Polarity of the Radical

The polarity of the free radicals is certainly one of the main factors determining the sensitivity to polar effects. Often the terms of polarity and electrophilicity and nucleophilicity of the free radicals are considered synonymous. 

This behavior is, however, not surprising for two reasons the ionization potentials are determined in gas phase and the reactions take place in polar solvents moreover there is good evidence that other factors play an important role in determining the sensitivity to polar effects. 

The identification of the polarity with electrophilicity and nucleophilicity of the free-radicals must be therefore considered an oversimplification the most polar radical is not necessarily alwa the most sensitive to the polar effects. 

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In the case of substituted aryl radicals, the results may be slightly different, depending on the polarity of the radicals. With electrophilic radicals the overall reactivity of the thiazole nucleus will decrease and the percentage of 5-substituted isomer (electron-rich position) will increase, in comparison with phenyl radicals. The results are indicated in Table III-28. 

Moving one electron to the a lone-pair orbital changes the polarity of the radical. The calculated gross atomic charges are now ... 

Azacyclohexatriene-2-ylidene (3), the 2-isomer of pyridine, has been generated by one-electron oxidation of the corresponding radical cation in neutralization-reionization mass spectrometry.17 It was determined by ab initio H-F calculations that the charge polarization of the radical fonned by H-abstraction from pyrazine can be... 

Since the amount of polarization that can be retained in the radical pair after the triplet reaction will depend upon both the chemical reaction rate and the intrinsic triplet spin lattice relaxation rate, the spin polarization of the radical pair will be given by

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At first, such CIDEP was considered to be due to the polarization transfer from an excited triplet molecule to a radical. The initial spin polarization of triplet benzophenone is emissive, but that of triplet pyruvic acid is absorptive. Similarly, the spin polarization of the radicals was always emissive regardless of the spin polarization of the triplet states [8]. Thus, such emissive signals were attributed to the T-D quenching as shown by Processes (13-8)... 

Another possible explanation for the solvent effect might be based on the difference in the chain transfer rate from the propagating radical to solvent and on the reinitiation rate by the resulting solvent radical. Let us discuss the effect of the solvent transfer reaction on kp under the following three aspects (1) likelihood of chain transfer to solvent, (2) stability of the resulting solvent radical, (3) polarity of the radical. 

The polarity of the solvent radical might participate in the solvent effect. Although the polarity of the radical is difficult to be directly estimated, Hammet s constants am or 0 are considered to become a measure of the polarity of the aromatic... 

At first the polarity of the radical was considered pre-eminent over all the other factors in determining the sensitivity to the polar effects, but soon it was realized that the polarity of the substrate is no less important, so that strong polar effects were observed not only with strongly polar radicals, but also with moderately polar radicals, if the aromatic substrate has a marked polar nature. 

The time resolution of the technique depends on many factors. Direct kinetic measurements using ESR has a time resolution of less than a microsecond. Multiple pulse and FDMR techniques can have time resolution of less than 100 nanoseconds. Because the radicals are often formed in a way in which a polarization of the radicals exist, it is difficult to determine the concentration of the species, since the polarization of the radicals leads to an increase in detection sensitivity. Also, the relaxation of the polarization changes the proportionality between signal and concentration. This makes the kinetics quite complicated and difficult to interpret. A recent review by Trifunac, et al 1986, gives a more complete description of the techniques an limitations (Trifunac etaL 1986). 

It is normal that the polarity of the radical and that of the monomer are defined by the same value since this polarity is determined by the electronic effect of the substituent which is the same for A and A. ... 

In the early 1990s, a new spin polarization mechanism was posPilated by Paul and co-workers to explain how polarization can be developed m transient radicals in the presence of excited triplet state molecules (Blattler et al [43], Blattler and Paul [44], Goudsmit et al [45]). While the earliest examples of the radical-triplet pair mechanism (RTPM) mvolved emissive polarizations similar in appearance to triplet mechanism polarizations, cases have since been discovered m which absorptive and multiplet polarizations are also generated by RTPM. 

Figure Bl.16.19. (a) CIDEP spectrum observed in die photolysis of xanthone (1.0 x 10 M) in cyclohexanol at room temperature. The stick spectra of the ketyl and cyclohexanol radicals with RPM polarization are presented, (b) CIDEP spectrum after the addition of hydrochloric acid (4.1 vol% HCl 0.50 M) to the solution above. The stick spectra of the ketyl and cyclohexanol radicals with absorptive TM polarization are presented. The bold lines of the stick spectra of the cyclohexanol radical show the broadened lines due to ring motion of the radical. Reprinted from [62].

Blattler C and Paul H 1991 CIDEP after laser flash irradiation of benzil in 2-propanol. Electron spin polarization by the radical-triplet pair mechanism Res. Chem. Intermed. 16 201-11... 

In this equation P and Q are parameters that describe the reactivity of the radical and monomer of the designated species, and the values of e measure the polarity of the two components without distinguishing between monomer and radical. 

Acylation. Aliphatic amine oxides react with acylating agents such as acetic anhydride and acetyl chloride to form either A[,A/-diaLkylamides and aldehyde (34), the Polonovski reaction, or an ester, depending upon the polarity of the solvent used (35,36). Along with a polar mechanism (37), a metal-complex-induced mechanism involving a free-radical intermediate has been proposed. 

Similarly, carboxylic acid and ester groups tend to direct chlorination to the / and v positions, because attack at the a position is electronically disfavored. The polar effect is attributed to the fact that the chlorine atom is an electrophilic species, and the relatively electron-poor carbon atom adjacent to an electron-withdrawing group is avoided. The effect of an electron-withdrawing substituent is to decrease the electron density at the potential radical site. Because the chlorine atom is highly reactive, the reaction would be expected to have a very early transition state, and this electrostatic effect predominates over the stabilizing substituent effect on the intermediate. The substituent effect dominates the kinetic selectivity of the reaction, and the relative stability of the radical intermediate has relatively little influence. 

On the basis of the reaction of alkyl radicals with a number of polycyclic aromatics, Szwarc and Binks calculated the relative selectivities of several radicals methyl, 1 (by definition) ethyl, 1.0 n-propyl, 1.0 trichloromethyl, 1.8. The relative reactivities of the three alkyl radicals toward aromatics therefore appears to be the same. On the other hand, quinoline (the only heterocyclic compound so far examined in reactions with alkyl radicals other than methyl) shows a steady increase in its reactivity toward methyl, ethyl, and n-propyl radicals. This would suggest that the nucleophilic character of the alkyl radicals increases in the order Me < Et < n-Pr, and that the selectivity of the radical as defined by Szwarc is not necessarily a measure of its polar character. 

The behavior of methyl and halomethyl radicals in their reactions with the fluoro-olefms (Table 1.2), can thus be rationalized in terms of a more dominant role of polar factors and the nucleophilic or electrophilic character of the radicals involved." Methyl radicals are usually considered to be slightly nucleophilic, trifluoromcthyl and triehloroincthyl radicals arc electrophilic (Tabic 1,4). 

Methanol is a very minor product and the observation that its polarization is more intense in N20-saturated solution than in He-saturated solution suggests that it is formed by reaction of OH" radical, probably by a degradation of the radical formed by the addition of OH to DMSO other than the main one given in reaction 21. 

The origin of postulate (iii) lies in the electron-nuclear hyperfine interaction. If the energy separation between the T and S states of the radical pair is of the same order of magnitude as then the hyperfine interaction can represent a driving force for T-S mixing and this depends on the nuclear spin state. Only a relatively small preference for one spin-state compared with the other is necessary in the T-S mixing process in order to overcome the Boltzmann polarization (1 in 10 ). The effect is to make n.m.r. spectroscopy a much more sensitive technique in systems displaying CIDNP than in systems where only Boltzmann distributions of nuclear spin states obtain. More detailed consideration of postulate (iii) is deferred until Section II,D. 

Provided that polarization arises predominantly as a result of the difference in the gr-factors of the components of the radical pair, similar considerations apply to systems where several coupled nuclei are polarized. Net polarization, A or E, results. This is indicated in Fig. 3b for the AB system of protons in a hypothetical product R2CHA. CHbRj... 

The lifetime of the radical pair is important since successive encounters of radicals [going between case (1) and case (2)] will give more Tq-S mixing and therefore greater nuclear spin polarization. 

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