Anion conjugate base

The electronegativity of this atom affects the polarity of the bond to the proton, and it affects the relative stability of the anion (conjugate base). 

The hydrogen phosphates are moderately strong acids and react with bases to form anions (conjugate bases). Repulsion between negatively charged species prevents further reaction between these anions and OH. ... 

Loss of a proton is relatively rare but may take place in cases where there is a positive charge on the complex (e.g. IrH2(diene)L in equation 8) or the anionic conjugate base is stabilized (e.g. HCo(CO)4 in equation 9). 

Iodination has been used to assess the effect of Ni(II) complexa-tion upon the reactivities of pyrazole and imidazole (66JA5537 72JA-2460), which curiously are opposite in effect. Iodination has been used to determine the relative reactivities of pyrazole and its anion (conjugate base) (67JA6218). Under alkaline conditions, pyrazole reacts as the anion which is so reactive that it may be tribrominated [55LA(593)179 ... 

A study of the iodination of pyrazole and 1-alkylpyrazoles has led to the conclusion that the reactivity of the anion (conjugate base) is enhanced relative to the neutral species by io9 5-12 8 (67JA6218). The reactivity of the 4(5)-positions of imidazole (statistically corrected) to that of the 4-position of pyrazole has been determined as 1.3 (64JA2857), which agrees with the localization energy calculated for these positions of -2.103 and -2.13(3, respectively (55AJC100). 

On the other hand, for groups such as the carboxyl, with a neutral acid form and an anionic conjugate base, a represents the fraction uncharged. The fractional charge is then ... 

Acids are proton donors (according to Bronsted) or electron acceptors (according to Lewis, this is a more general concept). Strong acids are completely dissociated in water, releasing protons (H+) and anionic conjugate bases. Acids have a sour taste. 

What about weak bases such as amines In aqueous solutions, water functions as the proton donor or acid (Rx. 2-5), producing the familiar hydroxide anion (conjugate base). 

Basic anions Conjugate bases of weak acids... 

Neutral anions -> Conjugate bases of strong acids Examples Cl", CIO4 NO3. ... 

HIn represents nonionized acid molecules, and In- represents the anion (conjugate base) of HIn. The essential characteristic of an acid-base indicator is that HIn and In must have quite different colors. The relative amounts of the two species determine the color of the solution. Adding an acid favors the reaction to the left and gives more HIn molecules (color 1). Adding a base favors the reaction to the right and gives more In ions (color 2). The ionization constant expression can be rearranged. 

Hydrogens alpha to carbonyl are unusually acidic because of resonance stabilization of the anionic conjugate base. 

A second type of behavior occurs for weak acids like HCN, H2S, and aromatic sulfonamides (ArS02NH2). Assuming that the anions (conjugated bases) bind the low-pH species of the enzyme, the bell-shaped plot of log K versus pH (Figure 2.15) can be accounted for. In fact, at low pH, the inhibitors are in the protonated form, which is not suitable for metal binding. At high pH the concentration of the low-pH species of the enzyme decreases. The maximal apparent affinity is experimentally halfway between the p/fa of the inhibitor and the p Ta of the enzyme, treated as if it were only one. The same type of curve is also expected if the high-pH species of the enzyme binds the weak acid. Indeed, kinetic measurements seem to favor this hypothesis for sulfonamides. ... 

The extent to which a weak base accepts a proton from water to form OH is expressed by a base-dissociation constant, K. Bronsted-Lowry bases include NH3 and amines and the anions of weak acids. All produce basic solutions by accepting from water, which yields OH and thus makes [HaO" ] < [OH ]. A solution of HA is acidic because [HA] [A ], so [HsO ] > [OH j. A solution of A is basic because [A ] >> [HA], so [OH ] > [H3O ]. By multiplying the expressions for Kg of HA and Kb of A , we obtain K. This relationship allows us to calculate either Kg of BH, the cationic conjugate acid of a molecular weak base B, or Kb of A , the anionic conjugate base of a molecular weak acid HA. 

It is apparent that acidity in organic molecules is influenced by several factors. Among these, inductive effects, solvent effects, and factors that influence the stability of both the acid and the anionic conjugate base are important. The base is obviously important since the strength of an acid is directly dependent on the strength of the base. 

The NH-acidity increases with the number of pyridine-like N-atoms, because the anions (conjugate bases) become more stable. The Ka values of tetrazoles thus correspond approximately to those of carboxylic acids. 

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