Equilibrium constants, of acetates

Flo. 33. Extrapolation to zero ionic strength of the equilibrium constant of acetic acid in aqueous solution at 25°C. 

Solid-State Structure and Ring-Chain Equilibrium Constants of Acet- and Benzamjdrazonium iodides (59A S9B,... 

The way in which dissociation constants are obtained from experimental data is illustrated in Table 1.7, in which the dissociation equilibrium constant of acetic acid is computed from molar conductivities. The average value... 

Table 1.7 Calculation of the dissociation equilibrium constant of acetic acid from measured values of molar conductivity...

Carboxylic acids and esters contain far less enol than aldehydes and ketones. So little enol is present that it is difficult to measure, and the 10 values for the enolization equilibrium constants of acetic acid and methyl acetate given in Table 20.2 are only approximate. The main reason for the decreased tendency of carboxylic acids and esters to enolize appears to be the stabilization of the carbonyl group of the keto form by electron release from the alkoxy oxygen. 

The rates of both formation and hydrolysis of dimethyl acetals of -substituted benzaldehydes are substituent-dependent. Do you expect to increase or decrease with increasing electron-attracting capacity of the pam substituent Do you expect the Ahydroi to increase or decrease with the electron-attracting power of the substituent How do you expect K, the equilibrium constant for acetal formation, to vary with the nature of the substituent ... 

EXAMPLE 20.7. Calculate the hydronium ion concentration of a 0.200 M solution of acetic acid, using the equilibrium constant of Example 20.6. 

The concept of using the base 10 logarithm to express the magnitude is a widespread practice today. Equilibrium constants of chemical reactions are often noted or compared as pK values where pK = — log 10 (magnitude of equilibrium constant). For example, the extent of dissociation of acetic acid, the acid in vinegar, is quantified by an equilibrium constant of 1.8 x 10-5. Here, then, pK = — log,o (1.8 x 1(T5) = 4.74. 

The first indication that A-acyloxy-A-alkoxyamidcs reacted by an acid-catalysed process came from preliminary H NMR investigations in a homogeneous D20/ CD3CN mixture, which indicated that A-acetoxy-A-butoxybenzamide 25c reacted slowly in aqueous acetonitrile by an autocatalytic process according to Scheme 4 (.k is the unimolecular or pseudo unimolecular rate constant, K the dissociation constant of acetic acid and K the pre-equilibrium constant for protonation of 25c).38... 

An enzymatic reaction intermediate formed by phospho-ryl transfer to a carboxyl group on an enzyme. Acyl-phosphates are structurally analogous to acid anhydrides (R—CO —O —CO—R ), and they are thermodynamically less stable than either of the two phosphoanhydride bonds in ATP. This is evident by the fact that the acetate kinase reaction (ADP + acetyl-phosphate = ATP + acetate) favors ATP formation with an equilibrium constant of about 3,000. Acetyl-phosphate can be chemically synthesized by reacting orthophosphate with acetic anhydride. 

In acetic acid it is possible to measure separately the equilibrium constant of proton transfer to form an ion pair and the constant for dissociation of ion pairs to form free ions. [I. M. Kolthoff and S. Bruckenstein, J. Amer. Chem. Soc., 78, I (1956) S. Bruckenstein and I. M. Kolthoff, J. Amer. Chem. Soc., 78, 10 (1956)]. G. W. Geska and E. Grunwald, J. Amer. Chem. Soc., 89, 1371, 1377 (1967) applied this technique to a number of substituted anilines and concluded that the equilibrium constant of the ionization step, rather than the overall acid dissociation constant, is the quantity that should be considered in discussions of effects of structural changes on acidity. 

The effect of water on the equilibrium constant for the reaction of 1 mol of ethanol, 1 mol of acetic acid, and 23 moles of water has been investigated. This mixture has an equilibrium constant of 3.56, compared with 3.79 for the reaction with anhydrous materials (7,37). 

Equilibrium constants between acetals and the corresponding enol ethers (60) were calculated as the ratios of the lyonium catalytic rate constants for the two reverse reactions (isotope exchange or bromination, on the one hand, and alcoholysis on the other). The data obtained (Kankaanpera et al., 1973 ... 

Don t be confused by this statement Acetal formation and hydrolysis are invariably carried out under thermodynamic control—what we mean here is that the equilibrium constant for acetal hydrolysis, which is a measure of rate of hydrolysis divided by rate of formation, turns out to be small because the rate of formation is large. 

To provide the relationships which enable calculation of a certain property (physical quantity) from other measured data. Examples (i) evaluation of the equilibrium constant of the dissociation of acetic acid from pH measurements, (ii) evaluation of surface reaction enthalpy from the temperature jump in a calorimeter, or from the temperature dependency of the point of zero charge. 

The assumption that all undissociated acetic acid in aqueous solutions is present only in a single form is erroneous because the equilibrium between the ionogen- and pseudo-forms is disregarded. Hence the true dissociation constant of acetic acid is much larger than that usually employed. 

The above reaction is called hydrolysis. As a result of this process, a solution of sodium acetate in water is basic because an excess of OH is produced. Note that the reaction (77-5) is the sum of the reactions (77-i) and 17-7). The equilibrium constants of these three reactions must then be related as follows (see Problem 16.14) ... 

Menschutkin made a comparative study of the relative rates of esterification and the equilibrium constants of a large number of acids and alcohols. He found striking differences among primary, secondary, and tertiary dcohols, both as to the rates and as to the limits of esterification. Table 12-1 gives some of his results for acetic miid heated to 155 C with equivalent amounts of various alcohols. 

CoMFA and related 3D QSAR approaches have been applied to correlate various physicochemical properties. Equilibrium constants of the hydration of carbonyl groups could be explained by a combination of C=0 bond order, steric, and electrostatic fields [1005]. 3D QSAR studies that correlate a, inductive, and resonance parameters of benzoic acids [1015, 1016] as well as pKg values ofclonidine analogs [1017] show that a H " field precisely describes such electronic parameters, e.g. (Jm.p of benzoic acids (n = 49 rpir = 0.976 snr = 0.082 Spress = 0.093). Steric parameters of benzoic acids, like surface area and van der Waals volume can be described by a steric field alone, while values of acetic acid methyl esters need a combination of both steric and electrostatic fields (n = 21 rpix = 0.984 Sfit = 0.133 SpREss = 0.209) [1016]. 

Here is an example of what has been said. With the help of the Table in Sect. A2.1 of the Appendix we will determine the acidity constant of acetic acid (CH3COOH), abbreviated to HAc, in an aqueous solution, i.e., the equilibrium constant for the dissociation of acetic acid in water... 

Conversely, Eq. (21.42) can be used to determine the degree of dissociation a of a weak electrolyte at a given concentration c by measuring the molar conductivity. Moreover, with the help of Eq. (21.41), the equilibrium constant of the substance becomes accessible. However, for these calculations we need the limiting molar conductivity A . This quantity is very difficult to find experimentally because the steep rise of the A at low concentrations makes an extrapolation to infinite dilution very uncertain. The law of independent migration of ions [Eq. (21.35)] offers a way out. hi the case of infinite dilution, the limiting molar conductivity of acetic acid is the sum of the contributions of cation and anion ... 

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