Nitrobenzene, atom polarization

As in the 1,2-dichloroethane case too, transient EMF and SHG responses to KSCN were observed for the nitrobenzene membranes without ionic sites. This suggests that here too not only SCN but also K ions are transferred into the nitrobenzene phase. Salt extraction into the bulk of the organic phase, in analogy to similar observations previously reported for neutral ionophore-incorporated liquid membranes without ionic sites [55], was indeed independently confirmed by atomic absorption spectrometry. Figure 15 shows the concentration of K in nitrobenzene equilibrated at room temperature with a 10 M aqueous solution of KSCN as a function of equilibration time. The presence of the ion exchanger TDDMA-SCN efficiently suppresses KSCN extraction into the organic phase but in its absence a substantial amount of KSCN enters the nitrobenzene phase. The trends of the EMF and the SHG responses are therefore very similar in spite of the different polarities of the plasticizers. 

Tetrabutylphosphonium hydrogen difluoride [Bu4PF (HF)] and dihydrogen trifluoride [Bu4PF (HF)2] have been shown to be expedient reagents for nucleophilic fluorination of aromatic substrates containing a chlorine or bromine atom or a nitro group under mild conditions in non polar solvents. Thus, for example l-chloro-4-nitrobenzene (28) is converted to l-fluoro-4-nitrobenzene (29) in 90% yield.219... 

Fig. 1.2. COSMO surface polarity of nitrobenzene, 4-amino-nitroben-zene, and p-dinitrobenzene (red = negative, green = neutral, blue = positive polarity). Due to intramolecular electron redistributions the oxygen atoms of the nitro group are much more polarized in 4-amino-nitrobenzene, and much less polarized in p-dinitrobenzene.

We must not overlook, however, the fact that when the field of a polar bond is sufficiently buried within the molecule its influence upon solubility tends to disappear. Thus, in spite of the polarity of the bond between the carbon atom and the nitro-group, as shown in nitrobenzene and nitromethane, we might expect tetranitromethane to behave in its solutions as a substance of low polarity. Again, stannic chloride is nonpolar but stannic fluoride, apparently on account of the smaller halogen atoms, is so polar as to form a high-melting solid. 

Activated ester method The reactivity of carboxylates and phosphates can be increased by electron drawn and polarizing compounds, l-chloro-4-nitrobenzene or chloroacetonitril are added to form a reactive ester that polarizes the 5 -phosphate group to a partially positive charge. As noted above, the positivated phosphor atom is susceptible to a nucleophilic attack forming a covalent bond. 

A reaction corresponding to Eq. (5-30) is the addition of nitrosyl chloride to alkenes such as cyclohexene or styrene [84, 85]. The reaction seems to be faster in polar solvents e.g. nitrobenzene and trichloromethane) than in less polar solvents e.g. toluene and tetrachloromethane). This is consistent with the view that the reaction involves an electrophilic attack of NO —Cl . The reaction was, however, also found to be very slow in diethyl ether. Presumably, this is due to strong bonding of the NO+ cation to the ether oxygen atom [84]. 

If the acid group is joined directly to the oxygen atom as an ester 6 we have the Fries rearrangement3 which can be controlled to give either the para or ortho products. In polar solutions such as nitrobenzene,4 the para product 5 is formed in high yield, e.g. R = Me, 75%, while in non-polar solvents or in the absence of solvent, the ortho product 7 is formed in similar yield, e.g. R = Me, 70%. 

Aromatic n or /values derived from meta and para substituents tend to be identical, but ortho substituents often give outlying values, e.g., when they permit internal hydrogen bonding, lipophilicity is increased. Apart from this, these n and / values are very sensitive to polar environments. For example, n for chlorine substituted in benzene is 0.71, but this becomes (insertion is in all cases, meta) 0.61 in nitrobenzene, 0.68 in phenylacetic acid, 0.83 in benzoic acid (all ionizable substances are corrected for ionization), 0.98 in aniline, and 1.04 in phenol. This difference of 0.43 between extremes is increased to 0.90 when nitro-group replaces chlorine in the same nuclei (Hansch, Leo et aLy 1973). Two highly polar substituents, particularly if both are nucleophilic, show enhanced lipophilicity (often 0.8) if separated by only one carbon atom, and about half this enhancement if separated by two carbons (Leo, Hansch, and Elkins, 1971 Rekker, 1977, pp. 49y 98, 293). 

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