The Position of Equilibrium in Acid-Base Reactions

Use the approach we developed in Section 2.4 to predict the position of equilibrium in acid-base reactions. Equilibrium favors reaction of the stronger acid and stronger base to form the weaker acid and the weaker base. It is helpful to remember that even though ammonium ions are positively charged, they are much weaker acids than carboxylic acids. 

Ethanol has about the same acidity as water. Higher-molecular-weight, water-soluble alcohols are slightly weaker acids than water. Thus, although alcohols have some acidity, they are not strong enough acids to react with weak bases such as sodium bicarbonate or sodium carbonate. (At this point, it would be wise to review Section 4.4, which discusses the position of equilibrium in acid-base reactions.)... 

Incorporation of lsO into the ketone occurs hardly at all under these conditions, i.e. at pH 7, but in the presence of a trace of acid or base it occurs [via the hydrate (13)] very rapidly indeed. The fact that a carbonyl compound is hydrated will not influence nucleophilic additions that are irreversible it may, however, influence the position of equilibrium in reversible addition reactions, and also the reaction rate, as... 

The position of equilibrium in an acid-base reaction lies to the side of the weaker acid. 

What then is the difference between an acid and an electrophile, or between a base and nucleophile No great difference until we try to use the terms in a quantitative sense. For example, if we refer to acid strength, or acidity, this means the position of equilibrium in an acid-base reaction. The equilibrium constant Ka for the dissociation of an acid FIA, or the pKa, is a quantitative measure of acid strength. The larger the value of Ka or the smaller the pKa, the stronger the acid. 

A strong acid is a substance that reacts completely with water, so that the acid ionization constant defined in Eq. (10) or (11) is effectively infinite. This situation can only be achieved if the conjugate base of the strong acid is very weak. A weak acid will be characterized by an acid ionization constant that is considerably less than unity, so that the position of equilibrium in the reaction represented in Eq. (8) favors the existence of unreacted free acid. 

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

The position of equilibrium in an acid-base reaction favors reaction of the stronger acid (lower value) with the stronger base to form fhe weaker acid (higher pK value) and the weaker base. 

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. 

In 1923, Brpnsted, following in the footsteps of Arrhenius who, apparently, was only concerned with the proton as the acidifying principle, proposed that an acid is a proton donor and a base is a proton acceptor. While we recognize that this is dramatically oversimplified given what we now know about solvent participation, it nonetheless remains a useful construct. Indeed, for materials that ionize, completely or otherwise, the add strength or addity is referred to as the position of equilibrium in the reaction of the acid with a base. Thus, for the generalized acid HA ionizing in water (H2O) Equation 5.6 can be written as... 

Draw the products of each of the following acid-base reactions, and then predict the position of equilibrium in each case ... 

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... 

These reactions are equilibria. What the rule actually says is that the position of equilibrium will be such that the weaker acid predominates. However, this needs to be taken into account only when the acid and base are close to each other in the table (within 2 p f units). 

This dissociation is an equilibrium reaction because it proceeds in both directions. Acetic acid is weak, so only a few ions dissociate. The position of equilibrium lies to the left, and the reverse reaction is favoured. In the reverse reaction, the hydronium ion gives up a proton to the acetate ion. Thus, these ions are an acid and a base, respectively, as shown in Figure 8.3. The acid on the left (CH3COOH) and the base on the right (CHsCOO") differ by one proton. They are called a conjugate acid-base pair. Similarly, H2O and are a conjugate acid-base pair. 

Aqueous ammonia is a weak base, so relatively few hydroxide ions form. The position of equilibrium lies to the left. In the forward reaction, the water molecule gives up a proton and acts as an acid. A substance that can act as a proton donor (an acid) in one reaction and a proton acceptor (a base) in another reaction is said to be amphoteric. (Water acts as an acid in tbe presence of a stronger base, and as a base in tbe presence of a stronger acid. 

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 . 

The effect of the medium on the position of equilibrium can be considered from two points of view (a) comparison of the gas-phase and solution equilibrium constants, and (b) comparison of the equilibrium constants for different solvents. Unfortunately, few equilibrium reactions have been studied both in the gas and liquid phases [5, 6j. These are primarily non-ionic reactions where the interaction between reacting molecules and solvent is relatively small e.g. the Diels-Alder dimerization of cyclo-pentadiene). In this chapter, therefore, equilibria which have been examined in solvents of different polarity will be the main topic considered (except for acid-base reactions described in Section 4.2.2). 

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