Carbanions relative basicity

In the previous section, anions of the same basicity in (CH3)2SO solution showed a different order of reactivities when the anions attacked hydrogen than when they attacked carbon. The major difference was an enhanced nucleophilicity for carbanions when forming a bond to carbon. This difference may have a thermodynamic origin because, in the gas phase, carbanions have an enhanced carbon basicity, relative to nitranions and oxanions, at the same hydrogen basicity. Although the rates (nucleophilicities) of the SN2 reactions for different donor-atom anions in (CH3)2SO were compared at the same hydrogen basicity (pKHA), the order is not intrinsic, that is, a nucleophilic order where the rates have been adjusted for AG° differences. Indeed, when the n-CH3(CH2)3Cl SN2 reactivities are adjusted for estimated... 

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

The basic principle of all diazotizations of aromatic amines with a hydroxy- or a sulfonamido group in the 4-position relative to the amino group involves a deprotonation of the OH or NH group, respectively, after diazotization of the amino group. There is also a case of a deprotonation of a CH group in the 4-position of an aniline derivative, namely in the diazotization of 4-aminophenylmalononitrile (2.41) which, by the sequence of steps shown in Scheme 2-23, yields 3-diazo-6-dicyanomethylene-1,4-cyclohexadienone (2.42), as found by Hartzler (1964). This product can also be represented by a zwitterionic carbanion-diazonium mesomeric structure. 

Considering the stepwise mechanism, different possibilities arise depending on the relative values for the rate constants ki, k i and k2- The application of the steady state condition to the carbanion yields equation (6) for the rate constant of the basic catalysis process. 

Base catalyzed condensation reactions of esters and ketones have an additional factor of importance in determining the product, and this is the fact that the overall reaction, as well as the intermediate steps, is highly reversible. The final product may be rate or equilibrium determined, and in the latter case the result may depend on the relative acidity of the various possible products. In a highly basic medium the product will be partly in the form of a salt and the stability of the salt is then a product-determining factor. Failure of a condensation to take place may be due either to an insufficiently high concentration of carbanions or to the instability of the product. The reactions of ethyl isobutyrate will illustrate both points.419... 

When a catalytic amount of base is used, the reaction proceeds with thermodynamic control of enolate formation. The most effective nucleophiles under these conditions are carbanions derived from relatively acidic compounds such as /i-kctocstcrs or malonate esters. The adduct anions are more basic and are protonated under the reaction conditions. Scheme 1.11 provides some examples. 

Compounds with a high HOMO and LUMO (Figure 5.5c) tend to be stable to selfreaction but are chemically reactive as Lewis bases and nucleophiles. The higher the HOMO, the more reactive. Carbanions, with HOMO near a, are the most powerful bases and nucleophiles, followed by amides and alkoxides. The neutral nitrogen (amines, heteroaromatics) and oxygen bases (water, alcohols, ethers, and carbonyls) will only react with relatively strong Lewis acids. Extensive tabulations of gas-phase basicities or proton affinities (i.e., —AG° of protonation) exist [109, 110]. These will be discussed in subsequent chapters. 

The reactions of carbanions show them to be very soft bases (relative to OH- for example) and consequently they react more rapidly than most nucleophiles with both soft and hard acids. Thus carbanions are more basic than OH- and react preferentially with carbonyl centres in the presence of hydroxide ions, e. g. in the Claisen condensation. 

Formaldehyde possessing a strongly polarised carbonyl bond is very susceptible to either cationic or anionic addition polymerization. With a basic initiator the active centre is an alkoxide ion in contrast to the carbanion of a vinyl monomer. The readiness with which polymerization occurs makes initiation by relatively weak covalent bases possible. 

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