Acidity delocalization effects

There are almost no studies of substituent effects on additions to carbon-carbon triple bonds extant in the literature. Bowden and Price (208) have reported a correlation of rates of addition of hydrogen bromide to 3-substituted propiolic acids with the Hammett equation using the Op constants. Unfortunately, there are only three substituents in the set. Sufficient data are available for a single set of 1,3-dipolar cycloaddition. The set studied is shown in Table XXXIII, and the results of the correlation are in Table XXXIV. The correlation was significant the delocalized effect is predominant in this set. 

Delocalization of charge in the conjugate base anion contributes to stabilization of the anion, and thus ionization of the acid is enhanced. Delocalization effects in bases are more likely to stabilize the base... 

Overview of Acid and Base Functional Groups Inductive Effects Delocalization Effects Proximity Effects... 

Picric acid is a strong acid with a pKa of 0.42. The acid-strengthening effect of the three nitro groups, which all can delocalize the election pair of the conjugate base, is quite large. 

The intrinsic delocalized (resonance) electrical-effect parameter. It represents the delocalized electrical effect in a system with zero electronic demand. cTg The electronic demand sensitivity parameter. It adjusts the delocalized effect of a group to meet the electronic demand of the system. cTj) A composite delocalized electrical-effect parameter which is a function of and cTg.Examples of constants are the ctr and ctr constants.The ctr constants, where k designates the value of the electronic demand rj, are also examples of Ojy constants. ctr a composite delocalized electrical-effect parameter of the ctd type with rj equal to 0.380. It is derived from 4-substituted benzoic acid pK values. 

Both the abundances and the values of acidic functional groups in humic substances are constrained and controlled by the compositional and structural properties of these substances. It is therefore important to be familiar with the principal structural properties that determine the acidities of organic acids. In this section of the chapter, compositional constraints on the abundances of acidic functional groups and a general discussion of statistical, electrostatic, and delocalization effects on the acidities of organic acids are presented. Specific applications to humic substances are given in the following sections of the chapter. 

The literature on proton binding by humic substances indicates that statistical effects, delocalization effects, and, probably most importantly, the effects of dipolar groups on the acidity of a functional group have generally been ignored. An attempt has been made in this chapter to provide the reader with a rather detailed discussion of the nature of substituent effects on the dissociation constants of organic acids. Statistical, electrostatic, and delocalization effects have been treated separately. 

He and his co-workers also observed that the F chemical shifts of 3-substituted fluorobenzenes are predominantly a function of the localized effect, and advocated their use in the determination of ff/ values (24c,24d). Many authors have determined localized effect parameters in this manner. This work of Taft and his group has been criticized by Dewar and his co-workers (24c,24/). Charton (22) has observed that a correlation of aj values obtained by Taft and his coworkers (24c) with those determined from acetic acid pX<,s shows that they are on a different scales. In view of the fact that 3-substituted fluorobenzenes are not entirely free From the delocalized effect, and for the other possible objections cited above, the F shielding parameters of this system should not be used to determine reliable values of the localized effect parameter. 

Suitable model systems for the estimation of Cl should be free of the delocalized effect and should have a geometry closely resembling that of the meta-or para-substituted benzoic acids. There are two model systems that meet these requirements. They are the 3- and 4-substituted methyl benzoic acids, VI and VII and the 3-substituted adamantane-1-carboxylic acids, VIII, and 4-substituted bicyclo[2.2.2]octane-l-carboxylic acids(I). In both model systems, the delocalized effect is unimportant. The geometry of the substituted methylbenzoic acids is only approximately similar to that of the corresponding benzoic acids. The geometry of I is very similar to that of 4-substituted benzoic acids, as was noted previously that of VIII is fairly close to that of 3-substituted benzoic acids. 

Electron Delocalization in the Conjugate Base With a of —1 4 nitnc acid is almost completely ionized m water If we look at the Lewis structure of nitric acid m light of what we have said about inductive effects we can see why The N atom m nitric acid IS not only electronegative m its own right but bears a formal charge of +1 which enhances its ability to attract electrons away from the —OH group... 

Carboxylic acids are weak acids and m the absence of electron attracting substituents have s of approximately 5 Carboxylic acids are much stronger acids than alcohols because of the electron withdrawing power of the carbonyl group (inductive effect) and its ability to delocalize negative charge m the carboxylate anion (resonance effect)... 

The results obtained for 1-phenylpyrazole (32) and its conjugate acid (34) are consistent with those of Minkin. The bond order between thd two rings decreases by protonation (from 0.341 to 0.241) and this is in agreement with the expected effect of the N-2 positive charge on the delocalization of the lone pair on N-1 over the phenyl ring. 

The triarylmethyl cations are particularly stable because of the conjugation with the aryl groups, which delocalizes the positive charge. Because of their stability and ease of generation, the triarylmethyl cations have been the subject of studies aimed at determining the effect of substituents on carbocation stability. Many of these studies used the characteristic UV absorption spectra of the cations to determine their concentration. In acidic solution, equilibrium is established between triarylearbinols and the corresponding carbocations. 

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