Polarizable groups

Further lowering the dielectric constants has been achieved by preparing highly fluorinated polyethers without any sulfone, ketone, or other polarizable groups.239 241 Typically, the /jara-lluorinc atoms on highly fluorinated aromatic compounds, such as hexafluorobenzene and decafluorobiphenyl, are activated and thus can go through aromatic nucleophilic substitution with HFBPA under typical reaction conditions (Scheme 6.31).217... 

For dinitro-substituted substrates, it has been recently shown that activation in 2,4-dinitrophenyl substrates is mainly due to the mesomeric effect of the 4-nitro group, thus reducing the electron density at the reaction site83. However, another important feature of highly polarizable groups, such as a nitro group in the ortho position, has been considered to be the field effect101. 

Dipole-induced dipole interaction energies are proportional to the product of the square of the dipole moment and the polarizability a of the atom/group with which the ion interacts, divided by the product of the square of dielectric constant D and the sixth power of the distance between the dipole and the polarizable group. Dispersion interactions have energies that are proportional to the product of polarizabilities i and 2. divided by the sixth power of the distance between two polarizable atoms (or groups of atoms). 

Does Professor Williams have good suggestions for highly polarizable groups that might be incorporated in ligands to obtain anion carriers ... 

The intramolecular ene reaction [198] proceeds in good reioselectivity if either the ene or the enophile component is linked to polarizable groups. Based on the direction of polarization an excellent synthesis of (—)-a-kainic acid [199] has been designed. 

The general validity of these equations is supported by a great deal of experimental data on aromatic substitution reactions of toluene. Examples of values for some reactions obtained from these equations are given in Table 11.4.78 For other substituents, the treatment works well with groups that, like methyl, are not very polarizable. For more polarizable groups the correlations are sometimes satisfactory and sometimes not, probably because each electrophile in the transition state makes a different demand on the electrons of the substituent group. 

Just because a molecule is long, narrow, and meets the requirement of geometric anisotropy docs not ensure that it will have a liquid crystal phase. The particular phase structure that occurs in a compound, i.e.. smectic, nematic, or chiral nematic, not only dc >cnds on the molecular shape hut is intimately connected with the strength and position of the polar or polarizable groups within the molecule, the overall polarizability of the molecule, and the presence ol chiral centers. 

If a highly polarizable group is introduced into a receptor molecule, substrate binding should cause substantial perturbations, so that the recognition event would be converted into a non-linear optical signal. Such recognition-dependent nonlinear optical probes may be derived for instance from polyenes such as those shown in Figure 20, from inclusion complexes [8.94a] or from donor-acceptor calixarenes [8.94b]. 

Because triphenylchlorosilane is much more reactive toward solvolysis than triisopropylchlorosilane (whereas on steric grounds they should be about equal) Allen and Modena (3) were further convinced that the presence on silicon of easily polarizable groups strongly facilitates formation of the transition state. Based on a study of solvolysis in dioxane at 25.1° they derived the following rate expression ... 

For olefins, the first term has been studied in terms of the symmetry D2 independently by two groups913. The second term, which was also called the one-electron term, is usually neglected in cases of electronically allowed transitions because it is small compared with the other two terms. It is even smaller in the olefinic case, where the molecules contain mainly nonpolar bonds. Scott and Yeh73 have therefore concentrated on the third term, which arises from the coupling of a considered transition a with all the transitions of different chromophores within the molecule. If transition a falls energetically below all the transitions of the other chromophores, the latter can be viewed as polarizable groups and their anisotropies fi determine the dynamic coupling74. The final expression obtained was... 

Figure 7. Complexation free energies AG of the porphyrin macrocycle with benzoic acid derivatives, showing the absence of hydrophobic effects with R = CH3, and the large effect of polarizable groups such as R = N02.

For the dispersion of polar or ionic solids in aqueous media the hydrophobic group of the surfactant should contain polar groups, such as ether linkages, or polarizable groups, such as aromatic rings, capable of adsorbing onto the solid. 

Earlier literature has used the term hydrophobic bond, but it is clear from the above discussion that no special hydrophobic force exists. Nonpolar groups self-associate in water because their dispersal throughout the solvent would be entropically unfavorable. Once they come together and water is largely excluded, enthalpically favorable interactions are possible, but these are just (for nonaromatic hydrocarbons) the normal weak London forces between any polarizable groups. There is no bonding that is specifically hydrophobic. The correct term is hydrophobic effect. 

Although the interactions of charged, dipolar or polarizable groups have been investigated for various purposes, they have not often been utilized in the context of stereoselectivity. In fact, when coulombic effects were considered in the SN2 or E2 processes, their role was regarded as unimportant (Ingold, 1953 Cristol, et al., 1951). In view of the substantial electrostatic (field) effects estimated for polar substituents on the pA s of carboxylic acids (Tanford, 1958), metal-ion coordination (Basolo and Pearson, 1967), etc., it will be interesting to see what effects there may be on SS. 

If there are easily polarizable groups at the ends of the chain (such as benzene rings), the potential energy of the tt electrons in the chain does not rise so sharply at the ends. In effect this lengthens the path L, and we can write... 

Electron-donating inductive effect (Section 7.14A) An inductive effect in which an electropositive atom or polarizable group donates electron density through O bonds to another atom. 

---