Conjugate acid-base pair cations

The definition of pH is pH = —log[H+] (which will be modified to include activity later). Ka is the equilibrium constant for the dissociation of an acid HA + H20 H30+ + A-. Kb is the base hydrolysis constant for the reaction B + H20 BH+ + OH. When either Ka or Kb is large, the acid or base is said to be strong otherwise, the acid or base is weak. Common strong acids and bases are listed in Table 6-2, which you should memorize. The most common weak acids are carboxylic acids (RC02H), and the most common weak bases are amines (R3N ). Carboxylate anions (RC02) are weak bases, and ammonium ions (R3NH+) are weak acids. Metal cations also are weak acids. For a conjugate acid-base pair in water, Ka- Kb = Kw. For polyprotic acids, we denote the successive acid dissociation constants as Kal, K, K, , or just Aj, K2, A"3, . For polybasic species, we denote successive hydrolysis constants Kbi, Kb2, A"h3, . For a diprotic system, the relations between successive acid and base equilibrium constants are Afa Kb2 — Kw and K.a Kbl = A w. For a triprotic system the relations are A al KM = ATW, K.d2 Kb2 = ATW, and Ka2 Kb, = Kw. 

In zeolite cages the atoms accessible to adsorbates are either the cations (protons, alkaline, alkaline-earth. .. cations) or the framework oxygen. The first species have an acidic character, protonic or Lewis, and the second ones are basic. It follows that, as in most oxides, acids and bases form conjugate acid-base pairs in zeolites (62). 

Timonen, J.T. and Pakkanen, T.T. 1999. A qualitative H NMR study of CHClj adsorption on conjugated acid-base pairs in cation exchanged Y-zeohtes. Micrvpor. Mesopor. Mater. 30 327-333. 

A para-substituent may stabilize mesomerically either the conjugate acid of an acid-base pair rather more than it stabilizes benzoic acid, or it may stabilize the conjugate base rather more than it stabilizes the benzoate anion. The first situation is found in car-boxonium ions [13], where the delocalization of the positive charge on to a mesomerically electron-donating substituent stabilizes the cation. A similar resonance in the benzoic acid molecule [14] involves a separation of charge and affects the binding of the proton... 

Alkane radical cations are very strong Bronsted acids and form an acid-base pair with neutral alkyl radicals as conjugate base according to the acid-base half-reaction... 

Carbocations are usually formed either by loss of a leaving group or by addition of an electrophile to a pi bond. Carbocation formation is very rare in nonpolar solvents that cannot stabilize the cation. Polar solvent molecules can cluster their lone pairs around the cationic center, forming an intermolecular donation of electron density that stabilizes the carbocation. Some carbocations are stable enough to be on the pA a chart the more stable ones are the weaker acids. Recall that when the pH equals the pATa. the conjugate acid and base concentrations are equal. A protonated amide with a pATa of -0.5 requires very acidic water before it is half-cation and half-neutral. A protonated ester with a pATa of -6.5 is much less stable. However, since most carbocations are not on the pAfa chart, we need another method to rank carbocation stability. 

In the other aqueous solution, the nitrogen has no free pair of electrons and the quaternary ammonium cation could not possibly act as a base. The hydroxide anion of this salt, however, can easily accept a proton and is a potent base. Thus, the basicities of the solutions is determined by the relative basicities of the hydroxide anion and trimethyl amine. Hydroxide anion is a much stronger base which can be verified by comparing the relative aci ties of their conjugate acids. H2O, the conjugate acid of 0DH is a much weaker acid than the quaternary ammonium cation, trimethyl amine s conjugate acid. In fact, we may write the following reaction ... 

A dissection of the influence of solvent and base should be possible from studies in dipolar aprotic solvents containing various bases in which the two reactants can be varied independently. Although bases whose conjugate acids have pATa values of less than 11 rarely induce elimination in protic media , halide ions have been used successfully in this connection in dipolar aprotic solvents . Of course in the latter solvents, which lack exchangeable hydrogens but possess atoms with lone pairs capable of solvating cations, pATa values may differ appreciably from those already derived in protic sol-vents . There is obviously ample scope for future research in this field. 

Molecules with acid-base properties can also be classified as a FIT reaction. For instance, the protonated base BH+ (neutral B) can be regarded either as a simple cation transfer reaction or as a proton transfer facilitated by the conjugated acid (67,68). Finally, the FIT by ion-pair formation can also be classified into this category (6). 

The NH group lends to pyrrole the functionality of a base as weU as that of an acid [99]. The basicity of pyrrole is very weak (pfQ, = —3.8 for the conjugated acid) compared to cyclic aliphatic amines (e.g., pyrrolidine, pfQ, = +11.3). This large difference is due to incorporation of the nonbonding N-electron pair into the 6it-electron system of pyrrole. In principle, reversible proton addition to pyrrole may occur at all positions, but leads to cations of different thermodynamic stability ... 

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