Base strength amines

Basicity constant Ki, (Section 1 14) A measure of base strength especially of amines... 

Most organic compounds are bases, that is, they are capable of accepting a proton. The best-studied organic bases are the moderately strong ones, which will receive a proton in dilute aqueous solutions amines are the most important examples. The pKa value of the protonated base, referred to the infinitely dilute aqueous solution, is the usual measure of base strength, and the pH of the solution is a quantitative measure of solvent acidity, or ability to transfer a proton. 

Recently Stamhuis et al. (33) have determined the base strengths of morpholine, piperidine, and pyrrolidine enamines of isobutyraldehyde in aqueous solutions by kinetic, potentiometric, and spectroscopic methods at 25° and found that these enamines are 200-1000 times weaker bases than the secondary amines from which they are formed and 30-200 times less basic than the corresponding saturated tertiary enamines. The baseweakening effect has been attributed to the electron-withdrawing inductive effect of the double bond and the overlap of the electron pair on the nitrogen atom with the tt electrons of the double bond. It was pointed out that the kinetic protonation in the hydrolysis of these enamines occurs at the nitrogen atom, whereas the protonation under thermodynamic control takes place at the -carbon atom, which is, however, dependent upon the pH of the solution (84,85). The measurement of base strengths of enamines in chloroform solution show that they are 10-30 times weaker bases than the secondary amines from which they are derived (4,86). 

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. 

Again, at its simplest, allow 5 to 7 ppm of 40% strength amine (any single amine or blended material) for every 1 ppm of C02 loading. For most applications, theory seldom matches practice, and the first feed rate calculation should be considered as simply a reference point to establish a base condensate pH level. 

W Williams, GT Young. Amino acids and peptides. XXXV. Effect of solvent on the rates of racemization and coupling of acylamino acid p-nitrophenyl esters. Base strengths of amines in organic solvents, and related investigations. J Chem Soc Perkin Trans 1 1194, 1972. 

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... 

The lone pair of electrons on the N atom of amines accounts for their base strength and nucleophilicity. They abstract protons from water, react with Lewis acids, and attack electrophilic sites such as carbonyl carbon. 

Since OCH3 is electron-donating through extended n bonding, it increases electron density on N and the base strength of the amine because the ring accepts less electron density from N. 

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]. 

Solvent effects and adduct formation of [VO(acac)2l and other [VO(j8-diketonato)2] complexes have been studied by several methods361,366,52 -532 and in coordinating solvents [VO(acac)2] is known to add a sixth ligand according to equation (36). Older reports include a spectrophotometric and calorimetric study of [VO(acac)2] and [VO(tfacac)2] adducts.521 With [VO(acac)2] in nitrobenzene, the enthalpy change for reaction (36) ranges from 44.3 kJ mol-1 for n-decylamine to 24.3 kJ mol-1 for methanol. Equilibrium constants K were between 1000 and —0.6. [VO(acac)2] is not a sensitive indicator of relative base strength.521 For [VO(acac)2], A0 decreases by —3 G (and go increases by —0.0004) as amine adducts are formed.522... 

A straightforward treatment of the base strength of aniline can be given. A saturated aliphatic amine such as methylamine has a base constant Kb of about 5 X 10 4, corresponding to the reaction H H... 

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. 

Exercise 23-11 Draw atomic-orbital models for benzenamine and its conjugate acid and describe the features of these models that account for the low base strength of benzenamine relative to saturated amines. 

Substantial differences in base strength are found between alkanamines and unsaturated amines that have the group C=N —. An example is azabenzene (pyridine, C3H5N), which is a nitrogen analog of benzene ... 

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