Acid—base reaction equilibrium position determination

First determine what type of reaction is occurring. When elemental sodium is used, an oxidation-reduction reaction takes place, producing a sodium alkoxide and hydrogen gas. In acid-base reactions, the position of the equilibrium resides on the side with the weaker acid and weaker base (i.e., the more stable species). 

Given the approximate pXa values shown below, for the following acid-base reactions (a) (e), determine whether the position of the equilibrium lies over to the reactant side or the product side. 

Equilibrium control the outcome of an acid-base reaction is determined by the position of an equilibrium. 

Proton transfer is one of the prominent representatives of an ion-molecule reaction in the gas phase. It is employed for the determination of GBs and PAs (Chap. 2.11.2) by either method the kinetic method makes use of the dissociation of proton-bound heterodimers, and the thermokinetic method determines the equilibrium constant of the acid-base reaction of gaseous ions. In general, proton transfer plays a crucial role in the formation of protonated molecules, e.g., in positive-ion chemical ionization mass spectrometry (Chap. 7). 

In determining the position of equilibrium for an acid-base reaction (i.e., whether reactants or products are favored at equilibrium), remember that the equilibrium favors reaction of Ae strong acid and strong base and formation of the weak acid and weak base. In other words, strong reacts to give weak. Thus, the equilibrium lies away from the stronger acid and toward the weaker acid. 

The reason for this is that the outcome of an acid-base reaction is determined by the position of an equilibrium. Acid-base reactions are said, therefore, to be under equilibrium control, and reactions under equilibrium control always favor the formation of the most stable (lowest potential energy) species. The weaker acid and weaker base are more stable (lower in potential energy) than the stronger acid and stronger base. 

How Do We Determine the Position of Equilibrium in an Acid-Base Reaction ... 

In the preceding two acid-base reactions, water was the base (proton acceptor). But what if we have a base other than water as the proton acceptor How can we determine which are the major species present at equilibrium That is, how can we determine whether the position of equilibrium lies toward the left or toward the right ... 

The rate and activation parameters have been determined for the reaction of potassium methanethiolate with various 2-fluoro- and bromo-pyridines. Although an ortAo-methyl group did not activate the 2- position in 2-bromo- or 2-fluoro-pyridine towards attack by the methanethiolate ion, deactivation of the ortho rather than the para position was observed. At 110°C for the bromo-compounds Ao-Me Xp-Me = 3-9, while Ao-Br A -Br = 2-2. The results have been compared with those obtained using methoxide and benzenethiolate anions in methanol. The relative rates observed in HMPA are the same as those in methanoP . Thio-phenol reacts faster than its anion with a bromopyridine, in methanol, due to a rapid acid-base pre-equilibrium in which the pyridine is protonated. An o-MeO substituent accelerates the replacement of Br, and a small increase is also noted on going from MeOH to DMSO as solvenpii. 

PRACTICE the skill 3.10 Determine the position of equilibrium for each acid-base reaction below ... 

Specific acid catalysis is observed when a reaction proceeds through a protonated intermediate that is in equilibrium with its conjugate base. Because the position of this equilibrium is a function of the concentration of solvated protons, only a single acid-dependent term appears in the kinetic expression. For example, in a two-step reaction involving rate-determining reaction of one reagent with the conjugate acid of a second, the kinetic expression will be as follows ... 

In their acidity, basicity, and the directive influence exerted on electrophilic substitution reactions in benzenoid nuclei, acylamino groups show properties which are intermediate between those of free amino and hydroxyl groups, and, therefore, it is at first surprising to find that the tautomeric behavior of acylaminopyridines closely resembles that of the aminopyridines instead of being intermediate between that of the amino- and hydroxy-pyridines. The basicities of the acylaminopyridines are, indeed, closer to those of the methoxy-pyridines than to those of the aminopyridines, the position of the tautomeric equilibrium being determined by the fact that the acyl-iminopyridones are strong bases like the iminopyridones and unlike the pyridones themselves. Thus, relative to the conversion of an... 

Reactions can be catalyzed by acid or base in two different ways, called general and specific catalysis. If the rate of an acid-catalyzed reaction run in a solvent S is proportional to [SH j, the reaction is said to be subject to specific acid catalysis, the acid being the lyonium ion SH. The acid that is put into the solvent may be stronger or weaker than SH , but the rate is proportional only to the (SH ] that is actually present in the solution (derived from S + HA SH + A ). The identity of HA makes no difference except insofar as it determines the position of equilibrium and hence the (SH ]. Most measurements have been made in water, where SH is H30 . 

As the C-H acid molecules approach some base, their equilibrium distance is determined by the sum of the corresponding van der Waals radii. In this case, the distance between two equilibrium positions of the proton is equal to about 1.6 A. For such a tunneling distance, the exponential term in (4.14) turns out to be extremely small, i.e. 10 . Clearly, the proton transfer reaction can take place only at much shorter distances for which the tunneling probability sharply increases in accordance with (4.14). However, the molecules are hindered from coming close to each other by the repulsive forces which sharply increase with decreasing distance. These forces may be described by the Born-Mayer potential[446] ... 

This is always the case for any two acids, and by measuring the positions of the equilibrium the relative strengths of acids and bases can be determined. Of course, if the two acids involved are close to each other in strength, a measurable reaction will occur from both sides, though the position of equilibrium will still be over to the... 

The data in Table 1.1 allow one to estimate the position of the equilibrium for any of the other carbon acids with a given base. It is important to keep in mind the position of such equilibria as other aspects of reactions of carbanions are considered. The base and solvent used will determine the extent of deprotonation. There is another important physical characteristic which needs to be kept in mind, and that is the degree of aggregation of the carbanion. Both the solvent and the cation will influence the state of aggregation, as will be discussed further in Section 1.6. 

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