Basic strength conjugate acids

Procedures to compute acidities are essentially similar to those for the basicities discussed in the previous section. The acidities in the gas phase and in solution can be calculated as the free energy changes AG and AG" upon proton release of the isolated and solvated molecules, respectively. To discuss the relative strengths of acidity in the gas and aqueous solution phases, we only need the magnitude of —AG and — AG" for haloacetic acids relative to those for acetic acids. Thus the free energy calculations for acetic acid, haloacetic acids, and each conjugate base are carried out in the gas phase and in aqueous solution. 

The UV spectra of 3,6-di-ter/-butyl-substituted 2H-, 3H- and 4//-azepines in various ethanol solutions have been recorded.30 On the basis of pH-dependent spectra, pATa values of 7.02, 4.54 and 6.81, respectively, have been calculated for the conjugate acids of these azepines. The increase in basic strength (2H > 4H > 3//) can be reconciled with mesomeric stabilization of the respective azepinium cations. 

To express the relative strengths of an acid and its conjugate base (a conjugate acid-base pair ), we consider the special case of the ammonia proton transfer equilibrium, reaction C, for which the basicity constant was given earlier (Kb = [NH4+l[OH ]/ NH3]). Now let s consider the proton transfer equilibrium of ammonia s conjugate acid, NH4+, in water ... 

We used DFT to optimize the geometries of various Hammett bases on cluster models of zeolite Brpnsted sites. For p-fluoronitrobenzene and p-nitrotoluene, two indicators with strengths of ca. -12 for their conjugate acids, we saw no protonation in the energy minimized structures. Similar calculations using the much more strongly basic aniline andogs of these molecules demonstrated proton transfer from the zeolite cluster to the base. We carried out F and experimental NMR studies of these same Hammett indicators adsorbed into zeolites HY and HZSM-5. 

Since the acid HX acts as a solvent, its activity may be regarded as constant and included in the equilibrium constant. is the mean activity coefficient of the cation acid and the stabilizing anion X. The way in which equations (5) and (6) are written define the corresponding equilibrium constants as basicity constants K. Their reciprocal corresponds to the acidity constant and gives the acid strength of the conjugate acid AH. ... 

This system has been almost completely dropped in favour of using pATa throughout the acidity-basicity scale. To measure the strength of a base, we use the pATa of its conjugate acid, i.e. we consider the equilibrium... 

Electron-donating groups on nitrogen are going to increase the likelihood of protonation, and help to stabilize the conjugate acid. They thus increase the basic strength. 

The enhanced acidity is partly due to very effective H-bonding in the conjugate base, which decreases its basic strength. 

Figure 2.5 The Br0nsted plot for the general-base catalysis of the hydrolysis of ethyl dichloroacetate. The logarithms of the second-order constants obtained from the plot of Figure 2.4 are plotted against the pAT s of the conjugate acid of the catalytic base. The slope is the /3 value. Note that the points for amine bases ( ) fall on the same line as those for oxyanion bases (O), showing that the catalysis depends primarily on the basic strength of the base and not on its chemical nature.

If there is a large number of conjugated multiple bonds or condensed aromatic rings in the hydrocarbon molecule (tt, -conjugation), then the basic strength is considerably enhanced and the hydrocarbon is converted into a carbonium ion by acids. 

As discussed in Section 3.10.3, in the gas phase the basicity of simple amines follows the order NMe3 > NHMe2 > NH2Me > NH3 because of the electron donating effect of the methyl (Me) groups. In solution, however, we can define a basicity constant as the equilibrium constant for the reaction shown in Equation 3.4. Note it is important to specify temperature, solvent (usually water) and solution ionic strength, 1 Basicity constants are related to the acid dissociation constants (/Q of the base s conjugate acid via the dissociation constant of water, K = 10 14 at 25 °C. Thus Kbx K = Kw. 

The conjugate base of a weak acid has base strength. Acetic acid is a weak acid. Therefore, the acetate ion has base strength. You probably noticed the stumbling language here. We didn t say, Since acetic acid is a weak acid, the acetate ion is a strong base, because that isn t true. However, a solution of sodium acetate will be basic. 

Probably the most important chemical reaction of amines, at least in medical applications, is the basicity of amines. Most amines have a noticeable base strength and will accept a proton from a strong acid to form its conjugate acid. The conjugate acid of an amine is called an ammonium salt. The ionic salt is much more soluble in water than the electrically neutral freebase form. So, by... 

The acid strength of a solid is defined as the ability of the surface to convert an adsorbed neutral base into its conjugate acid (2). If the reaction proceeds by means of proton transfer from the surface to the adsorbate, the acid strength is expressed by the Hammett acidity function Ho, Ho = pKa + log[B]/[BH + ], where [B] and [BH + ] are, respectively, the concentrations of the neutral base (basic indicator) and its conjugate acid, and pK.d is p BH +. If the reaction takes place by... 

The acid or base strength is defined by comparing the interaction between the sites and reference (probe) molecules. For example, the acid strength is the ability of the site to convert an adsorbed reference neutral base into its conjugate acid. This process may be monitored by following the colour changes of the indicators or by such techniques as IR or NMR spectroscopies. Several criteria must be considered in the selection of these probes their pK value must be adapted to the acidity or basicity of the surface under study the size of the probe molecules should ensure that all sites are readily accessible finally, the probe should be selected so that its interaction with the surface is easily studied by the available experimental techniques. Catalytic test reactions are sometimes used... 

Structural features are not the only factors that affect acidity or basicity. The same compound can have its acidity or basicity changed when the conditions are changed. The effect of temperature (p. 364) has already been mentioned. More important is the effect of the solvent, which can exert considerable influence on acid and base strengths by differential solvation. If a base is more solvated than its conjugate acid, its stability is increased relative to the conjugate acid. For example,... 

It is often more convenient to talk about basicities than acidities. In this textbook, the pk b of a base is defined as the pKu of its conjugate acid. For example, according to this book s definition, NH3 has a pk h of 10 (because NH4+ has a pKa of 10) and a pKu of 35. The strength of a base correlates directly with the weakness of its conjugate acid. Factors that increase acidity decrease basicity, and factors that decrease acidity increase basicity. For example, EtS- is less basic than EtO, just as EtSH is more acidic than EtOH. 

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