Basic strengths, relative order

The bipyridines are dibasic, and the two acid dissociation constants Ki and K2, for all the bipyridines have been determined. Typical values are recorded in Table I. There has been considerable interest in the first dissociation constants Ki of 2,2 -bipyridine and substituted 2,2 -bipyridines because of their use as metal complexing agents. In general, the order of relative basic strengths of derivatives of 2,2 -bipyridine is as expected. Electron-attracting substituents reduce the basicity, whereas electron-donating substituents increase the basicity of the molecule. " The dissociation constants of several substituted bipyridines correlate well with the Hammet equation. 2,2 -Bipyridines with an electron-donating substituent at position 4 are monoprotonated at N-1 and not at... 

If we compare the acid strengths Ka) of a series of substituted benzoic acids with the strength of benzoic acid itself (Table 26-4), we see that there are considerable variations with the nature of the substituent and its ring position, ortho, meta, or para. Thus all three nitrobenzoic acids are appreciably stronger than benzoic acid in the order ortho para > meta. A methoxy substituent in the ortho or meta position has a smaller acid-strengthening effect, and in the para position decreases the acid strength relative to benzoic acid. Rate effects also are produced by different substituents, as is evident from the data in Table 26-5 for basic hydrolysis of some substituted ethyl benzoates. A nitro substituent increases the rate, whereas methyl and methoxy substituents decrease the rate relative to that of the unsubstituted ester. 

Hexafluoroantimonic acid forms salts with pentafluoropyridine, tetra-fluoropyrazine, tetrafluoropyridazine, 3,S>dichlorotrifluoropyridine, hepta-fluoroquinoline, and heptafluoroisoquinoline the P n.m.r. spectra of these salts in various solvents have been studied, and the order of basicity of Scheme 53 was deduced. The most important factor which changes the relative order of base strengths from the hydrocarbon to the fluorocaiimn series is the number of fluorine atoms vdiidi flank the ring nitrogen in eadi compound. 

The functioning of an acid depends on the presence of basic molecules to which protons can be transferred. Although the three factors discussed above have relevance also for acidic and basic electrolytes, the overriding influence on the ionisation, and conductivity, of an acid will be the basic strength of the solvent, and vice versa. In acetic acid or sulphuric acid, therefore, substances may ionise as bases which are non-electrolytes in water. In the same way, substances whose acid properties are too feeble to be evidenced in aqueous solutions can be studied, and put in relative order of acid strength, by conductance... 

The relative basicity of carbonyl oxygen atoms can be measured by studying strength of hydrogen bonding between the carbonyl compound and a hydrogen donor such as phenol. In carbon tetrachloride, values of for 1 1 complex formation for the compounds shown have been measured. Rationalize the observed order of basicity. 

Evidence that the butadiene is jr-complexed to cobalt prior to addition to the growing cation was obtained by van de Kamp (365). Aromatic jr-electron donors were found to compete with the butadiene for the cobalt in an equilibrium reaction which decreased polymerization rate. The order of jr-donor strength, K, was determined for a variety of substituted aromatics, taking butadiene as unity. Except for the inversion of mesitylene and durene, these constants are in the same order as the relative basicities determined by equilibrium studies in strong acids (84). 

The basic idea in defining the donicity is based on the work of Lindqvist (29), who obtained from calorimetric measurements an order of relative, solvent, EPD strengths toward SbClg. It was then supposed that the relative EPD properties toward different EPA units (neutral and cationic) could be predicted (at least qualitatively) from the trends observed toward SbClg. This is, indeed, true for a large number of both neutral and ionic substrates (25-28). Figure 1 shows the relationship between F chemical shifts of CF3I dissolved in various EPD solvents as a function of solvent donicity DN (30). Nucleophilic attack of an EPD at the iodine atom causes an electron shift from iodine to the... 

Thus either the acidity or basicity constant describes fully the protolysis properties of an acid-base pair. The stronger the acidity of an acid, the weaker the basicity of its conjugate base, and vice versa. For illustration purposes Table 3.2 lists a series of acids and bases in the order of their relative strength. 

In contrast to these gas-phase findings, the oxygen protonation was found to be favoured in various solutions . The influence of the solvent is known to be a crucial factor determining the strength of bases. In some cases, the relative basicity ordering is even reversed by external effects. 

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