Anilinium cations

Aniline will be used as a second example. It has a pK of 4.60 at 25° in H2O. If it is placed in aqueous solution at pH 1.60 it will exist almost completely (99.9%) as the anilinium cation. This solution can then be extracted with solvents e.g. diethyl ether to remove neutral impurities. The pH of the solution is then adjusted to 7.60 whereby aniline will exist as the free base (99.9%) and can be extracted into diethyl ether in order to give purer aniline. 

If aniline is protonated, any such interaction, with resultant stabilisation, in the anilinium cation (14) is prohibited, as the electron pair on N is no longer available ... 

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. 

The surface is initially covered by the BARF(-) anion and by an anilinium(+) cation and a floating cation is grafted here the coordination of the floating cation to surface oxygens should be avoided. Indeed results indicate the complexity of the surface. 

Figure 11.9 Sorption isotherms for some charged organic compounds interacting with natural solids (a) quinolinium cation on a subsoil of /oc = 0.024 and cation exchange capacity of 84 mmol/kg (Zachara et al., 1986), ( >) anilinium cation on a surface soil with /oc = 0.013 and cation exchange capacity of 112 mmol/kg (Lee et al., 1997), and (c) sorption of 4-(2,4-dichloro-phe-noxy)-butyrate anion on a sediment with/oc = 0.015 and unknown anion exchange capacity (Jafvert, 1990).

It is also possible to prepare chiral PANI by in situ polymerisation with CSA, and in this case the reaction can afford chiral nanotubes [63]. The optically active materials contain nanotubes with 80 to 200 nm outer diameter and an internal diameter of between 20 and 40 nm, as revealed through microscopy images. A self-assembly process was proposed in which anilinium cations and CSA anions form micelles which act as templates for the growing polymer chains. Nanotubes are also formed when (R)- or (S)-2-pyrrolidone-... 

The previous argument would suggest that in even less basic anilines the reaction of the nitrating acid should be exclusively directed towards the small fraction of unprotonated amine. This would mean that anilines of this type would be nitrated in ortho- and/or /mra-position instead of the meta-position. This idea is illustrated in the nitration of sulfanilic acid (Figure 5.22), which proceeds ortho to the H2N group and meta to the S03H of the free amine B instead of meta to the H3N group of the anilinium cation D. 

The five parent compounds in Table III are arranged in order of increasing pKa of their ionizable protonic groups. For phenoxyacetic acid (pKa = 3.17) and phenylacetic acid (pKa = 4.31), the primary ionization is that of the carboxylic acid side chain. The acidity of the TFMS parent compound (pKa = 4.45) is attributable to the loss of the relatively labile proton from the parent side chain (< -NH-SOo-CF3 < -N -S02-CF3 -f- H+). For aniline, the process with pKa = 4.63 is associated with protonic ionization of the anilinium cation. The pKa = 9.89 process in phenol refers to the formation of phenolate anion. 

Conversion of an acidic carbon (Cl) to a basic carbon (C3—E) increases the point of zero charge from 3 to 10, and this is detrimental for the adsorption of anilinium cations at low pH but it also enhances the n electron density in the graphene layers and thus increases the dispersive potential, say. 

Mykhalichko, B. M. Copper(l) acetylenide complexes, synthesis and structure of cluster p compound (AnH)2[Cu4Cl6(CCCH20H)] (AnH = anilinium cation). Russ. J. Coord. Chem. (Translation of Koordinatsionnaya Khimiya) 1999, 25, 336-341. 

According to the Brpnsted definition, the acidity of a molecule is associated with its capacity to give up a proton Ph—NH2 — Ph—NH +H+. The change of standard enthalpy or free energy of this deprotonation reaction is a measure of the intrinsic acidity. As discussed above, in solution, the propensity of an aniline derivative is to accept a proton. The measured dissociation constant (pATa) is related to the basicity of the neutral molecule (or the acidity of the anilinium cations). As a consequence, relatively little is known about their acidity and/or the anilinide anions. However, the NH acidities have been well established in hydroxamic acids even though the latter usually behave as O-acids134. It is therefore of interest to get some insight into the deprotonation of aniline in the gas phase. 

H. Nakajima and G. Matsubayashi, Intercation/polymerization of the anilinium cation in the VOPO4 interlayer space, Chem. Lett., 22,423-426 (1993). 

Substituent (X) Phenoxide anion Anilide anion Anilinium cation ... 

Mulliken Charges and Overlap Populations for Substituted Anilinium Cations... 

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