Bases acetate ions

Example A common misconception is that the conjugate base of a weak acid is strong This is sometimes but not always true It is true for example for ammo nia which is a very weak acid (pK 36) Its conjugate base amide ion (H2N ) is a much stronger base than HO It is not true however for acetic acid both acetic acid and its conjugate base acetate ion are weak The conjugate base of a weak acid will be strong only when the acid is a weaker acid than water... 

It may be noted parenthetically that species functioning as bases (acetate ions or water) have been included in the lost three examples. 

Fiqure 2.4 Determination of the rate constants for the general-base catalysis of the hydrolysis of ethyl dichloroacetate. The first-order rate constants for the hydrolysis are plotted against various concentrations of the base. The slope of the linear plot is the second-order rate constant (k2). The intercept at zero buffer concentration is the "spontaneous hydrolysis rate constant for the particular pH. A plot of the spontaneous rate constants against pH gives the rate constants for the H+ and OH" catalysis. It is seen that pyridine is a more effective catalyst than the weaker base acetate ion. [From W. P. Jencks and J. Carriuolo, J. Am. Chem. Soc. 83,1743 (1961).]... 

To predict the pH of mixtures of weak acids or bases and their salts quantitatively, we set up an equilibrium table, as described in Toolbox 10.1. Then we use the acidity or basicity constant to calculate the concentration of hydronium ions present in the solution. The only difference is that now the conjugate acid and base are both present initially, so the first line of the table must have their initial concentrations. For instance, in the mixed acetic acid/sodium acetate solution, both acetic acid and its conjugate base, acetate ions, are present initially. In the ammonia/ammo-nium chloride solution, both the base (ammonia) and its conjugate acid (the ammonium ions) are present initially. 

In the Bronsted-Lowry scheme, every acid has a conjugate base and every base has a conjugate acid. Thus NH4 + is the acid conjugate to the base NH3, and OH- is the base conjugate to the acid H20. Similarly, acetic acid, CH3COOH, donates its acidic proton to H20 to produce the conjugate base, acetate ion, CH3C02 ... 

In other words, the unshared electron pair of the base, acetate ion, is delocalized (spread over both oxygens) by resonance. This electron pair is stabilized and less available for bonding to the proton, which localizes this electron pair in the sigma bond and costs resonance energy. The most common effect of resonance on an acid-base reaction is to delocalize and stabilize the unshared electron pair of the conjugate base, resulting in a stronger acid. 

Ethoxide ion, a strong base, removes a proton from acetic acid. The formation of the weak base, acetate ion, in this step drives the equilibrium to the final products, the alcohol and the carboxylate anion. 

Thus the acetic acid in the first reaction becomes its conjugate base acetate ion, while the base, hydroxide ion, becomes its conjugate acid, water. In the reverse reaction the nomenclature also reverses. Note that a molecule such as water can be both an acid, donating a proton to become its conjugate base hydroxide ion, or it can be a base, accepting a proton to become its conjugate acid, a hydronium ion. 

Conventionally, some chemicals are called adds, whereas others are called bases. This convention is based on the form the chemical takes in its ur chaiged state or when it Is not in contact with water. For example, although the acetate ion that is formed when acetic add dissociates is a base, acetate ion generally is not called "acetic base "... 

A buffer solution consists of 0.5 M acetic acid CH3COOH (Ka value of 1.8 10 M) and 0.5 M sodium acetate CHsCOONa. This solution consists of a weak acid (acetic acid) ad its corresponding weak base (acetate ion). As the amount of the weak acid and weak base are similar (it is sufficient if there just in the same order of magnitude) we have a buffer system. 

The key catalytic intermediate in these processes is a palladium(II)- 7r-allyl complex. There are, in principle, two possible pathways for the formation of this intermediate-removal of a proton from a palladium(II)-olefin complex by a base (acetate ion) or oxidative addition of an olefin to a coordinatively unsaturated palladium(O) center (Figure 1). 

Analogous transformations of [Co(L245)] + are also well documented. In this case in the presence of an excess of base (acetate ion) an internal rearrangement of one peripheral secondary amine moiety of [Co(L245)] + occurs and this allows an additional nitrogen atom to coordinate to the metal centre, forming a five-coordinate cobalt(n) complex [Co(L"245)] (Scheme 4-15) [132]. 

All of the previous acid-base examples dealt with situations in which an acid and its conjugate base were close to equilibrium. In the following example, we consider the addition of a strong acid to an acetic acid/acetate ion buffer. Hydro-nium ion reacts with the conjugate base (acetate ion) in the buffer, so that the con-centtations of species in solution change markedly from their initial value. (If you add a strong base to this buffer, hydroxide ion reacts with acetic acid in the buffer.)... 

Solution We know that the first reaction has to be a radical halogenation because that is the only reaction that an alkane undergoes. Bromination will lead to a greater yield of the desired 2-halo-substituted compound than chlorination will because a bromine radical is more selective than a chlorine radical. To maximize the yield of the desired substitution product over the elimination product, a weak base (acetate ion) is used for the substitution reaction (Section 10.9). The ester is hydrolyzed to an alcohol that forms the target molecule when it is oxidized. 

We have seen that pH = pKa = 4.74 when acetic acid and acetate ion are present in equal concentrations. Here the concentration of the conjugate base (acetate ion) is greater than that of the acetic acid. The solution should be somewhat more basic (less acidic) than pH = 4.74. A pH of 4.80 is a reasonable answer. 

The most common charge types for the acid HB and its conjugate base B are CH3COOH = H+ -f CH3C00 (acetic acid, acetate ion)... 

When an acid and a base react, the products are a new acid and base. For example, the acetate ion, C1T3COO-, in reaction 6.7 is a base that reacts with the acidic ammonium ion, N1T45", to produce acetic acid and ammonia. We call the acetate ion the conjugate base of acetic acid, and the ammonium ion is the conjugate acid of ammonia. 

Weak bases only partially accept protons from the solvent and are characterized by a base dissociation constant, kj,. For example, the base dissociation reaction and base dissociation constant for the acetate ion are... 

Nucleophilicity roughly parallels basicity when comparing nucleophiles that have the same reacting atom. For example, OH- is both more basic and more nucleophilic than acetate ion, CH3CO2-, which in turn is more basic and more nucleophilic than H20. Since "nucleophilicity" is usually taken as the affinity of a Lewis base for a carbon atom in the Sfj2 reaction and "basicity" is the affinity of a base for a proton, it s easy to see why there might be a correlation between the two kinds of behavior. 

Notice that this reaction is the reverse of the reaction of the weak base QH - (the acetate ion) with water (Chapter 13). It follows from the reciprocal rule that for this reaction,... 

We saw in Section J that a salt is produced by the neutralization of an acid by a base. However, if we measure the pH of a solution of a salt, we do not in general find the neutral value (pH = 7). For instance, if we neutralize 0.3 M CHjCOOH(aq) with 0.3 M NaOH(aq), the resulting solution of sodium acetate has pH = 9.0. How can this be The Bronsted-Lowry theory provides the explanation. According to this theory, an ion may be an acid or a base. The acetate ion, for instance, is a base, and the ammonium ion is an acid. The pH of a solution of a salt depends on the relative acidity and basicity of its ions. 

Acetate ions and the other ions listed in last row of Table 10.8 act as bases in water. 

We have seen how to estimate the pH of a solution of a weak acid or base (Chapter 10), but suppose that a salt of the acid or base is also present. How does that salt affect the pH of the solution Suppose we have a dilute hydrochloric acid solution and add to it appreciable concentrations of the conjugate base, the Cl- ion, as sodium chloride. Because the acid is strong, its conjugate base is extremely weak and so has no measurable effect on pH. The pH of 0.10 M HCl(aq) is about 1.0, even after 0.10 mol NaCl has been added to a liter of the solution. Now suppose instead that the solution contains acetic acid to which sodium acetate has been added (the acetate ion, CH jC()2, is the conjugate base of CH COOH). Because the conjugate base of a weak acid is a base, we can predict that adding acetate ions (as sodium acetate) to a solution of acetic acid will increase the pH of the solution. Similarly, suppose we have a solution of ammonia and add ammonium chloride to it. The... 

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