Nitroalkanes, ionization

The Hel and Hell spectra of some simple nitroalkanes are depicted in Figure 17. IP values are collected in Table 16. The assignment of the characteristic IPs is analogous to those of nitromethane (Table 15). Comparison of the data reveals that the orbitals are destabilized in a manner parallel to the electron-donating power of the alkyl groups143. The gap between the first two ionizations remains essentially constant, between 0.41 and 0.44 eV, in the series. 

Kinetic and equilibrium acidities of several families of nitroalkanes have been discussed extensively in the chapter by Lewis172, where the effects of changing substituents and the nature of the base, together with the role of the solvent on rates of ionization and equilibria, have been considered. 

Table 18 shows the free energies and enthalpies of ionization of some nitroalkanes. 

TABLE 18. Thermodynamic properties for ionization of nitroalkanes in DMSO at 25 °C... 

Here we are concerned mainly with alkylamines. Detailed SCF charge analyses [43] indicate that any gain in total charge translates into a downfield shift, which is the trend exhibited by alkylamines, but also by nitroalkanes and isonitriles. Examples are offered in Table 6.5 (see also Fig. 6.6), along with pertinent ionization potentials, indicated in kcal/mol ... 

The /V-(2,6-dimethy 1-4-oxopyridin-1 -yl)pyridinium salts (15)24 have proved to be versatile intermediates for the regiospecific synthesis of 4-substituted pyridines (17) via attack by the appropriate carbon nucleophiles, e.g. ionized ketones,90 nitroalkanes,91 esters and nitriles,92 and a-diketones, a-keto esters, a-diesters, disulfones etc. (Scheme 10).93 Aromatization of the intermediate 1,4-dihydro adduct (16) was generally achieved under free radical conditions. 

Evans, A. G., et al. Ionization of triphenyl methyl chloride with nitroalkanes. Trans. Faraday Soc. 47, 711 (1951). 

Salts of nitro alkanes show a considerable deviation from the non-ionized nitroalkanes. Novikov and co-workers [7] give the frequencies of mononitro anion as follows ... 

Simple nonconjugate alkyl halides participate in the reaction according to the same pattern. The naked nitrite ion reacts with alkyl halides giving only nitroalkanes (103). On the contrary, the use of AgNOj usually leads to nitrite esters as the softer nitrogen center is tied up by Ag . The cation also assists ionization of the alkylating agents when the latter are halides. 

A good example of behaviour of this kind is provided by the ionization of carbonyl compounds (equation 6), to which additional data for proton transfer from nitroalkanes to various bases may be added [13, 14]. The isotope effect on these reactions rises to a maximum of 10 at AG° = 0, just where the Bronsted exponent is one-half, and it falls off to considerably lower values on either side of this point. The endothermic side of AG° = 0 is particularly well documented here kyjk-o drops to about 3 when AG° 25 kcal mole" and the Bronsted exponent becomes ca. 0.9. Aromatic hydrogen exchange shows a similar correspondence between isotope effect [15] and Bronsted exponent [16], and additional examples may be found in the hydrolysis of vinyl ethers [17] and diazocompounds [18], as well as in the diazo-coupling reaction [19]. 

It is convenient, for the purpose of seeing how this difference in charge type operates, to view the aromatic protonation reaction in the reverse direction. The process then becomes, just like the ionization of a nitroalkane or of a carbonyl compound, a proton transfer from saturated carbon situated next to some group Z into which the electron pair left behind can delocalize (equation 11). In the reaction... 

An invariance of kyjk y for very large ranges of reactivity and equilibrium constant is clearly not implicit in Westheimer s treatment. Further consideration of this behaviour is postponed, but an additional difficulty with the ionization of ketones and nitroalkanes should be emphasized. Although a broad correlation exists between /chAd and ApK, there is essentially no correlation between ApX and reactivity spanning the full range of substrates considered [52, 66]. This represents a departure from the Hammond postulate, and indicates that either the rates or the equilibria, or both, fail to reflect the extent of proton transfer in the transition state. Almost certainly this is a contributing factor to the scatter in Figure 6, but it may also be responsible for more systematic departures from normal behaviour. 

Bronsted exponents for the ionization of a series of nitroalkanes and ketones, measured mainly by Bell and his collaborators [48, 55], are shown in Table 6. Although a common set of bases was not used in all cases, the bases were confined to carboxylate anions. The values are compared with isotope effects determined for ionization of the substrates with H2O as the base [48,55], and it is apparent that kff/ko increases steadily with )8, with only ethyl acetoacetate and acetylcyclohexanone out of line, and that the correlation with p is better than that with either the reactivity or pK of the substrate. The isotope effects are rather small because water is a weak base and in most cases the transition states should be strongly asymmetric in structure. Moreover because )8 refers to the much stronger carboxylate anions the absence of an isotope maximum at a = 0.5 is not surprising [48]. As would be expected, available measurements for carboxylate anions give large values of k k, but the results are too fragmentary to permit any further conclusion. 

Isotope effects and Bronsted exponents for the ionization of ketones and nitroalkanes ... 

II.8 Intramolecular catalysis in enolization reactions and in ionization reactions of nitroalkanes... 

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