Proton affinities aniline

Evaluation of the only appropriate Fukui function is required for investigating an intramolecular reaction, as local softness is merely scaling of Fukui function (as shown in Equation 12.7), and does not alter the intramolecular reactivity trend. For this type, one needs to evaluate the proper Fukui functions (/+ or / ) for the different potential sites of the substrate. For example, the Fukui function values for the C and O atoms of H2CO, shown above, predicts that O atom should be the preferred site for an electrophilic attack, whereas C atom will be open to a nucleophilic attack. Atomic Fukui function for electrophilic attack (fc ) for the ring carbon atoms has been used to study the directing ability of substituents in electrophilic substitution reaction of monosubstituted benzene [23]. In some cases, it was shown that relative electrophilicity (f+/f ) or nucleophilicity (/ /f+) indices provide better intramolecular reactivity trend [23]. For example, basicity of substituted anilines could be explained successfully using relative nucleophilicity index ( / /f 1) [23]. Note however that these parameters are not able to differentiate the preferred site of protonation in benzene derivatives, determined from the absolute proton affinities [24],... 

It is a well established fact that the preferred site of protonation of aniline in solution is the nitrogen atom, due to the higher stabilization upon solvatation of the N-protonated species with respect to the ring protonated ion (/). In the gas phase however, the situation is less clear, and the preferred site of protonation has been the subject of many experimental and theoretical studies. Based on proton transfer equilibria at 600 K and upon correlation of the proton affinities... 

Table I Computed total atomization energy at 0 K of aniline and proton affinities at 298 K for protonation at different sites. Proton affinities for N-protonation of aniline. All values are given in kcal/mol.

Now the ratio Kb for aniline/iq, for prim, aliph. amine amounts to 4.6 io 10/3 to 5 io 4 — 1.5 to 0.9 io 6. This indicates a difference in AF of 8.4 to 7.gkcal/mole (T= 298° K). Since there is no reason to assume any essential difference in AS, the proton affinity of aniline, A 7, will also be 8 kcal lower, which is practically equal to the above-mentioned extra R.E. deduced from the heat of combustion. 

TABLE 6. Calculated proton affinities (PA, kJ mol-1 at 0 K) of halogenated anilines... 

Proton affinity (PA) is the negative of the enthalpy change defined at 298 K for the protonation reaction (equation 2)6. As seen from the values of representative monosubstituted anilines (Table 1), PA values of anilines are quite high. 

TABLE 1. Ionization energies (IE) and proton affinities (PA) of aniline and selected derivatives... 

Substituent on aniline Ionization energy (eV) (method) Proton affinity (kJ mol 1)... 

The N—H N hydrogen bond is responsible for the formation of the complexes between aniline and aliphatic amines (ammonia, methylamine, dimethylamine and tri-methylamine) which act as proton acceptors. Infrared photodissociation spectra and DFT calculation indicate208 that the clusters [aniline/ammonia]+ and [aniline/methylamine]+ have a non proton transferred (without the proton donation from the aniline moiety to the amine molecule) structure, while the complexes [aniline/dimethylamine]+, [aniline/ trimethylamine]+ possess a proton transferred structure. Reasonably, the proton transfer increases on increasing the proton affinity of the amine used as solvent. 

Hillebrand, C., Klessinger, M., Eckert-Maksic, M. and Maksic, Z.B. (1996) Theoretical model calculations of the proton affinities of aminoalkanes, aniline and pyridine. The Journal of... 

Parthaserathi et al. [238] have used the group philicity index to estimate the pK s of several phosphoric acid esters and related compounds They included the calculations in a group with anilines and alcohols, but the numerical results for the phosphoric acids appear generally good Moser, Range, and York [301] have calculated gas-phase proton affinities and basicities for a large collection of acyclic and cyclic phosphates. 

Proton affinity and protonation sites of aniline. Energetic behavior and density functional reactivity indices ... 

Both proton affinities and acidities are calculated from the difference in energy of the lowest energy conformations of both neutral and ionic species. This in itself poses a problem for systems with many atoms - how to ensure the lowest energy conformer. For example, there are five possible sites of protonation in aniline, protonation of the carbons at the ortho. 

On the other hand, in glacial acetic acid, aniline has a much greater proton affinity than does the lyate ion CH3COO" and so behaves as a strong base... 

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