Anion basicity

According to the principle of extraction, the organic extractants can be classified into four classes (i) solvating (neutral) (ii) cationic (acidic) (iii) anionic (basic) and (iv) chelating. The trade names and chemical names of the various extractants have been presented in the figures illustrating the classification. 

Any hydrophobe can yield each of the main (i.e. anionic, cationic, nonionic or amphoteric) types of surfactant in much the same way as the same chromogenic system can be used in anionic, basic or disperse dyes. This will be demonstrated in the following sections, dealing with each class of surfactant, using the cetyl-containing (C16H33) hydrophobe. 

In solution, lithium hexamethyldisilazide (LiHMDS) is a strong enough base to deprotonate esters, ketones, and alcohols, with a pK of about 27 in DMSO solvent. In the gas phase, the bare anion is too weak to deprotonate methanethiol, much less the ketones, esters, and comparable carbon acids. The change in relative anionic basicity is on the order of 14 kcal/mol. 

Bruice and Schmir (3) have shown that for a series of imidazole derivatives, klm depends on the base strength of the catalyst and since pKA is an approximate measure of base strength, the value of klm should increase with increase in pKA. Table I shows that this is indeed the case. Imidazole, pKA = 7.08, has a catalytic constant eight times larger than that of benzimidazole, pKA = 5.53. Bronsted and Guggenheim (2) have obtained a linear relationship between log k/ and pKA for a series of carboxylic acids in the pKA range of 2 to 5, where kB is the carboxvlate anion basic catalytic constant for the mutarotation of glucose and Ka is the acid dissociation constant of the acid. Our results for imidazole and benzimidazole fit fairly well into the Bronsted plot. 

This somewhat simplified picture of possible transitions from one mechanism to another can be expanded and supplemented by a finer differentiation of the factors influencing bond strength and catalyst acid-base properties. Such structural parameters are the number and nature of substituents on Ca and Cp and the nature of the group X. The action of a catalyst depends on its cation charge and radius, on anion basicity and on lattice and surface arrangement (for some details see ref. 67). A temperature increase usually shifts the mechanism in the direction of El. 

Good linear correlations of BE with gas-phase basicities of the anionic Lewis bases (the analysis included not only the halides F, CL, Br and F, but also other anions such as CN , N02, H , HCT, CH3 and CF3) were observed for a few Lewis acids (C02, S02, Et3B)305. The predictive power of these plots of anion BE vs anion basicity for obtaining estimates of new thermochemical data for novel anions has been pointed out305. 

Use mechanisms to show how monomers polymerize under acidic, basic, or free-radical conditions. For chain-growth polymerization, determine whether the reactive end is more stable as a cation (acidic conditions), anion (basic conditions), or free radical (radical initiator). For step-growth polymerization, consider the mechanism of the condensation. 

However, reaction of acyclic dienamines with hydrazoic acid gives a mixture of products derived by 1,2-, 1,4- and 3,4 + 1,2-addition of HN3 to the diene system. In this case C-protonation is followed immediately by addition of the strongly nucleophilic azide anion, so that equilibrium of the C-protonated enamines cannot occur3c. Treatment of the morpholine dienamine of isophorone with trichloroacetic acid in boiling benzene resulted in decarboxylation and the 1,4-addition of a proton and the trichloromethyl anion. Basic hydrolysis of the adduct gave dienoic acid 54 (Scheme 4). 

Nitrostyrenes are easily polymerized anionically, basically due to the strong electron-withdrawing effect of the nitro group on the double bond, both by induction and resonance effects. The double bond is electron-deficient and susceptible to nucleophilic attack. Carter et al. [117] synthesized and polymerized a number of substituted nitrostyrenes and measured their reactivity. Their results are summarized in Table 6. 

Lewis acids, should therefore be used in carbocationic polymerizations to prevent /3-proton abstraction. Hexafluoroantimonate is probably the least basic and least nucleophilic counteranion, whereas the conjugate bases of oxy acids are more basic. Anion basicity, which correlates reciprocally with the acid strength, decreases in the order HOCIO3 = HO-S02CF3 > H0S02C1 > HI > HBr > S02(0H)2 > HC1 > H0(0)CCH3. As discussed in Section B.l, triflate, and perchlorate anions are the least basic and least nucleophilic of the oxy-acid anions. 

Esters are hydrolyzed in aqueous base to form carboxylate anions. Basic hydrolysis of an ester is called saponification. 

In conclusion, the comparison of the experimental methods (HS-GC, H NMR) shows that, with regard to the anion s interaction with alcohols, excellent correlation in the relative order of the anions is found [74, 92], which also reflects the trends for anion basicity established using solvatochromic dyes [17]. In contrast, quite diverging information on cation interactions are obtained by HS-GC and II NMR, which may be due to a lower sensitivity of HS-GC towards such interactions. Furthermore, the lack of correlation to other cation-dependent studies, e.g. ESI-MS, is still inconclusive [28],... 

Kinetics Proton transfer catalyzes many reactions. Proton transfer between heteroatom lone pairs is very fast, often at the diffusion-controlled limit. Under reversible (equilibrium) conditions, the most acidic proton is removed preferentially. However, if the deprotonation is done under irreversible conditions, the proton removed is determined by kinetics, not thermodynamics (Section 9.3). Anion basicity always competes with nucleophilicity. Proton transfer is slow enough between organometallics and protons adjacent to carbonyls (carbon bases with carbon acids) that addition of the organometallic to the carbonyl is the dominant process, path AdN. 

Suba C, Niyazymbetov ME, Evrms DH (1997) Addition of electrochemicaUy-generated anions to aldehydes emd olefins effect of reaction medium and anion basicity. Electrochim Acta 42 2247-2255... 

Why are most anions basic in H2O Give formulas of four anions that are not basic. 

Complexes with carboxylates and related ligands Complexes with inorganic oxo anions Basic salts of mercury (II)... 

Of course, the intrinsic presence of neutral molecules in pure ionic liquids becomes more and more relevant if the equilibrium constant is less and less in favor of the ionic compound. For the N-protonated l-alkyUmidazolium salts the amount of neutral, unprotonated alkylimidazole is obviously a function of the anion s basicity. Here, the wide range of potential anion basicities has provoked the question as to whether these systems should at all be considered as ionic liquids. However, some convincing applications of these substances in catalysis [28] and organic synthesis [29] (see BASIL process in Chapter 9) and physical measurements [30] have led to the now widely accepted view that at least combinations of 1-alkylimidazoles with strong acids can be regarded as ionic liquids. 

The negatively charged base reacts with the arylpalladium(II) halide to give the arylpalladium hydroxide or alkoxide complex, which is able to form the dimeric palladium-boron complex XXIII what is crucial for the transmetallation process [2-6]. It is apparent that the metal cation (from the base) accelerates the formation of the latter, as clearly showed by Zhang and coworkers [15]. They have developed the SM coupling procedure for sterically bulky arylboronic acids when the clear influence of the anion basicity and the cation effect were discovered. The cationic radius is presumably an important parameter which influences the formation of dimeric... 

For weak bases having neutral acidic and anionic basic species (or weak bases having cationic acidic and neutral basic species), increases with... 

Let us look at the p Ca of acetic acid pJCa 4.7 (Fig. 7.2). When pH of the solution is 7, that is, higher than pkfa, most of the acid is in the dissociated form, CH3COO . On the other hand when pH of the solution is lower than pJCa. like in this problem, pH = 3.9, most of the CH3COOH remains undissociated. It seems to me there is a lesson to be learned here the higher the pH of the medium the more dissociated (anionic, basic, deprotonated) the acid-base molecule. At lower, more acidic pH, the acid remains mostly undissociated, as acid. The sweet spot is when pfCa equals pH CHsCOO" and CH3COOH are then equal, that is, 50 50. This lesson applies nicely to the acid-base reactions of biological molecules as the next riddle tells us. 

---