Ionization of Substituted Benzoic Acids

SAMPLE SOLUTION (a) Think of the two compounds as substituted derivatives of acetic acid. A fert-butyl group is slightly electron-releasing and has only a modest effect on acidity. The compound (CH3)3CCH2C02H is expected to have an acid strength similar to that of acetic acid. A trimethylammonium substituent, on the 

Another proposal advanced to explain the acid-strengthening effect of polar substituents holds that the electron-withdrawing effect is transmitted through the water molecules that surround the carboxylate ion rather than through successive polarization of a bonds. This is referred to as a field effect. Both field and inductive contributions to the polar effect tend to operate in the same direction, and it is believed that both are important. 

It is a curious fact that substituents affect the entropy of ionization more than they do the enthalpy term in the expression 

A considerable body of data is available on the acidity of substituted benzoic acids. Benzoic acid itself is a somewhat stronger acid than acetic acid. Its carboxyl group is attached to an. yp -hybridized carbon and ionizes to a greater extent than one that is attached to an. 577 -hybridized carbon. Remember, carbon becomes more electron-withdrawing as its. y character increases. 

Oxalic acid is poisonous and occurs naturally in a number of plants including sorrel and begonia. It is a good idea to keep houseplants out of the reach of small children, who might be tempted to eat the leaves or berries. 

PROBLEM 19.5 What is the most acidic neutral molecule characterized by the formula C3HXO2 I 

Closely related to the inductive effect, and operating in the same direction, is the field effect. In the field effect the electronegativity of a substituent is communicated, not by successive polarization of bonds but through the medium, usually the solvent. A substituent in a molecule polarizes surrounding solvent molecules and this polarization is transmitted through other solvent molecules to the remote site. 

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The best-known equation of the type mentioned is, of course, Hammett s equation. It correlates, with considerable precision, rate and equilibrium constants for a large number of reactions occurring in the side chains of m- and p-substituted aromatic compounds, but fails badly for electrophilic substitution into the aromatic ring (except at wi-positions) and for certain reactions in side chains in which there is considerable mesomeric interaction between the side chain and the ring during the course of reaction. This failure arises because Hammett s original model reaction (the ionization of substituted benzoic acids) does not take account of the direct resonance interactions between a substituent and the site of reaction. This sort of interaction in the electrophilic substitutions of anisole is depicted in the following resonance structures, which show the transition state to be stabilized by direct resonance with the substituent ... 

For many groups NX X2 there are values of am and/or ap based on the ionization of substituted benzoic acids in 1 1 EtOH-H20 or other aqueous organic solvents230 237 238. The significance of some of these values is doubtful, because corrections for tautomerism should have been applied. 

Shorter, J., Values of am and op based on the ionization of substituted benzoic acids in water at 25°C , Pure Appl. Chem., 66, 2451-2468 (1994). 

The 2-furyl group has been characterized as a substituent in classical Hammett-type studies involving the ionization of substituted benzoic acids, the ionization of substituted phenols, and the solvolysis of 1-arylethyl acetates. The a values found (<7 +0.06, dp +0.02 +0.11, [Pg.402]

McDaniel, D.H., Brown, H.C. (1958) Extended table of Hammett substituent constants based on the ionization of substituted benzoic acids. J. Org. Chem. 23, 420. 

The reason for employing alternative treatments to ortho-substituted aromatic molecules is that changes in rate or ionization constants mediated by meta or para substituents are mostly changes in or because substitution does not affect AS or AS". Ortho substituents affect both enthalpy and entropy the effect on entropy is noteworthy because entropy is highly sensitive to changes in the size of reagents and substituents as well as degree of solvation. Bolton et al. examined the ionization of substituted benzoic acids and measured accurate values for AG, and A S... 

McDaniel, D.H. and Brown, H.C. (1958). An Extended Table of Hammett Substituent Constants Based on the Ionization of Substituted Benzoic Acids. J.Org.Chem., 23,420-427. 

Hammett LFER. An LFER with broad implications was developed in the 1930s by Hammett (1937) to explain substituent effects on reactions of meta- or para-substituted benzene compounds. Hammett found that hydrolysis of substituted ethyl benzoates, ionization of substituted benzoic acids, and many related reactions are affected by nature and position of aromatic substituents, and that the effect could be predicted quantitatively. Using rate and equilibrium constants for ionization of benzoic acid as references, Hammett defined the LFER ... 

The a values are always those determined from the ionization of substituted benzoic acids, but the p values are different for each substrate type and also for each reaction type because they reflect the sensitivity of the substrate to the electronic effect of the substituent. With p > 1, the ionization or reaction rate is more sensitive to substituent s electronic effects than is the ionization of benzoic acids. With p < 1, electron-withdrawing groups will increase the equilibrium constant or rate, but less than in benzoic acid dissociation. A negative p indicates that electron-donating groups increase the reaction constant. In consideration of reaction rates, a small r often indicates that the reaction involves either radical intermediates or some other pathway that requires little charge separation. 

The symbol k0 is an intercept term that is equal to k for the parent (unsubstituted) compound. The reaction constant p depends on reaction conditions such as solvent and temperature, representing the susceptibility of the reaction to environmental effects. In contrast, the substituent constant up is a measure of the electronic effect of replacing hydrogen by a given substituent, and is assumed to be independent of the reaction conditions. By defining p = 1 for the room temperature ionization of substituted benzoic acids in water, Hammett calculated op values directly for 13 substituents, and predicted those for a further 17 substituents by applying the primary rrP values to other reactions. Later work increased the number of aP values to 44 and the number of reaction series to 51 [35]. 

Icq and Kq are, respectively, the rate and equilibrium constants for the unsubstituted parent compound that is, the substituent is H. The standard substituent H is assigned a cr value of 0. The standard reaction, assigned a value of p = 1.0, is the ionization of substituted benzoic acids. 

Substituents and Acid Strength 742 ionization of Substituted Benzoic Acids 744... 

Figure 1 Linear free energy relationship for basic hydrolysis of substituted ethyl benzoates and ionization of substituted benzoic acids. k values (1 mol" s", 85% ethanol w/w, 25°C) ...

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