Ionization constant of benzoic acid

Strong et al. [8] have determined the ionization constant of benzoic acid in H2O as a function of ternperamre by conductance methods. Their data are listed in Table 20.3, with the pK based on a hypothetical standard state of 1 mol dm. Temperature was controlled to +0.002°C. 

TABLE 20.3. Ionization Constants of Benzoic Acid as a Function of Temperature... 

More important and more widely applicable criteria of protonation sites are available in the effects of more distant substituents, in particular meta and para in benzene derivatives, which are as a rule purely electronic in origin. Exceptionally, they may be complicated by steric effects (e.g., in polysubstituted derivatives). The most widely useful approach is that of Hammett (1940), who suggested that the effects of substituents on the ionization constants of benzoic acid may be taken as a measure of their effectiveness in other systems involving other reaction centres and in reactions other than acid-base equilibria. He thus defined substituent constants, a, by the equation... 

The aromatic acids resemble closely in chemical properties the acids derived from the paraffins, and yield similar derivatives. On account of the negative nature of the phenyl radical they are stronger than the fatty acids. The ionization constant of benzoic acid, CgHb.COOH, for example, is 0.0060, while that of acetic acid is 0.0018. 

Chem. Soc., 561-568 (1933) (4.190) Brockman EG and Kilpatrick M, The thermodynamic dissociation constant of benzoic acid from conductance measurements,/ACS, 56, 1483-1486 (1934) (4.199) Saxton B and Meier HE, The ionization constants of benzoic acid and die three monochlorbenzoic acids, at 25°, from conductance measurements, JACS, 56, 1918-1921 (1934) (4.200) Dippy JFJ and Williams ER, Chemical constitution and die dissociation constants of monocarboxylic acids. II,/CS, 1888-1892 (1934) (4.203) Jeffery GH and Vogel AI, Thermodynamic dissociation constant of benzoic acid at 25° from conductivity measurements.P/iiZ. Mag., 18, 901-909 (1934) (4.196). 

Robinson RA and Stokes RH, Electrolyte Solutions, Revised Edn., Butterwordis (Lond.), Appendix 12.1. Cited Briscoe HT and Peake JS, Measurements of the ionization constant of benzoic acid using silver chloride electrodes, /. Phys. Chem., 42,637-640 (1938) (4.218) ... 

Hammett s reaction constants p for various aromatic parent compounds were obtained by letting p = 1 for the ionization constant of benzoic acid in an aqueous solution. Based on this assumption, values of p have been extensively tabulated (Jaffe, 1953 McDaniels and Brown, 1958 Wells, 1968 Shorter, 1982). His constants tr for different substituents to these aromatic parent compounds are also included in these references, along with exact procedures for finding the correct values. 

The variation of the electronic effects of amino substituents on the ionization constants of benzoic acids has been shown to be the result of steric inhibition of resonance, originating in the presence of adjacent groups . Another interesting phenomenon is the fact that the effect of the NHg group in position 4 of biphenylcarboxylic acid is transmitted in a predictable manner, but it is quantitatively less than in benzoic acids . 

Pronk JT, Liem K, Bos P, Kuenen JG (1991) Energy transduction by anaerobic ferric iron respiration in Thibacillus ferrooxidans. Appl Environ Microbiol 57 2063-2068 Read AJ (1981) Ionization constants of benzoic acid from 25 to 250 C and to 2000 bar. J Solution Chem 10 437-450... 

Log-log plot of ionization constants of benzoic and phenyl-acetic acids in water at 25°. (From Physical Organic Chemistry by J. S. Hine. Copyright 1962. Used with permission of McGraw-Hill Book Company.)... 

Probably the most reliable value for the first ionization constant of isophthalic acid is that determined by Ang in 1958103. When the pKa value of 3.70 (water, 25 °C) is combined with a value of 4.205 for benzoic acid and a statistical correction is applied for the presence of two ionizable groups, the value of 0.204 for am is obtained. It is generally believed among physical organic chemists that CO2H should differ but little from CC Me in net electron-attracting ability and therefore the above am value is anomalously low by at least 0.1 unit (see Section IV.A). The explanation of this is not clear. There does not appear to be any information available for the first ionization of terephthalic acid of reliability comparable with that for isophthalic acid. 

If we take a series of compounds which are related by structural substitutions, such as benzene derivatives, then we can assign the effects of the structural changes to the ratio KBJKB. if we select this property as a standard. Such a system of correlation was first proposed by Hammett and revised and extended by Jaff " to account for the effects of meta and para substituents on the reactivity of benzene derivatives. For convenience, the ionization constant of benzoic acia queous solution at 25°C was chosen as standard and for each meta or para substituent a, a value of ionization constant for benzoic acid and Ka the ionization constant of the corresponding, substituted benzoic acid. 

The ionization of monoprotic acids and bases and hence their solubility and absorption is dependent on their pK, the pH at which the drug is 50% ionized. First developed by Hammett and published in 1940, the relation between the dissociation constants of benzoic acid derivatives and the longer range electronic (inductive, mesomeric and field) effects is linear and additive. 

Lines 21-40. Physical data. The usual crystalline shape, density (note two values reported,), sublimation notation, boiling point data, and so on. K at 25" is the ionization constant of the acid the pH ofthe saturated solution (2.8 at 25°C) is given. The solubility data (Soly) are very complete, including water solutions at various temperatures, a bit about the phase diagram of the compound, and solubility in other solvents. Note that numerical data are given where possible. Lines 41-67. Properties of some slots of benzoic acid. 

In application of the Hammett correlation a value of unity is commonly assigned to the ionization constant for benzoic acid in aqueous solution at 25°C. From this we can write a relationship for a... 

In 1934 Saxton and Meier [1] published a report in which they described measurements of the acid dissociation constants of benzoic acid and its three monochloro derivatives. The results showed an interesting pattern all of the chloro-derivatives were more acidic than the parent compound, with the ortlio-substituted compound most acidic and the meta-chloro derivative least acidic. However, the authors closed their report with the lament that at present, there is no adequate theoretical method of calculating the ionization of a weak electrolyte from its structure. ... 

At this point, equation 6.85 is one equation with four unknowns. Therefore, it is necessary to establish a reference system with defined values of a and p before using the relationship for other reactions. Hammett adopted the ionization of benzoic acid derivatives in water as the reference reaction and arbitrarily set the value of p for that equilibrium to 1.0. Therefore, Kq is the ionization constant for benzoic acid itself. Hydrogen is the substituent in the "unsubstituted" compoxmd, so the substituent constant cr for hydrogen was set equal to 0. Then the a values for another substituent could be determined from the value of K for the substituted benzoic acid bearing that substituent from the relationship... 

Table 19 3 lists the ionization constants of some substituted benzoic acids The largest effects are observed when strongly electron withdrawing substituents are ortho to the carboxyl group An o nitro substituent for example increases the acidity of benzoic acid 100 fold Substituent effects are small at positions meta and para to the carboxyl group In those cases the values are clustered m the range 3 5-4 5... 

The numerical values of the terms a and p are defined by specifying the ionization of benzoic acids as the standard reaction to which the reaction constant p = 1 is assigned. The substituent constant, a, can then be determined for a series of substituent groups by measurement of the acid dissociation constant of the substituted benzoic acids. The a values so defined are used in the correlation of other reaction series, and the p values of the reactions are thus determined. The relationship between Eqs. (4.12) and (4.14) is evident when the Hammett equation is expressed in terms of fiee energy. For the standard reaction, o%K/Kq = ap. Thus,... 

Reactions that occur with the development of an electron deficiency, such as aromatic electrophilic substitutions, are best correlated by substituent constants based on a more appropriate defining reaction than the ionization of benzoic acids. Brown and Okamoto adopted the rates of solvolysis of substituted phenyldimeth-ylcarbinyl chlorides (r-cumyl chlorides) in 90% aqueous acetone at 25°C to define electrophilic substituent constants symbolized o-. Their procedure was to establish a conventional Hammett plot of log (.k/k°) against (t for 16 /wcra-substituted r-cumyl chlorides, because meta substituents cannot undergo significant direct resonance interaction with the reaction site. The resulting p value of —4.54 was then used in a modified Hammett equation. 

Next we turn to the magnitudes of the p constants. Evidently if p = 0, there is no substituent effect on reactivity. Moreover because p = -I-1.000 by definition for the aqueous ionization of benzoic acids, we have a scale calibration of sorts. Wiberg gives examples of p as a measure of the extent of charge development in the transition state. McLennan" has pointed out that p values must first be adjusted for the transmission factor before they can be taken as measures of charge devel-... 

The ortho effect may consist of several components. The normal electronic effect may receive contributions from inductive and resonance factors, just as with tneta and para substituents. There may also be a proximity or field electronic effect that operates directly between the substituent and the reaction site. In addition there may exist a true steric effect, as a result of the space-filling nature of the substituent (itself ultimately an electronic effect). Finally it is possible that non-covalent interactions, such as hydrogen bonding or charge transfer, may take place. The role of the solvent in both the initial state and the transition state may be different in the presence of ortho substitution. Many attempts have been made to separate these several effects. For example. Farthing and Nam defined an ortho substituent constant in the usual way by = log (K/K ) for the ionization of benzoic acids, postulating that includes both electronic and steric components. They assumed that the electronic portion of the ortho effect is identical to the para effect, writing CTe = o-p, and that the steric component is equal to the difference between the total effect and the electronic effect, or cts = cr — cte- They then used a multiple LFER to correlate data for orrAo-substituted reactants. 

LFER. Consider the rate of base hydrolysis of a series of ethyl benzoates given by d[C2HsOH]/dt = LfdALQ COOQHsHOH"]. Show that a plot of log Jfcx versus log Ka, where Ka is the ionization constant of the parent benzoic acid, should be linear, and relate its slope to that given by a conventional Hammett plot of log k versus a. 

An example of a reaction series in which large deviations are shown by — R para-substituents is provided by the rate constants for the solvolysis of substituted t-cumyl chlorides, ArCMe2Cl54. This reaction follows an SN1 mechanism, with intermediate formation of the cation ArCMe2 +. A —R para-substituent such as OMe may stabilize the activated complex, which resembles the carbocation-chloride ion pair, through delocalization involving structure 21. Such delocalization will clearly be more pronounced than in the species involved in the ionization of p-methoxybenzoic acid, which has a reaction center of feeble + R type (22). The effective a value for p-OMe in the solvolysis of t-cumyl chloride is thus — 0.78, compared with the value of — 0.27 based on the ionization of benzoic acids. 

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