Amines base strength, table

Table 24.1 lists pXa values of some ammonium ions and indicates that there is a substantial range of amine basicities. Most simple alkylainines are similar in their base strength, with p/q/s for their ammonium ions in the narrow range 10 to 11. Aiylctmines, however, are considerably less basic than alkylamines, as are the heterocyclic amines pyridine and pyurole. 

Pyridine has practically the same ionization potential as benzene it does not, however, necessarily follow that the flrst ionization potentials (Table 9) relate to an electron of the highest occupied 7r-orbital. These compounds are bases, though weaker than aliphatic amines, and parallels have been noted between base strength and ease of ionization (Nakajima and Pullman, 1958 Krishna and Chowdhury, 1963), which might indicate that the flrst ionization potential is that of a lone-pair... 

Other examples of the catalytic activity of amines and other catalysts [9] are given in Tables II and III, which show that their activity is not dependent on base strength [10]. 

The base strengths of simple alkanamines usually are around Kb = 10-4 (Ka = 10-10) in water solution, and vary within perhaps a factor of 10 from ammonia to primary, secondary, and tertiary amines, as can be seen from the data in Table 23-1. Cyclohexanamine has about the same base strength as methanamine, whereas the effect on the basic nitrogen of being in a saturated ring, as in azacyclohexane, increases the base strength somewhat. 

Later work by Farkas and Strohm [121], however, related the catalytic strength of an amine to its F1/2 value, the potential at the halfneutralization point in a potentiometric titration in ethyl acetate. In this analysis a high potential indicates low basicity. The F, y 2 value should be a better measure of the basic strength of the amine as a catalyst for the isocyanate/hydroxyl reaction than pK measures of base strength, since the pK value is obtained in aqueous media. The Fjy2 values do, indeed, correlate well with catalytic effect, and explain the unusual catalytic strength of triethylene diamine, as shown in Table 10. The rate data in this table are for the reaction between phenyl isocyanate and 2-ethyl-hexanol. 

Table 7.3.2 Base strengths of some common amines...

Craven studied the reactions of phenyl and o- and p-tolyl isocyanates with aniline, o- and p-toluidine and o-chloroaniline in dioxane solution mainly in the absence of catalysts. The reactivity in these systems agreed with the classical electronic picture according to which electron donating group that increase the nucleophilicity or base strength increase the reactivity of the amine. Substituents in ortho position, particularly on the isocyanate, cause steric hindrance and reduce the reactivity (see Table XIX). 

The catalytic constants for the base-catalyzed reaction in toluene calculated according to kc = kj[c2ii] and the base strengths of the amines are summarized in Table XX and Fig. 3. It is immediately apparent that base... 

Jensen et al. [23], studied more than 60 different hydroxylated piperidine and hexahydropyridazines, and the measured stereoelectronic effects are summarized in Table 5.1. The value of each substituent is the basicity decreasing effect in pK units. As can be seen, the axial OH decreases the base strength of the amine by 0.5 and 0.2 units when it is in the p- and y-positirai, respectively, relative to the amine, while the equatorial OH decreases the pKa by 1.3 and 0.6 in those instances. Thus, the equatorial OH is significantly more electron-withdrawing than the axial OH. 

The catalytic activity of the tertiary amines generally parallels their base strength, except when steric hindrance is pronounced. Hn compounds exert much stronger catalytic effects on the reactions than do tertiary amines. This is illustrated in Table 6.7. The mechanism of catalysis by metal salts is believed to operate as follows ... 

For a series of amine catalysts, the catalytic activity generally closely parallels the base strength of the amine except when steric effects become pronounced. The latter effects are illustrated in Table 14.2 in which it is seen that activity... 

The introduction of a trimethylsilyl group into an alkyl group increases the base strength, the pKg, values of both trimethylsilyl-methylamine and di(trimethylsilylmethyl) amine being appreciably higher than the values for methylamine and dimethylamine, respectively. The effect diminishes, however, the further the trimethylsilyl group is removed from the amino group (Table 5). 

The silver complexes of amines of the general type [Ag(RNH2)] provide some features of interest in relation to the base strengths of the amines in protonic solvents. Some comparative data are shown in Table 18. Although there are slight discrepancies between the data from the different sourcesthere is an evident parallelism . 

---