Triphenylamine, basicity

Various basic substances, such as aromatic amines (naphthyl-amines dissolve with difficulty in dil. HCl, diphenylamine only in cone. HCl, triphenylamine insoluble) nitro-anilines some amino-carboxylic acids. 

Conjugation of the amino group of an aiylfflnine with a second aiomatic ring, then a thud, reduces its basicity even further. Diphenylamine is 6300 times less basic than aniline, whereas triphenylamine is scarcely a base at all, being estimated as 10 ° times less basic than aniline and 10 " times less basic than ammonia. 

The aniline molecule is thus stabilised with respect to the anilinium cation, and it is therefore energetically unprofitable for aniline to take up a proton it thus functions as a base with the utmost reluctance (p Ka = 4 62, compared with cyclohexylamine, pKa = 10-68).Thebase-weakening effect is naturally more pronounced when further phenyl groups are introduced on the nitrogen atom thus diphenylamine, Ph2NH, is an extremely weak base (pKa = 0-8), while triphenylamine, Ph3N, is by ordinary standards not basic at all. 

Some apparent anomalies in Table XII are the divergent results of reactions 1 and 4. In reaction 1 the alkyl sodium was generated in situ and the difference in the result may be due to the reaction of something other than the alkyl sodium. The exclusively meta dimetallation reported for reaction 4 is also anomalous since a purely inductive deactivation of the ring should give at least some of the para isomer. Another anomaly is the meta orientation in reaction 8. The nitrogen of triphenylamine is not very basic, however, and might not participate in the special mechanism proposed for reaction at the ortho position. 

Phenyl dialkylamines react with nitrous acid to give nitroso compounds, the NO group entering the p-position, if this is free, to the basic group. These nitroso compounds, generally blue or green in colour, give yellow or red salts with mineral acids. Purely aromatic tertiary amines, e.g., triphenylamine, do not yield such nitroso compounds. 

Substitution by an aromatic group has a marked effect on solution base strength. One might be tempted to attribute the low basicity of aniline, di- and triphenylamines, and phenol compared with reference compounds (Table 3.17) to partial delocalization of the nonbonded electron pair on the nitrogen or oxygen into the n orbital system of the ring. But gas-phase results indicate the basicity... 

In diphenylamine and finally triphenylamine the basicity is still lower, so that this last amine only forms salts with the strongest acids of all, such as HC104. A condition for maximum resonance is a completely plane structure, which can, however, not be satisfied in the last-mentioned substance. This is also the case with dimethyl 0-toluidine, which is a weaker base than dimethylaniline, while 0-toluidine has practically the same strength as aniline. 

The gas-phase basicity of triphenylamine (876.4 kJ/mol) is less than that of triphenylphosphine (940.4 kJ/mol). Do you expect electronic or steric factors to contribute more to this difference Explain. (Data from C. Laurence and J.-F. Gal, Lewis Basicity and Affinity Scales Data and Measurement, John Wiley and Sons, United Kingdom, 2010, p. 5.)... 

Purely aromatic tertiary amines, e.g. triphenylamine, do not dissolve in water or in dilute acids, but dissolve in concentrated sulphuric acid. They are almost devoid of basic properties and so do not give quaternary ammonium iodides (see below). They are not often encountered, the common tertiary amines being of the classes ... 

Attempts failed to prepare 9-azatriptycene (304) analogously from acridine or acridine A -oxide. However, the targeted compound can be readily obtained, in 54% yield, by treating 9-(2-chlorophenyl)-9,10-dihydroacridine with potassium amide for 90 min in refluxing liquid ammonia (Scheme 1-237). 9-Azatriptycene (304) shows all attributes of a basic amine. For example, it combines with acids and with methyl iodide, whereas the noncaged analogue triphenylamine even resists protonation by concentrated sulfuric acid. 

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