Stability of anions

The use of liquid salts, based on anodi-cally stable cations such as 1,2-dimethyl-3-propylimidazolium (Dmpi) without added solvent, allows the investigation of the electrochemical stability of anions [75],... 

Substitution of anions by electron-withdrawing substituents, such as F or CF3,increases the anodic stability of anions [78,82,139] Figure7 shows an example. 

The anodic stability of anions on glassy carbon is the following order BR4 < C104 [Pg.476]

Lithiation of alkyl groups is also possible and again a combination of donor chelation and polar stabilization of anionic character is required. Amides and carbamates can be lithiated a to the nitrogen. 

The introduction of powerful electron-withdrawing substituents (NO2, CN, a polynitrogen heterocycle) at the a-C atom of AN stabilize anions of AN, thus facilitating their generation. At the same time, stabilization of anions of AN leads to a decrease in their reactivity and such anions act as milder nucleophilic agents. It will be seen from the following that this leads to an increase in the contribution of C-alkylation. 

In contrast to the high stability of anionic species, cations possessing a positive charge on the -conjugated system of a fullerene cage are uncommon. The chemistry of such cations have remained unexplored for nearly a decade, since C6o became available in macroscopic quantities in 1990 (70), in spite of their potential significance in both fundamental and application studies. 

Another family of lithium salts is based on the conjugate bases of the organic superacids, where acid strength is increased because of the stabilization of anions by the strongly electron-withdrawing groups, usually perfluorinated alkyls. In these anions, the... 

Nucleic Base Acidities. The N1 and N3 Acidities of Uracil. Published in J. Am Chem. Soc. 2000, 122, 6258-6262. Our progress to date involves calculations and experiments for the determination of nucleobase activities. We have learned that there is an enormous difference in the inherent stabilities of anions at the N1 (7) and N3 (8) positions in uracil (3a, Figure 1). 

Stability of anionic compounds with such halogen equivalent groups Aould be sufficiently high. 

The following groups of compounds illustrate the profound effect that resonance delocalization has on the stability of anions and hence the acidity of the conjugate acids. To compare the acidities of these acids, the conjugate bases can be ranked according to their resonance stabilization and that ranking of anion stabilization is predictive of the acidity orders. 

The topic of the stability of anionic centers in hydrocarbon solvents was apparently first addressed by Ziegler and Gellert 282) in 1950 for ethyl- and n-butyllithium. n-Butyllithium was found to decompose at temperatures above 100 °C to yield... 

All these features are also crucial in enzyme catalysis. The role of hydrogen bonding in stabilizing reactive intermediates has been recognized (196) and experimental studies on the stabilization of anionic species are numerous (197). 

LEWIS ACID-BASE CHARACTER AND STABILITY OF ANION AND CATION RADICALS. 

In studying the relationships between functional groups and proton acidities, we will first look at carboxylic acids. As illustrated in Scheme 2.2, carboxylic acids dissociate to form protons and carboxylate anions. Furthermore, as shown in Scheme 2.3, the carboxylate anion is stabilized through two resonance forms. It is this resonance stabilization that serves as the primary driving force behind the acidic nature of carboxylic acids. Further evidence of the relationship between resonance stabilization of anions and acidity can be seen when comparing the pKa values of carboxylic acids to the pKa values of alcohols. 

Resonance, as introduced in Chapter 2, explains stability of anions and rationalizes trends in pKa values. However, resonance can also be used to rationalize the stability of cations (positively charged ions). As shown in Scheme 4.7, the stability of the cyclohepta-triene cation is explained by its resonance forms. There is, of course, another reason for the stability of the cycloheptatriene cation, which relates to the principles of aromaticity and which will not be discussed in detail in this book. 

As this chapter develops you will see other examples of the versatility of sulfur. You will see how it takes part readily in rearrangements from the simple cationic to the sigma tropic. You will see that it can be removed from organic compounds in either an oxidative or a reductive fashion. You will see that it can stabilize anions or cations on adjacent carbon atoms, and the stabilization of anions is the first main section of the chapter. 

In this chapter we shall discuss some of the rich and varied chemistry of these, and other, organosul-fur compounds. The stabilization of anions by sulfur is where we begin, and this theme runs right through the chapter. We will start with sulfides, sulfoxides, and sulfones. Sulfur has six electrons in its outer shell. As a sulfide, therefore, the sulfur atom carries two lone pairs. In a sulfoxide, one of these lone pairs is used in a bond to an oxygen atom—sulfoxides can be represented by at least two valence bond structures. The sulfur atom in a sulfone uses both of its lone pairs in bonding to oxygen, and is usually represented with two S=0 double bonds. 

Yet the attached oxygen atoms cannot be the sole reason for the stability of anions next to sulfur because the sulfide functional group also acidifies an adjacent proton quite significantly. There is some controversy over exactly why this should be, but the usual explanation is that polarization of the sulfur s 3s and 3p electrons (which are more diffuse, and therefore more polarizable, than the 2s and 2p electrons of oxygen) contributes to the stabilization. 

These reactions are also useful syntheses of vinyl phosphine oxides and of vinyl silanes. The stabilization of anions is weak—weaker than from phosphorus or sulfur—but still useful. The Wittig reagent used to make allyl silanes earlier in this chapter illustrates this point. 

The stability of anionic systems is governed by several factors (a) carbon hybridization (b) effective overall n-conjugation (c) inductive effects (d) aromatic stabilization and (e) environmental factors, e.g., ion-solvation equilibrium. 

CID methods have proven to be very useful in measuring the stabilities of clusters of bare transition metal atoms, providing many more thermochemical values than photodissociation methods. In our laboratory, we have used CID to study the cationic clusters of ten different transition metal elements, including TiJ (x=2-22),VJ (x=2-20), CrJ (x=2-21),Mn, FeJ (x=2-19),CoJ (x=2-18),NiJ (x=2—18), and CuJ the second row transition metal clusters of NbJ (x=2-ll) and the third row transition metal clusters of Taj (x=2-4). These results have been summarized and trends analyzed previously [176,177]. CID methods have also been used by Ervin et al. to measure the stabilities of anionic clusters of the coinage metals Cu (x=2-8) [178], Ag (x=2—11) [179], and Au (x=2-7) [180] and group 10 metals Pd [181] and Ptx (x=3-6) [182]. A multiple collision-in-... 

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