Conjugate acids strength

In general, as base strength increases, conjugate acid strength decreases. Explain why the conjugate acid of the weak base NH3 is a weak acid. 

The significance of the possible diprotonation of water under extremely acidic conditions directly affects the question of acid strength achievable in superacidic systems. The leveling effect mentioned above limits the acidity of any system to that of its conjugate acid. Thus, in... 

Tabulating Values for K and Kb A useful observation about acids and bases is that the strength of a base is inversely proportional to the strength of its conjugate acid. Consider, for example, the dissociation reactions of acetic acid and acetate. 

The most important appHcation of metal alkoxides in reactions of the Friedel-Crafts type is that of aluminum phenoxide as a catalyst in phenol alkylation (205). Phenol is sufficientiy acidic to react with aluminum with the formation of (CgH O)2Al. Aluminum phenoxide, when dissolved in phenol, greatiy increases the acidic strength. It is beheved that, similar to alkoxoacids (206) an aluminum phenoxoacid is formed, which is a strong conjugate acid of the type HAl(OCgH )4. This acid is then the catalyticaHy active species (see Alkoxides, metal). 

Effect on Oxide—Water Interfaces. The adsorption (qv) of ions at clay mineral and rock surfaces is an important step in natural and industrial processes. SiUcates are adsorbed on oxides to a far greater extent than would be predicted from their concentrations (66). This adsorption maximum at a given pH value is independent of ionic strength, and maximum adsorption occurs at a pH value near the piC of orthosiUcate. The pH values of maximum adsorption of weak acid anions and the piC values of their conjugate acids are correlated. This indicates that the presence of both the acid and its conjugate base is required for adsorption. The adsorption of sihcate species is far greater at lower pH than simple acid—base equihbria would predict. 

An inflection point in a pH-rate profile suggests a change in the nature of the reaction caused by a change in the pH of the medium. The usual reason for this behavior is an acid-base equilibrium of a reactant. Here we consider the simplest such system, in which the substrate is a monobasic acid (or monoacidic base). It is pertinent to consider the mathematical nature of the acid-base equilibrium. Let HS represent a weak acid. (The charge type is irrelevant.) The acid dissociation constant, = [H ][S ]/[HS], is taken to be appropriate to the conditions (temperature, ionic strength, solvent) of the kinetic experiments. The fractions of solute in the conjugate acid and base forms are given by... 

Phenol has different chemical properties from those of typical alcohols. Display the electrostatic potential map for phenol. Does this suggest that phenol is likely to be a stronger or weaker acid than any of the compounds discussed above Compare the electrostatic potential map for 4-nitrophenol to that for phenol. What effect does substitution by nitro have on acid strength Explain your result by considering charge delocalization in the conjugate base. Draw all reasonable Lewis structures for phenoxide anion and for 4-nitrophenoxide anion. Which is more delocalized Is this consistent with experimental pKa s ... 

Bronsted-Lowry acid, 49 conjugate base of, 49 strengths of, 50-52 Bronsted-Lowry base, 49 conjugate acid of, 49 strengths of, 50-52 Brown, Herbert Charles. 223 Butacetin, structure of. 833 1,3-Butadiene, 1,2-addition reactions of, 487-489... 

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. 

Because conjugate acids and bases are in equilibrium in solution, we use the equilibrium constant for proton transfer between the solute and the solvent as an indicator of the strength of an acid or a base. For example, for acetic acid in water,... 

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 ... 

TABLE P a 10.3 Conjugate Acid-Base Pairs Arranged by Strength Acid name Acid formula Base formula Base name PKb... 

The azide ion is a weak base and accepts a proton to form its conjugate acid, hydrazoic acid, HN3. Hydrazoic acid is a weak acid similar in strength to acetic acid. 

By definition, every carbanion possesses an unshared pair of electrons and is therefore a base. When a carbanion accepts a proton, it is converted to its conjugate acid (see Chapter 8). The stability of the carbanion is directly related to the strength of the conjugate acid. The weaker the acid, the greater the base strength and the lower the stability of the carbanion. Here, by stability we mean stability toward a proton donor the lower the stability, the more willing the carbanion is to accept a... 

Unfortunately, it is not easy to measure acid strengths of very weak acids like the conjugate acids of simple unsubstituted carbanions. There is little doubt that these carbanions are very unstable in solution, and in contrast to the situation with carbocations, efforts to prepare solutions in which carbanions such as ethyl or isopropyl exist in a relatively free state have not yet been successful. Nor has it been possible to form these carbanions in the gas phase. Indeed, there is evidence that simple carbanions such as ethyl and isopropyl are unstable toward loss of an electron, which converts them to radicals. Nevertheless, there have been several approaches to the problem. Applequist and O Brien studied the position of equilibrium for the reaction... 

Acid strength may be defined as the tendency to give up a proton and base strength as the tendency to accept a proton. Acid-base reactions occur because acids are not equally strong. If an acid, say HCI, is placed in contact with the conjugate base of a weaker acid, say acetate ion, the proton will be transferred because the HCI has a greater tendency to lose its proton than acetic acid. That is, the equilibrium... 

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. 

Figure 15.1 Conjugate acid-base pair strengths.

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