Halogen substituents structural parameters

In addition to the peraryl and peralkyl derivatives already described, the crystal structures for a number of distannanes with mixed types of substituents, including halogens, have been determined (Table 1). For the most part, the geometric parameters and solid-state conformations of these compounds are in keeping with what has been presented so far. However,... 

The synthesis of a series of l//-pyrazolo[3,4-3]quinoxalines (flavazoles) 55 by acylation, alkylation, halogenation, and aminomethylation of the parent compound was reported and their structures were investigated by H, and N NMR spectroscopy <2005T2373>. Restricted rotation about the partial C-N double bond of the A -acyl derivatives was studied by dynamic NMR spectroscopy and the barriers to rotation were determined. N NMR data of a series of 3-alkyl [aryl]-substituted 5-trichloromethyl-l,2-dimethyl-l//-pyrazolium chlorides 56 (where the 3-substituents are H, Me, Et, -Pr, -Bu, -Pent, -Hex, (CH2)sC02Et, CH2Br, Ph, and 4-Br-C6H4) were reported <2002MRC182>. The N substituent chemical shift (SCS) parameters were determined and these data were compared with the SCS values and data obtained by molecular orbital (MO) calculations. 

Figure 10 Theoretical (a) and practical (b) representation of QSARs. Panel b describes a QSAR for the methanotrophic oxidation (activity of methane monooxygenase) of 6>r /i6>(Ci2)-substituted biphenyls. The structural backbone was biphenyl, and the substituents considered included all halogens, methyl-, methoxy-, hydroxyl-, nitro-, and amino-moieties (Lindner et al, 2003). The molecular descriptors used in (b) are (charge on the ortho-csubon), (Taft s steric parameter), and log ow...

They clearly indicate that the pf is not the same as solvent hydrogen bond basicity, /ii, because the pf value treats the solvent as a solute in the chemical interactions, and that the pf and fli scales are relatively collinear but not interchangeable . (The latter scale is based on the comparison of the indicators p-nitroaniline and p-nitro-Af, N-dimethylaniline.) Neither is the value connected to solute proton-transfer basicity . They have established that this pf value is relatively constant for homologous series of solvents, and that substituents on the parent structure of the solvent do not overly influence the pf value in terms of inductive or polar effects, unless the substituent is halogenated, in which case the will decrease. Chain branching of the parent also has little effect on the pf value. This makes it possible to predict average P2 values for solutes whose ATg values are not known. Correlations of kinetic data with pf are not always accurate because the pf parameter does not take into consideration solvent size, which can lead to steric hindrance of hydrogen bond formation . ... 

The effect of substituents on the rate of addition of carbanions to nitroarenes and the rate of p-elimination of HL from the o adducts have also been studied [8, 30, 31]. The former effect is an important parameter, because it is, in fact, a measure of influence of substituents on electrophilic activity of nitroaromatic rings. The effect of substituents on rate of the S Ar reactions of o- and p-halonitrobenzenes has been thoroughly studied [2, 32]. However, since the S Ar of halogen is a secondary process, the obtained data cannot be used as a real measure of electrophilicity of halonitroarenes. We have determined the effects of substituents and the ring structure on the rate of the VNS reactimi of nitroarenes with the carbanion of chloromethyl phenyl sulfone by using competitive experiments under the conditions, which assure a fast p-eUmination of HL from the o adducts [30, 31]. The values of VNS rates obtained under such conditions proved to correlate with those of the addition step. Selected values of the relative rate constants in relation to nitrobenzene as the standard are shown in Fig. 1. 

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