Basic strength aliphatic amines

The existence of the alkylamines was predicted by J. von Liebig,1 1837 in 1849, A. Wurtz obtained the primary alkylamines and in 1851, A. W. Hofmann prepared the primary, secondary, and tertiary alkylamines, and also the alkylammonium bases or the quarternary ammonium bases in which all four atoms of the ammonium group are replaced by the alkyl radicles. The basic character of ammonia is retained by the amines, and since the alkyl radicles are themselves basic, the aliphatic amines are even more basic than ammonia itself. W. Ostwald made an estimate of the basicity of the ethyl ammonias, and found the relative strengths of these bases ... 

Since the discovery of the exceptional basicity of l,8-bis(dimethylamino)-naphthalene (Alder et al., 1968) and its unusual kinetic behaviour (Hibbert, 1973, 1974, 1975) there has been considerable interest in the acid-base properties of hindered diaminonaphthalenes. 1,8-Bis(dimethylamino)naph-thalene (pK — 12.1 at 25°C and ionic strength 0.1 mol dm-3) (Alder et al., 1968 Hibbert, 1974 Chiang et al., 1980) is more basic than most aliphatic amines and the pAT-values of the partially methylated diamines [52] illustrate the dramatic effect of introducing the fourth methyl group (Alder et al., 1968). Reaction of protonated l,8-bis(dimethylamino)naphthalene with... 

Arylamines. Arylamines like aliphatic amines are basic. A lone pair of nonbonding electrons on nitrogen can bond to acids, yielding an arylam-monium salt. Base strength of arylamines are lower than aliphatic amines. A stronger base corresponds to a less acidic ammonium ion (higher pK. A weaker base corresponds to a more acidic ammonium ion (lower pK. ... 

The chemistry of amines is dominated by the lone-pair electrons on nitrogen, which makes amines both basic and nucleophilic. The base strength of arylamines is generally lower than that of aliphatic amines because the nitrogen lone-pair electrons are delocalized by interaction with the aromatic tt system. Electron-withdrawing substituents on the aromatic ring further weaken the basicity of a substituted aniline, while electron-donating substituents increase basicity. 

In addition to the determination of total base, it is also possible to titrate mixtures. This can be done in two ways. One is to titrate mixtures based on the type of amines present. For example, one can distinguish between primary, secondary, and tertiary amines. This is done simply. Acetylate the primary and secondary amines in the mixture with acetic anhydride. They are converted to amides which are only weakly basic. Tertiary amines are not affected and titrate very well. A further differentiation can be made, however. The primary amine can be reacted with salicyl aldehyde to form a Schiff base. The secondary and tertiary amines are unaffected as far as basic strength is concerned, so that one can titrate the sum of secondary and tertiary amines. By these two titrations plus a determination of total amine, one can resolve the mixture. This approach works well for aliphatic amines, but not for aromatic amines. 

Another method of analyzing a mixture of bases is to utilize the difference in the basicity of its components. As an example, let us take a mixture of aromatic and aliphatic amines. Since aliphatic amines are more strongly basic, one would expect to get a titration curve with two breaks, one for the aliphatic amine and one for the aromatic amine. However, you must not use glacial acetic acid for this titration because you will get a curve similar to curve B in Fig. 1. In other words, you get one potentiometric end point for the sum of the two. The reason is that glacial acetic acid reacts with aliphatic amines to form the acetate ion, which has about the same basic strength as the aromatic amine. Glacial acetic acid levels these two amines to the same strength. What you have to do is employ a nonaqueous solvent like acetonitrile and titrate with perchloric acid dissolved in dioxane. If you do this. 

Pyridine is a much weaker base than the aliphatic amines (pA"a 10), a fact which has been attributed to the greater s character of the nitrogen lone-pair electrons in pyridine. Electron-releasing substituents increase the basic strength, electron-attracting ones decrease it. 

Solution of sulfur in very pure aliphatic tertiary amines produces only a moderate amount of heat and the solutions contain very few ions. However, the color of sulfur is a little more intense. The enhancement depends upon the base strength (trialkylamines>4-picoline> pyridine) of the amine. Steric requirements can reverse the basicity order (pyridine>2,6-lutidine). The most consistent explanation involves a contact charge-transfer complex which denotes the electrons on the nitrogen only as the light wave passes... 

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