Hydrogen bonding basicity scale

A hydrogen-bond basicity scale has been developed that can be used to determine the relative basicity of molecules. Table 8.2 gives the p hb values for several common heteroatom containing molecules. This is obtained from the protonated form (conjugated acid) of the base in question. The larger the number, the more basic is that compound. 

Recently195, the hydrogen bond basicity scale (p.K hb as logarithm of the formation constant of 4-fluorophenol/base complexes in carbon tetrachloride, equilibrium 21) has been measured for several nitro derivatives (nitromethane, nitrobenzene, IV-nitrocamphorimine, 2-nitropropane, 4-nitro-o-xylene, 4-nitroanisole, lV,./V-diethyl-4-nitroaniline, l-dimethylamino-2-nitroethylene, l-piperidino-2-nitroethylene) ... 

TABLE 1. p Thb, Ar(OH) and hydrogen-bond basicity scales for phenols and, for comparison, water and alcohols... 

VII. CONSTRUCTION OF HYDROGEN-BOND BASICITY SCALES FROM PHENOLS... 

TABLE 15. Hydrogen-bonding basicity scale constmcted from 4-fluorophenol... 

In 1989 a log solute hydrogen-bond basicity scale was constructed for 91 bases. It was scaled to 4-nitrophenol as hydrogen-bond donor in 1,1,1-trichloroethane (equations 38 and 39) and was explicitly targeted to the needs of the medicinal chemist. To this end, measurements were made in 1,1,1-trichloroethane, a solvent considered a better model for real biological phases than the non-polar tetrachloromethane. In addition, data are given for molecules of special interest to the medicinal chemist, for example many heterocycles never before investigated. The log and pA ne scales have a similar meaning and it is not unreasonable to find a fair correspondence between 24 common values (equation 40). 

Hydrogen-Bond Basicity Scales Early Works... 

The 4-Fluorophenol Hydrogen-Bond Basicity Scale 4.3.1 Definition... 

Table 4.3 Equilibrium constants KM (I mol j and hydrogen-bond basicity scales, pKsnx SLnd AC° (kj mol ), for the complexes of 4-fluorophenol with carbon it bases, in CO4 at 25 °C. 

Table 4.7 Total equilibrium constants, Kc(t) (I mol ), and hydrogen-bond basicity scales, rKbhx anc/ 4C° (kl moH ), for the n and N complexes of4-fluorophenol with arylamines, in ecu, at 25 "C [90].

Table 4.11 Hydrogen-bond basicity scales, pKgnx and AC° at 25 °C. 

Table 4.17 Hydrogen-bond basicity scales, pKgi-ix and AC° (kj mol j, of esters, lactones and carbonates. In CCI4 at 25 °C. 

Table 4.19 Hydrogen-bond basicity scales, pKgnx and AC° (kj moH ), of nitro and nitroso compounds, in CCU at 25 °C.

Table 4.22 Hydrogen-bond basicity scales, pKgnx nd AG° (kj moi j, of oxides of organoderivatives of group 15 eiements, in CCb at 25 °C [94, 96].

Table 4.23 Hydrogen-bond basicity scales, pKbhx and AG° (kl moH ),... 

Table 4.25 Hydrogen-bond basicity scales, pKgnx a.nd AC° (kj mol ), of haloalkanes, in CCl4at 25°C[83, 88, 91],...

Chardin, A., Berthelot, M., Laurence, C. and Morris, D.G. (1995) Tributylammonium cyanami-date BusN+N CN a super-basic nitrile group more basic, on the hydrogen-bond basicity scale, than any amine or pyridine. J. Phys. Org. Chem., 8, 626-628. 

Here,. 4v(OH - OMe) is determined for three nitriles chloroacetonitrile, acetonitrile and dimethylcyanamide. The results are analysed in terms of substituent effects on the basicity of the nitrile group and compared with other hydrogen-bond basicity scales, in order to test the significance of electronic spectral shifts as basicity parameters. 

Table 7. 8 Wavenumbers (cm ) of the longest wavelength tt tt transition of 4-nitrophenol, v OH), and 4-nitroanisole, v(OMe), dissolved in chloroacetonitrile, acetonitrile and dimethylcyanamide. Corresponding hydrogen-bond shifts, Av(OH — OMe) (cm ). Comparison with other hydrogen-bond basicity scales.

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