Basicity of alkylamines

Predict the basicity of alkylamines, arylamines and heterocyclic amines. 

In general, there is always an attenuation in effects from gas phase to solution (slope < 1), but this has been found to be particularly large for substituent effects of the basicity of alkylamines" and pyridines in water. 

Just as aniline is much less basic than alkylamines because the unshared electron parr of nitrogen is delocalized into the tt system of the nng p nitroamlme is even less basic because the extent of this delocahzahon is greater and involves the oxygens of the nitro group... 

ALkylamines are corrosive to copper, copper-containing alloys (brass), aluminum, 2inc, 2inc alloy, and galvani2ed surfaces. Aqueous solutions of aLkylamines slowly etch glass as a consequence of the basic properties of the amines in water. Carbon or stainless steel vessels and piping have been used satisfactorily for handling aLkylamines and, as noted above, some aLkylamines can act as corrosion inhibitors in boiler appHcations. 

The small differences in basicity between fflumonia and alkylamines, and fflnong the various classes of alkylamines (primary, secondary, tertiary), come from a mix of effects. Replacing hydrogens of ammonia by alkyl groups affects both sides of the acid-base equilibrium in ways that largely cancel. 

Nonaiomatic heterocyclic compounds, piperidine, for exanple, aie similar in basicity to alkylamines. When nitrogen is part of an aromatic ring, however, its basicity decreases markedly. Pyiidine, for exanple, resembles aiylamines in being almost 1 million times less basic than piperidine. 

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. 

As noted previously, arylamines are generally less basic than alkylamines. Anilinium ion has pKa = 4.63, for instance, whereas methylammonium ion has pfCa = 10.64. Arylamines are less basic than alkylamines because the nitrogen lone-pair electrons are delocalized by interaction with the aromatic ring tt electron system and are less available for bonding to H+. In resonance terms, aryl-amines are stabilized relative to alkylamines because of their five resonance forms. 

Another fallacy to be refuted by PA data was the widespread belief that the low basicity of arylamines compared to alkylamines simply reflects delocalization of the lone pair of electrons on the nitrogen atom within the aromatic ring. Thus, the low basicity of aniline (pKa = 4.58) compared to ammonia (pKa = 9.27) is often attributed to such a conjugative... 

The gas-phase basicity order of alkylamines can be reproduced in terms of softness of the alkyl groups (using the Fukui function and local softness parameter ), this being far more important than group electronegativity58. 

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 ... 

The pathway of the Kabachnik-Fields reaction depends on the nature of the substrates. The amine and hydrophosphoryl compound form a complex in which either one of the partners may react with the carbonyl compound. Often, the basicity of the amine determines the reaction pathway. Weakly basic amines such as anilines, which can act as proton donors, favour the formation cf an imine, whereas alkylamines such as cyclohexylamines do not form imines ... 

As soon as the first SMA was prepared, the question arose as to the structure and consequently of the basicity of this new type of amine. As is true for N-alkylamines, it could be expected that the silylmethyl group would make MSMA more basic than ammonia through its +1 effect. Conversely, the ability of a silicon atom to accept electrons from a nucleophile, as is the case with silatranes and o-silylbenzylamines,20... 

The apparent basicity anomaly of alkylamines can now be understood in terms of two opposing influences, one base-strengthening (due to increasing alkylation of the amine), and the other base-weakening (because of reduced solvation of the ammonium ions with increasing alkylation). For solvation effects on the basicities of alkyl amines in solvents other than water, see reference [243]. 

As shown in Table 24.1, pyridine (pK = 5.25) is a stronger base than pyrrol hut a weaker base than alkylamines. The diminished basicity of pyridine com pared with an alkylaminc is due to the fact that the lone-pair electrons on tin pyridine nitrogen are in an sp orbital, while those on an alkylamine n trogei are in an sp- orbital. Because s orbitals have their maximum electron density a the nucleus but p orbitals have a node at the nucleus, electrons in an orbital wit) more. s character are held more closely to the positively charged nucleus and art less available for bonding. As a result, the hybridized nitrogen atom V character) in pyridine is less basic than the. s/Ahybridized nitrogen in an alkylamine (25% s character). 

To compare the basicity of an alkylamine (RNH2) and an amide (RCONH2), we must once again compare the availability of the nonbonded electron pair on nitrogen. With RNH2, the electron pair is localized on the N atom. With an amide, however, the electron pair is delocalized on the carbonyl oxygen by resonance. This decreases the electron density on N, making an amide much less basic than an alkylamine. 

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