Anilines acid-base properties

As we will see in Chapter 8, the delocalization of the unshared electron pair in aniline has an important impact on the acid/base properties of anilines as compared to aliphatic amino compounds. 

To clarify the interrelation between the acid and base properties of a solid and its catalytic efficiency, the ring opening of epoxides was investigated by the use of zeolites with different acid-base properties (25). Table VI summarizes the results for ring openings of unsymmetrical epoxides with aniline. 

The general mechanism of anodic oxidation and the main products of dimerization are similar for various C-substituted anilines, but detailed steps for a given reactant and the products distribution depend strongly on the nature and position of the substituent as well as on the medium used. It becomes comprehensible when taking into account that substituents exert a strong influence on the acid-base properties of the neutral reactants as well as on the reactivity of the electrogenerated radical cations, and that protona-tion/deprotonation steps participate in reaction schemes. 

The first topic to be dealt with in this article dates back to the early days of pulse radiolysis and is concerned with intermediates generated from organic nitro and nitroso compounds in some elementary redox processes. This will be followed by a presentation of some most recent results on aminyl radicals derived from amino acids, exemplifying the diversity of possible reactions of a seemingly simple type of radicals. The third example on aniline and aniline radicals aims to demonstrate the potential of time-resolved resonance Raman spectroscopy. A common message of all these studies on N-centered radicals hints at the importance of acid/base properties of radicals. The aniline system, in particular, also draws attention to spin and charge distribution, and possible implications to the chemistry of radical species. 

The modification of ZSM-5 with metal oxides and variation of Si/Al ratio in ZSM-5 catalysts result in ZSM-5 catalysts of different acid-base properties. In Fig. 4.5, the correlation between the aniline conversion and the acid amount is shown for Na—ZSM-5 of different Si/Al ratios. Correlation is clearly observed between the conversion and the acid amount, indicating the presence of weak acid sites being required. Besides acid sites, the presence of base sites is required. The ZSM-5 catalysts modified with MgO or CS2O which possess large quantity of basic sites show hig conversions... 

We have discussed the effect of structure on the acidity of alcohols and on the basicity of amines. The acid-base properties of their aromatic cousins, phenols and anilines, are affected by the aromatic ring. Both the pif values of phenols and the values of anilines illustrate this effect. 

ACID-BASE PROPERTIES OF PHENOLS AND ANILINES Phenols... 

Eurukawa and co-workers [81] state that PANI is an interesting material because it is not only an ECP but is also a good material to use as an electrode of a secondary battery with aqueous or non-aqueous electrolytes. PANI polymerised from aniline in an aqueous acid solution is converted to several forms with different electrical properties by acid/base treatments and oxidation/reduction. The as-polymerised form gives high electrical conductivity ( 5 S/cm). It becomes insulating when treated with an aqueous alkaline solution or is reduced electrochemically in an aqneons acid solution. Reduced-alkali-treated PANI is also insulating and is unstable in air its colour changes from white to blue upon exposure to air. PANI doped with electrolyte anions is obtained by electrochemical oxidation [82]. It was found in this work to be a new conductivity form (o = 5.8 S/cm). Recently, a secondary lithium battery with a reduced alkali pellet as the cathode, and non-aqueous electrolytes has been developed as a power source of memory back up and a maintenance-free power source combined with a solar battery. 

Separations based upon differences in the chemical properties of the components. Thus a mixture of toluene and anihne may be separated by extraction with dilute hydrochloric acid the aniline passes into the aqueous layer in the form of the salt, anihne hydrochloride, and may be recovered by neutralisation. Similarly, a mixture of phenol and toluene may be separated by treatment with dilute sodium hydroxide. The above examples are, of comse, simple apphcations of the fact that the various components fah into different solubihty groups (compare Section XI,5). Another example is the separation of a mixture of di-n-butyl ether and chlorobenzene concentrated sulphuric acid dissolves only the w-butyl other and it may be recovered from solution by dilution with water. With some classes of compounds, e.g., unsaturated compounds, concentrated sulphuric acid leads to polymerisation, sulphona-tion, etc., so that the original component cannot be recovered unchanged this solvent, therefore, possesses hmited apphcation. Phenols may be separated from acids (for example, o-cresol from benzoic acid) by a dilute solution of sodium bicarbonate the weakly acidic phenols (and also enols) are not converted into salts by this reagent and may be removed by ether extraction or by other means the acids pass into solution as the sodium salts and may be recovered after acidification. Aldehydes, e.g., benzaldehyde, may be separated from liquid hydrocarbons and other neutral, water-insoluble hquid compounds by shaking with a solution of sodium bisulphite the aldehyde forms a sohd bisulphite compound, which may be filtered off and decomposed with dilute acid or with sodium bicarbonate solution in order to recover the aldehyde. 

Several additional studies have confirmed interannular electronic interactions in ferrocene. Early investigations by Nesmeyanov and coworkers and by others demonstrated the electron-donor properties of the ferrocenyl group (71). Aminoferrocene, for example, is approximately 20 times as strong a base as aniline, while benzoic acid is several times more acidic than ferrocenecarboxylic acid. Moreover, acidities and basicities of this sort are markedly affected by substituents on the opposite cyclopentadienyl ring. 

The solvent needed to do the job is one that has appreciably stronger acid properties than water. One such solvent, in which aniline is a strong base, is liquid acetic acid or any other acidic solvent with greater acid properties than water. 

Aniline does not give water-stable salts with weak acids like acetic acid. Some aromatic amines do give crystalline acetates with acetic acid, but these are decomposed by water with separation of the free base. This property is utilized industrially in the isolation of m-xylidine from a mixture containing its isomers. Hence, free aniline separates out of an aqueous solution of an aniline salt with a mineral acid if the mineral acid anion is replaced by the acetate ion, for example, by the addition of sodium acetate in sufficient concentration. 

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