Function, acidity

Following the pioneering work of Hammett and coworkers (1932,1970), in establishing the acidity fnnctions, // , H, and so on, it became apparent that organic bases of 

It wonld, of course, be desirable to define a single scale that measures proton activity, such that in dilute aqueous solution, it reduces to the accepted scale pH = -logjo ). Writing for a particular base A in acid solution the equilibrium 

P ah 3T0 known, lies in the behavior of the last term, containing the activity coefficients. Hammett s method depends on the assumption that, for a series of progressively weaker bases used to ascend the scale by overlapping their ranges of measurable ionization ratios, the activity coefficient ratios for two bases A and B in the same solution are equal. (Cox and Yates (1983) describe this assumption as the zero-order approximation. ) For the second base, B, 

To extend this function to higher levels of acidity requires a succession of suitable weak bases. Each should have a p value close enough to that of its predecessor that an overlapping range of acidity exists wherein the ratios for both can be measured spectroscopically. It has been determined over a large range of acidities, from dilute aqueous solutions into the superacid range. 

The foregoing assumes that A and B are of similar structure. If they are not, the assumption fails. Consider A and B as representing bases of two different structural classes. Most obviously, if the charges on the bases are different, the interionic parts of the activity coefficient ratios will differ. Using Davies s Equation 2.23 as a rough guide, if the interionic part of the activity coefficient of a univalent ion is given by yf, then log(/p=z log(yP, and 

Considering the limited applicability of the pH scale, a quantitative scale is needed to express the acidity of more concentrated or nonaqueous solutions. 

Bearing in mind that the proton is solvated (AH2+) and that AH is the solvent, the equilibrium can be written as in Eq. (1.11). 

The corresponding thermodynamic equilibrium constant is Khi[i, which is expressed as in Eq. (1.12), in which a is the activity, C the concentration, and/the activity coefficient. 

Because the firstratio represents the degree of protonation, Hammett and Deyrup8,9 defined the acidity function H0 by Eq. (1.14). 

Equation (1.14) can be written for further discussion in the more usual form of Eq. (1.15). 

Most organic compounds are bases, that is, they are capable of accepting a proton. The best-studied organic bases are the moderately strong ones, which will receive a proton in dilute aqueous solutions amines are the most important examples. The pKa value of the protonated base, referred to the infinitely dilute aqueous solution, is the usual measure of base strength, and the pH of the solution is a quantitative measure of solvent acidity, or ability to transfer a proton. 

Consider a neutral base B of such strength that it can be protonated in dilute aqueous solution in the acidic range, say pH 1-2. In the conventional manner the acid dissociation constant /ibh + is defined. 

Select now a second neutral indicator base C that is weaker than B by roughly an order of magnitude thus, a solvent can be found of such acidity that a significant fraction of both B and C will be protonated, but this will no longer be a dilute aqueous solution, so the individual activity coefficients will in general deviate from unity. For this solution containing low concentrations of both B and C, 

If B and C are not only of the same charge type but also of the same structural type, it is reasonable to postulate that the ratio/c/BH + //cH+/B will not be markedly different from unity. Let us make this cancellation assumption then Eq. (8-84) becomes 

Because the concentration ratios can be measured spectrophotometrically (in separate solutions usually) and pA BH+ is known, the unknown p Tch+ is obtained. 

As indicated by equation 7.14, we can represent the protonation equilibrium for a base by the relationship 

H represents the protonated solvent. Letting the symbol I stand for the ratio of the concentrations of protonated to nonprotonated base, ([BH + ]/[B]), we may write 

If [BH ] is equal to [B], then log I is 0, and pA sH + is equal to pH. In most acidic solutions, however, the activity coefficients cannot be ignored. Therefore, the acidity function Hq is defined as 

Equation 7.30 provides a way to determine Hq values for highly acidic solutions. We begin with a base having a known pATbh+ in aqueous solution. Then the ratio I is determined spectroscopically in a series of solutions having slightly different acidities. The protonated and nonprotonated bases show different absorption spectra, so UV-vis spectroscopy indicates the concentration of each species as a function of solution composition. The Hq values 

Correlation of log ([BH + ]/[B]) with percent H2SO4 for a series of nitroanilines. (Reproduced from reference 80. The figure also includes data from reference 79. The compounds are (a) 2-nitroani-line, (b) 4-chloro-2-nitroaniline, (c) 2,5-dichloro-4-nitroaniline, (d) 2-chloro-6-nitroaniline, (e) 2,6-di-chloro-4-nitroaniline, (f) 2,4-dini-troaniline, (g) 2,5-dinitroaniline, (h)4-chloro-2,6-dinitroaniline, (i) 

Secondary salt effects on the decomposition of diazoacetic ester in 0.05 M acetic acid, at 15 °C 

HjNH + H+ QHjNHj The equilibrium constant of such reactions is 

If [BH ] and [B] can be distinguished spectroscopically, as is the case for the anilines, and if K can be measured in more dilute solutions, then all the quantities in the left-hand side of eq. (13.67) are experimentally accessible. The right-hand side, where a j+ is the activity of H+, is defined as the acidity function Hq 

Acid-Base Catalysis and Proton-Transfer Reactions 

This acidity function can be measured in any acidic solution by introducing a suitable indicator and measuring the concentration of [BH+] and [B], For example, the mechanism 

---

The Hq acidity function relates to indicators ionising according to the different scheme B + H+ BH+... 

Hji function. A better correlation, up to nearly 89% sulphuric acid, is obtained by comparing the results at 25 °C with the acidity function — (/f + log % q). si, 42a, 43a these comparisons a straight line of approximately unit slope is obtained (fig. 2.4), although for the nitration of benzene in acidities greater than 68% sulphuric acid, the slope becomes i-20 (fig. 2.5). 

The correlations of rates with acidity functions provide a convenient means of treatii results, and their uses will frequently be illustrated. However, their status is empirical, for whilst the acidity dependence of nitration becomes less steep with increasing temperature, the slope of... 

If the concentration of effective aromatic species does vary with acidity, as sometimes happens if the compound is substantially proto-nated, then the acidity-dependence of the rate will be less steep than usual, because the concentration of the active free base diminishes significantly with increasing acidity. This situation has been observed in certain cases ( 8.2). The fall in the concentration of the active species can be allowed for from a knowledge of its pK and the acidity function which, for the particular compound, gives the best measure of the acidity of the medium. Then the corrected acidity-dependence of the rate resembles that observed with compounds the concentration of which does not change significantly with acidity. The nitration of minor species is discussed later ( 8.2). 

The ionisation ratio (/ = [SH+]/[S]) can be calculated from a knowledge of the acidity function (hj.) followed by the substrate, and the acidity constant of the conjugate acid. Thus, when I p i ... 

Further problems arise if measurements of the rate of nitration have been made at temperatures other than 25 °C under these circumstances two procedures are feasible. The first is discussed in 8.2.2 below. In the second the rate profile for the compound imder investigation is corrected to 25 °C by use of the Arrhenius parameters, and then further corrected for protonation to give the calculated value of logio/i fb. at 25 °C, and thus the calculated rate profile for the free base at 25 °C. The obvious disadvantage is the inaccuracy which arises from the Arrhenius extrapolation, and the fact that, as mentioned above, it is not always known which acidity functions are appropriate. 

Acidity function plots have been used in another way. In 65-90 % sulphuric acid the concentration of the nitronium ion is not equal to the concentration of nitric acid the quantity... 

In applying this criterion, obs. must be compared with calc, for the same temperature. In general this entails knowledge of the temperature dependence of the relevant acidity function and of the ionisation constant. The latter factor has sometimes been allowed for (as in the calculation of calc, for the nitration of 2,4,6-trimethylpyridine in 98 % sulphuric acid at 80 °C) by using the approximate relationship, -d pKf) dT = (p, -o-9)/T. 

These equations, relating to oi,s, and E t,g to Egy, show that 3od can be calculated for a reaction proceeding through the equilibrium concentration of a free base if the thermodynamic quantities relating to the ionisation of the base, and the appropriate acidity function and its temperature coefficient are known (or alternatively, if the ionisation ratio and its temperature coefficient are known under the appropriate conditions for the base. )... 

With the oxides which are nitrated as the cations the difficulties are much less serious for the use of an acidity function is not involved. Comparison of 2,6-dimethoxy- and 3,5-dimethoxy-pyridine i-oxide with wt-dimethoxybenzene (which is nitrated at the encounter rate)... 

Another method for deallylation of ally esters is the transfer of the allyl group to reactive nucleophiles. Amines such as morpholine are used[415-417], Potassium salts of higher carboxylic acids are used as an accepter of the allyl group[418]. The method is applied to the protection and deprotection of the acid function in rather unstable /f-lactam 664[419,420]. 

The acid function of an aliphatic chain bonded to a thiazole ring can be esterified. The corresponding acid chloride can also be prepared by the action of thionyl chloride, though the reaction is often accompanied by secondary reactions and gives poor yields (49, 74). 

Converting aldehydes and ketones to cyanohydrins is of synthetic value for two reasons (1) a new carbon-carbon bond is formed and (2) the cyano group in the prod uct can be converted to a carboxylic acid function (CO2H) by hydrolysis (to be discussed in Section 19 12) or to an amine of the type CH2NH2 by reduction (to be discussed m Section 22 9)... 

Hydroxy acids compounds that contain both a hydroxyl and a carboxylic acid function have the capacity to form cyclic esters called lactones This intramolecular esterification takes place spontaneously when the ring that is formed is five or six membered Lac tones that contain a five membered cyclic ester are referred to as 7 lactones, their six membered analogs are known as 8 lactones... 

Ammo acids are carboxylic acids that contain an amine function An amide bond between the carboxylic acid function of one ammo acid and the ammo nitrogen of another is called a peptide bond... 

Ammo acids are classified as a p 7 and so on according to the location of the amine group on the carbon chain that contains the carboxylic acid function... 

In the Strecker synthesis an aldehyde is converted to an a ammo acid with one more carbon atom by a two stage procedure m which an a ammo nitrile is an mterme diate The a ammo nitrile is formed by reaction of the aldehyde with ammonia or an ammonium salt and a source of cyanide ion Hydrolysis of the nitrile group to a car boxylic acid function completes the synthesis... 

Ammo acids undergo reactions characteristic of both their amine and carboxylic acid functional groups Acylation is a typical reaction of the ammo group... 

Aldaric acid (Section 25 19) Carbohydrate in which car boxyhc acid functions are present at both ends of the chain Aldanc acids are typically prepared by oxidation of aldoses with nitnc acid... 

Cation exchange resins — gel type—strongly acidic—sulfonic acid functionality ... 

Cation exchange resin— -macroreticular type- — sulfonic acid functionality... 

Since this intermediate involves an additional equivalent of acid functional groups, the rate law for the disappearance of A groups becomes... 

Acid-C t lyzed Chemistry. Acid-catalyzed reactions form the basis for essentially all chemically amplified resist systems for microlithography appHcations (61). These reactions can be generally classified as either cross-linking (photopolymerization) or deprotection reactions. The latter are used to unmask acidic functionality such as phenohc or pendent carboxyhc acid groups, and thus lend themselves to positive tone resist apphcations. Acid-catalyzed polymer cross-linking and photopolymerization reactions, on the other hand, find appHcation in negative tone resist systems. Representative examples of each type of chemistry are Hsted below. 

Carboxylic Acid Functional Group Reactions. Polymerization is avoided by conducting the desired reaction under mild conditions and in the presence of polymeriza tion inhibitors. AcryUc acid undergoes the reactions of carboxyUc acids and can be easily converted to salts, acryhc anhydride, acryloyl chloride, and esters (16—17). 

Direct, acid catalyzed esterification of acryhc acid is the main route for the manufacture of higher alkyl esters. The most important higher alkyl acrylate is 2-ethyIhexyi acrylate prepared from the available 0x0 alcohol 2-ethyl-1-hexanol (see Alcohols, higher aliphatic). The most common catalysts are sulfuric or toluenesulfonic acid and sulfonic acid functional cation-exchange resins. Solvents are used as entraining agents for the removal of water of reaction. The product is washed with base to remove unreacted acryhc acid and catalyst and then purified by distillation. The esters are obtained in 80—90% yield and in exceUent purity. 

Eats and oils from a number of animal and vegetable sources are the feedstocks for the manufacture of natural higher alcohols. These materials consist of triglycerides glycerol esterified with three moles of a fatty acid. The alcohol is manufactured by reduction of the fatty acid functional group. A small amount of natural alcohol is also obtained commercially by saponification of natural wax esters of the higher alcohols, such as wool grease. 

For anhydrous hydrogen fluoride, the Hammett acidity function Hq approaches —11. The high negative value of Hq shows anhydrous hydrogen fluoride to be in the class of superacids. Addition of antimony pentafluoride to make a 3 Af solution in anhydrous hydrogen fluoride raises the Hammett function to —15.2, nearly the strongest of all acids (34). 

Hammett s logarithmic acidity function is generally used (212). 

Accuracy and Interpretation of Measured pH Values. The acidity function which is the experimental basis for the assignment of pH, is reproducible within about 0.003 pH unit from 10 to 40°C. If the ionic strength is known, the assignment of numerical values to the activity coefficient of chloride ion does not add to the uncertainty. However, errors in the standard potential of the cell, in the composition of the buffer materials, and ia the preparatioa of the solutioas may raise the uacertaiaty to 0.005 pH unit. 

Bitumen Ionomers. Moisture-resistant asphalts (qv) have been prepared by reaction of metal oxides with acid-functionalized bitumens (75). Maleic anhydride or sulfur trioxide/trimethylamine complexes have been used successfully for introduction of acid groups into asphaltic bitumens. 

Coolants or cutting duids containing animal or vegetable oil must be avoided. The carboxyUc acid functions present can undergo reaction with the magnesium on standing. 

Millet Jelly Production. Starch powder is heated together with oxahc acid and hydrolyzed to produce millet jelly. Oxahc acid functions as a hydrolysis catalyst, and is removed from the product as calcium oxalate. This apphcation is carried out in Japan. 

---