Strongly dissociating base

As noted in the discussion of ( )-selective alkene formation, Kishi has found that a-substituted aldehydes reacted with trimethylphosphonopropionate and KOBu to produce the (Z)-alkene selectively. A strongly dissociating base is critical to this approach. In addition to the examples already presented in the discussion of ( )-alkene formation, the (Z)-selective reaction has recently been applied to the synthesis of macrolide antibiotics. In this example, a trisubstituted alkene was formed and closed to the lactone (148 equation 33). In an application to diterpenoids. Piers encountered an example of how substrate-specific the alkene formation can be. With a-dimethoxyphosphonyl-y-butyrolactone (150), the reactions with simple aldehydes proceeded with very high selectivity [(Z) ( ) = 99 1]. On application of the reaction to the more complex aldehyde (149) the (Z) ( ) stereoselectivity dropped to 3 1 in 58% yield (equation 34). No selectivity was observed on reaction with benzaldehyde. Although for hindered substrates, strongly basic conditions with a dimethyl phosphonate can be a simple and effective method for the synthesis of (Z)-isomers, the reaction is not general. In 1983, Still and coworkers introduced methodology that used bis(trifluoroethyl)phosphonoesters (153) to provide a facile approach to (Z)-aIkenes (154) when reacted with aldehydes (equation 35). " ... 

Weak acids other than carbonic that, together with their salts, are used as buffers include acetic and boric acids. The sodium and potassium salts of phosphoric acid, namely mono- and di-potassium (or sodium) phosphate are also very commonly used buffers in the laboratory and greenhouse. Neither strongly dissociated acids, such as hydrochloric, nitric and sulfuric, nor strongly dissociated bases, such as sodium and potassium, act as buffers. Stated another way, it is the acids that have low active acidity but high potential acidity that are suitable for use as buffers. Likewise, the bases must have low active basicity and high potential basicity. 

Phosphonates. In a Homer-Emmons-based synthesis of di- and trisubstituted (Z)-Q(, -unsaturated esters, the strongly dissociated base system of potassium bis(trimethylsilyl)amide/18-Crown-6 was used to prepare the desired phosphonate anions. This base system, coupled with highly electrophilic bis(trifluoro-ethyl)phosphono esters, gave phosphonate anions which, when allowed to react with aldehydes, gave excellent selectivity for the (Z)-Q , -unsaturated esters (eq 15). ... 

Consider now the salt of a strong acid and a weak base class (3). Here the initial high concentration of cations M + will be reduced by combination with the hydroxide ions of water to form the little-dissociated base MOH until the equilibrium ... 

Acids and bases may be strong, dissociating completely, or weak, partially dissociating and forming an equilibrium system. (See Chapter 15 for the details on weak acids and bases.) Strong acids include ... 

If you add a strong base such as sodium hydroxide (NaOH) to this mixture of dissociated base (A ) and undissociated acid (HA), the base s hydroxide is absorbed by the acidic proton, replacing the exceptionally strong base OH with a relatively weak base A and minimizing the change in pH ... 

The concentration of acidic groups can be determined by neutralization with a base, using solutions of sodium bicarbonate, sodium carbonate, sodium hydroxide or sodium ethoxide as titrants. Only those acidic groups which are much more strongly dissociated than the conjugate acids of the bases are completely neutralized. Thus, the differences in the amount of base required may be used to characterize the acidity of surface groups. Boehm (12) identified four different type of surface groups by this technique. 

In a M solution of a strong monobasic acid (supposing that the dissociation is complete) the hydrogen-ion concentration is 1 mol -1. On the other hand, in a M solution of a strong monovalent base the hydroxyl-ion concentration is lmol -1, thus the hydrogen-ion concentration is 10"14 mol C-. The hydrogen-ion concentration of most of the aqueous solutions dealt with in chemical analysis (other than concentrated acids, used mainly for dissolution of samples), lies between these values. 

A substance that produces OH (aq) ions in aqueous solution. Strong soluable bases are soluble in water and are completely dissociated. Weak bases ionize only slightly. 

If a is the initial concentration (molality) of the weak or moderately weak acid HA, and h is the amount of strong, monoacid base MOH added at any instant, then h is also equal to mM > the molality of ions at that instant, since the salt MA produced on neutralization may be taken as being completely dissociated. The acid HA is only partially neutralized to form A ions, and so... 

This dissociation, or ionization, increases as the dilution increases, and in the case of strong acids, bases, and their salts the dissociation is nearly complete even in moderately concentrated solutions. The percentage of dissociation of some familiar substances (normal solutions at 18°) is shown in the following —... 

Fortunately, there is such a method, which is both simple and generally applicable, even to mixtures of polyprotic acids and bases. It is based on the fact that we have available a closed-form mathematical expression for the progress of the titration. We can simply compare the experimental data with an appropriate theoretical curve in which the unknown parameters (the sample concentration, and perhaps also the dissociation constant) are treated as variables. By trial and error we can then find values for those variables that will minimize the sum of the squares of the differences between the theoretical and the experimental curve. In other words, we use a least-squares criterion to fit a theoretical curve to the experimental data, using the entire data set. Here we will demonstrate this method for the same system that we have used so far the titration of a single monoprotic acid with a single, strong monoprotic base. 

As repeatedly mentioned in the previous parts, the correctness of the constants obtained for the Lux acid-base equilibria depends essentially on the correctness of the assumption about the completeness of dissociation under the experimental conditions of the strong Lux bases, which are used for potentiometric titration of the studied acids and for calibration of the potentiometric cell with the indicator oxygen electrode. For the latter factor, the accordance to the corresponding Nernst equation of the potentialdetermining process at the indicator electrode is less important than the completeness of the oxide-ion donor s dissociation with the formation of O2-. 

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