and of pyridine bases (Tables VII and VIII) as a class, the less basic anilines are much more reactive. This difference in reactivity is based on a lower energy of activation as is the reactivity sequence piperidine > ammonia > aniline. Further relationships among the nucleophiles found in this work are morpholine vs. piperidine (Table III) methoxide vs. 4-nitrophenoxide (Table II) and alkoxides vs. piperidine (Tables II, III, and VIII). Hydrogen bonding in the transition state and acid catalysis increase the rates of reaction of anilines. Reaction rates of the pyridine bases are decreased by steric hindrance between their alpha hydrogens and the substituents or... 

Does this model give us a practical solution for the synthesis of monosubstitution products in high yields The model teaches us that reactions are not disguised by micromixing if the intrinsic rate constant (in Scheme 12-84 k2o and k2v>) is significantly less than 1 m-1s-1. As discussed in Section 12.7, the intrinsic rate constant refers to unit concentrations of the acid-base equilibrium species involved in the substitution proper, not to analytical concentrations. Therefore, if the azo coupling reaction mentioned above is not carried out within the range of maximal measured rates (i.e., with the equilibria not on the side of the 1-naphthoxide ion and... 

First, the simple thermodynamic description of pe (or Eh) and pH are both most directly applicable to the liquid aqueous phase. Redox reactions can and do occur in the gas phase, but the rates of such processes are described by chemical kinetics and not by equilibrium concepts of thermodynamics. For example, the acid-base reaction... 

Reactions can be catalyzed by acid or base in two different ways, called general and specific catalysis. If the rate of an acid-catalyzed reaction run in a solvent S is... 

Such behavior is more common than the full rate/pH profile of (1.207). Equation (1.208) is observed in acid catalysisand (1.209) in base catalysis. The rate constant for the reaction of only one of the two forms can be obtained directly, that is, in (1.208) and in (1.209). Ancillary information on is required to assess the rate constant of the acid-base partner. The absence of reliable data on A jj can pose a problem in assessing the missing rate constant. 

Reactions can be catalyzed by acid or base in two different ways, called general and specific catalysis. If the rate of an acid-catalyzed reaction run in a solvent S is proportional to [SH j, the reaction is said to be subject to specific acid catalysis, the acid being the lyonium ion SH. The acid that is put into the solvent may be stronger or weaker than SH , but the rate is proportional only to the (SH ] that is actually present in the solution (derived from S + HA SH + A ). The identity of HA makes no difference except insofar as it determines the position of equilibrium and hence the (SH ]. Most measurements have been made in water, where SH is H30 . 

The acid-base reaction involves the lattice O2. For a given hydrocarbon, the proton transfer rate is enhanced by an increased basicity of O2-. Indeed,... 

For confidence in the accuracy of /cHa.b> it is important that the curvature in Fig. 11.2A corresponds to an increase of two-fold or greater in the slope. Too often, the third-order term in Equation 11.2 (/cha b) is inaccurate because it derives from a very small fraction of the total reaction flux, and needs confirmation. Data for the enolisation of acetone are reproduced in Fig. 11.3, which shows that substantial differences between observed rate constants and calculated values not including the third-order term are seen only at high concentration of the acid-base pair [2]. Concentrations of acid-base pairs are between 0 and 2 M, and there are no major specific salt or solvent effects. This is important as the high... 

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