Polyfunctional acids and bases

We have so far considered acids and bases with a single ionizable group, and have rationalized the measured values in relation to structural features in the molecule. This additional structural feature could well have its own acidic or basic properties, and we thus expect that such a compound will be characterized by more than one pK value. 

Before we consider polyfunctional organic compounds, we should consider the inorganic acids sulfuric acid and phosphoric acid. Sulfuric acid is termed a dibasic acid, in that it has two ionizable groups, and phosphoric acid is a tribasic acid with three ionizable hydrogens. Thus, sulfuric acid has two pMa values and phosphoric acid has three. 

We can generalize that it is going to be more difficult to lose a proton from an anion than from an uncharged molecule. This is also true of polyfunctional acids, such as dicarboxylic acids. However, it is found that this effect diminishes as the negative centres become more separated. Thus, pATa values for some simple aliphatic dicarboxylic acids are as shown, loss of the first proton being represented by pATai and loss of the second by pATa2- 

CO2H HO2C CO2H oxalic acid malonic acid 

It can be seen that the difference between the first and second pATa values diminishes as the number of methylene groups separating the carboxyls increases, i.e. it becomes easier to lose the second proton as 

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NMR longitudinal relaxation times can be used for determination of the site of protonation in polyfunctional acids and bases (93JCS(P2)283). Thus, the 14N NMR spectrum of 4-aminopyridine shows clearly from the sharpening of the signal for the ring nitrogen that protonation has occurred here. This procedure is an important innovation in the elucidation of heterocyclic tautomeric structures, especially for the cases of fast exchange. 

Polyfunctional acids and bases play important roles in many chemical and biological systems. The human body contains a complicated system of buffers within cells and within bodily fluids, such as human blood. The photo is a scanning electronmicro-... 

This chapter describes polyfunctional acid and base systems, including buffer solutions. Calculations of pH and titration curves are also described. 

Thus far, we have not considered how to calculate the pH of solutions of salts that have both acidic and basic properties—that is, salts that are amphiprotic. Such salts are formed during neutralization titration of polyfunctional acids and bases. For example, when 1 mol of NaOH is added to a solution containing 1 mol of the acid... 

Note that these expressions are analogous to the a expressions we wrote for polyfunctional acids and bases except that the equations here are written in terms of formation equilibria while those for acids or bases are written in terms of dissociation equilibria. Also, the master variable is the ligand concentration [L] instead of the hydronium ion concentration. The denominators are the same for each a value. Plots of the a values versus p[L] are known as distribution diagrams. 

Polyfunctional acids and bases Species that contain more than... 

Chapter 8 Polyfunctional Acids and Bases Chapter 15 Poly functional Acids and Bases Chapter 13 Polyfunctional Acids and Bases... 

The common method of naming aldehydes corresponds very closely to that of the related acids (see Carboxylic acids), in that the term aldehyde is added to the base name of the acid. For example, formaldehyde (qv) comes from formic acid, acetaldehyde (qv) from acetic acid, and butyraldehyde (qv) from butyric acid. If the compound contains more than two aldehyde groups, or is cycHc, the name is formed using carbaldehyde to indicate the functionaUty. The lUPAC system of aldehyde nomenclature drops the final e from the name of the parent acycHc hydrocarbon and adds al If two aldehyde functional groups are present, the suffix -dialis used. The prefix formjlis used with polyfunctional compounds. Examples of nomenclature types are shown in Table 1. 

In addition to the use of peroxides for crosslinking, metal oxide, polyfunctional alcohols, amines and epoxide resin cure systems can be used with CSM rubbers. In the metal oxide based cure systems it is usual to add a weak acid, such as stearic acid, and accelerators, such as MBT, MBTS or TMTD magnesium or lead oxides are generally used. 

The polyester type polyols used in polyurethane laminating adhesives are produced by the direct esterification of polyfunctional carboxylic acids and glycols. Polyester polyols provide the soft segment in polyurethane products giving the adhesive flexibility. Ester groups of the polyol also contribute to adhesion. Polyester polyols provide limited wetting and adhesion of olefinic surfaces with amide slip additives (in contrast to polyether polyols). Typical examples include adipic acid, caprolactone, maleic acid and isophthalic based polyester polyols. 

Consider the simple alkyd recipe shown in Table 5-1, part (i). Alkyds are polyesters produced from polyhydric alcohols and polybasic and monobasic acids. They are used primarily in surface coatings. The ingredients of these polymers contain polyfunctional monomers and it is possible that such polymerizations could produce a thermoset material during the actual alkyd synthesis. This is of course an unwanted outcome, and calculations based on the Carothers equation can be used to adjust the polymerization recipe to produce a finite molecular weight polymer in good yield. The recipe can also be adjusted to provide other desirable characteristics of the product, such as an absence of free acid groups which may react adversely with some pigments. 

The pH of polyfunctional systems, such as phosphoric acid or sodium carbonate, can be computed rigorously through use of the systematic approach to multiequilibrium problems described in Chapter 11. Solution of the several simultaneous equations that are involved is difficult and time consuming, however. Fortunately, simplifying assumptions can be invoked when the successive equilibrium constants for the acid (or base) differ by a factor of about 10 (or more). With one exception, these assumptions make it possible to compute pH data for titration curves by the techniques we have discussed in earlier chapters. 

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