Acid-base reactions favored products/reactants

Whether an acid-base reaction favors formation of the products or formation of the reactants can be determined by comparing the pK value of the acid that loses a proton in the forward direction with the p a value of the acid that loses a proton in the reverse direction. The equilibrium will favor the reaction of the stronger acid to form the weaker acid. The following reaction favors formation of the reactants, because CH3OH2 is a stronger acid than CHsCCKlH. 

For example, when hydroxide ion or an alkoxide ion is used to remove an a-hydrogen from cyclohexanone, only a small amount of the carbonyl compound is converted into the enolate ion because the product acid (H2O) is a stronger acid than the reactant acid (the ketone). (Recall that the equilibrium of an acid-base reaction favors dissociation of the strong acid and formation of the weak acid see Section 2.5.)... 

Show the products of these acid-base reactions and predict whether the equilibria favor the reactants or the products ... 

Write equations for the following acid-base reactions. Use the information in Table 1-5 to predict whether the equilibrium will favor the reactants or the products. 

Complete the following proposed acid-base reactions, and predict whether the reactants or products are favored. 

Draw the products of each acid-base reaction, and using the pKg tabie in Appendix A, determine if equilibrium favors the reactants or products. 

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. 

Give the products of the following acid-base reactions, and indicate whether reactants or products are favored at equilibrium (use the p fa values that are given in Section 1.17) ... 

As discussed in this chapter, enolate anions are formed when a carbonyl compound containing an a-hydrogen is treated with a base such as hydroxide or an alkoxide. We noted earlier that a-hydrogens normally are considerably less acidic than water or alcohols, so the position of equilibrium in this acid-base reaction greatly favors the reactants rather than enolate products. 

Deciding Whether Reactants or Products Are Favored in an Acid-Base Reaction... 

Deciding whether reactants or products are favored in an acid-base reaction Given an acid-base reaction and the relative strengths of acids (or bases), decide whether reactants or products are favored. (EXAMPLE 163)... 

This observation leads to a second important general conclusion proton-transfer reactions favor the production of the weaker acid and the weaker base. For an acid-base reaction to form products completely, the reactants must be much stronger as acids and bases than the products. 

The reaction of an acid with a base is called an acid-base reaction or a proton transfer reaction. Both an acid and a base must be present in an acid-base reaction, because an acid caimot lose a proton unless a base is present to accept it. Acid-base reactions are reversible. Two half-headed arrows are used to designate reversible reactions. In Section 2.5 we will see how we can determine whether reactants or products are favored when the reaction has reached equilibrium. 

Ethyne has a p value of 25, water has a pK value of 15.7, and ammonia (NH3) has a p a value of 36. Draw the equation, showing equilibrium arrows that indicate whether reactants or products are favored, for the acid-base reaction of ethyne with a. HO. b. NH2. 

Many students think of acid-base reactions as all or nothing. They erroneously assume that if the reaction is favorable (e.g., NH3 + HF, which is downhill by 12 pA a units of energy) then all NH3 and HF molecules will be consumed during the reaction. This is not the case with this reaction (and, in fact, no reaction goes 100.000...etc. percent to completion. There is always an equilibrium point where the concentration of products causes the reaction to stop. At this point there are always some reactant molecules still around. The more the energy difference between reactants and products (the more downhill the reaction), the fewer reactant molecules remaining when the equilibrium point is reached. 

Write the equations for the reaction between each of the following acid-base pairs. For each reaction, predict whether reactants or products are favored (using the values of K given in Appendix 2). 

This causes a slight excess of base in the reaction, but it doesn t ciffect pH significantly. You can think of the undissociated acid as a reservoir of protons that are available to neutralize any strong base that may be introduced to the solution. As we explain in Chapter 14, when a product is added to a reaction, the equilibrium in the reaction changes to favor the reactants or to undo the change in conditions. Because this reaction generates A , the acid dissociation reaction happens less frequently as a result, further stabilizing the pH. 

It is known [41] that partial oxidation reactions in heterogeneous catalysis involves redox properties of the solid catalysts, allowing the well known Mars-van Krevelen mechanism [42] to occur, or at least to be facilitated. Acid-base properties are also an important feature, as they play a determining role in the activation of the reactants and in the desorption of the intermediate compounds, for instance, an acid surface will favor desorption of acid products, thus avoiding further over-oxidation, while a basic surface will favor desorption of basic products as olefins. It follows that heteropolyoxometallate compounds, in particular TMSP, appear as potential... 

A great many reactions are carried out in a convenient solvent for reactants and products. Dissolved reactants can be rapidly mixed, and the reaction process is easily handled. Water is a specially favored solvent because its polar structure allows a broad range of polar and ionic species to be dissolved. Water itself is partially ionized in solution, liberating and OH ions that can participate in reactions with the dissolved species. This leads to the important subject of acid-base equilibria in aqueous solutions (see Chapter 15), which is based on the equilibrium principles developed in this chapter. We limit the discussion in this subsection to cases in which the solvent does not participate in the reaction. 

Write equations for the following add-base reactions. Label the conjugate acids and bases, and show any resonance stabilization. Predict whether the equilibrium favors the reactants or products. 

As implied by the double arrows in reactions (16.1) and (16.2), the ionization of an acid or a base in water is a reversible reaction that reaches a state of dynamic equilibrium. In Section 16-3, we will consider some new ideas that will help us decide whether, for a given acid or base, equilibrium favors reactants (the un-ionized acid or base) or products (the ionized form of the acid or base). For now, it will be helpful to summarize some key aspects of the Bronsted-Lowry theory. 

Summary of the Relationship between Diastereoselectivity and the Transition Structure. In this section we considered simple diastereoselection in aldol reactions of ketone enolates. Numerous observations on the reactions of enolates of ketones and related compounds are consistent with the general concept of a chairlike TS.35 These reactions show a consistent E - anti Z - syn relationship. Noncyclic TSs have more variable diastereoselectivity. The prediction or interpretation of the specific ratio of syn and anti product from any given reaction requires assessment of several variables (1) What is the stereochemical composition of the enolate (2) Does the Lewis acid promote tight coordination with both the carbonyl and enolate oxygen atoms and thereby favor a cyclic TS (3) Does the TS have a chairlike conformation (4) Are there additional Lewis base coordination sites in either reactant that can lead to reaction through a chelated TS Another factor comes into play if either the aldehyde or the enolate, or both, are chiral. In that case, facial selectivity becomes an issue and this is considered in Section 2.1.5. 

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