Phenol electron distribution

Phenol radical cations exist only in strong acidic solutions (pKa -1) [1, 2]. However, in non-polar media phenol radical cations with lifetimes up to some hundred nanoseconds were obtained by pulse radiolysis [3], The free electron transfer from phenols (ArOH) to primary parent solvent radical cations (RX +) (1) resulted in the parallel and synchroneous generation of phenol radical cations as well as phenoxyl radicals in equal amounts, caused by an extremely rapid electron jump in the time scale of molecule oscillations since the rotation of the hydroxyl groups around the C - OH is strongly connected with pulsations in the electron distribution of the highest molecular orbitals [4-6]. 

Because the p value is positive, negatively charged carbon ions are considered to be the primary transition state complex (TSC). The TSC will dissociate to a substituted phenol radical and a stable anion. It may also be neutralized by the toluene, resulting in the addition of a proton, H+ (Arai and Dorfman, 1964) therefore, eaq is considered to interact with the ir-orbital of the ring as in electrophilic substitution rather than to affect electron distribution and polarizability of a certain substituent. 

The 1H NMR shifts of phenol give us an indication of the electron distribution in the n system. The more electron density that surrounds a nucleus, the more shielded it is and so the smaller the shift (see Chapter 11). All the shifts for the ring protons in phenol are less than those for benzene (7.26 p.p.m.), which means that overall there is greater electron density in the ring. There is little difference between the ortho and the para positions both are electron-rich. 

In the description of diffusion-controlled reactions, the reactants interact by several collisions (up to 100) before the successor situation can be reached. However, if each collision results directly in products, in the case of a rapid electron transfer, such an efficient process takes place in times < I fs. This is at least one hundred times faster than the mentioned rotation of the substituent in the donor molecule and, therefore, the variety of rotation states, i.e. of electron distributions, might be recognized. For the example of phenol, it means that different ionization products would be formedh... 

Since the exact electron distribution In 2 is not known both for the phenolic and for the phenolate-carboxonium form, the formulas should be regarded as more or less arbitrary mesomeric structures. Intermolecular proton transfer is not likely In nonpolar solvents furthermore, the emission of MSA In O.IM NaOH/CHjOH has its maximum at 24,000 cm l (23), compared to the observed maximum at 22,600 cm. ... 

As in the case of phenol or monosubstituted benzenes12, ANI exhibits six C(C) and one C(N) core basins, six V(C,C), five V(C,H), one V(C,N), two V(N,H) and one V(N, lone pair) localization domains. Integration of the density over each domain allows the amount of electrons present there to be determined. Thus the C—C bonds are nearly equivalent and each V(C,C) basin classically contains about 2.7-2.9 electrons (close to 3.0), whereas the V(N) and V(C,N) basins have 1.8 and 1.9 electrons, respectively (Figure 5). Relative to benzene, an increase in the population of the V(Cor//1<>—Cm< M) basin is noticeable (0.1 e) whereas the populations of other domains remain almost unchanged. Overall, the above analysis is in line with the classical picture that chemists always have about the electron distribution in aniline. 

It is clear from Table 7 that most of the phenoxyl radicals protonate on the oxygen to form phenol radical cations with pX a values about —1 to —2, i.e. 10 units lower than the pX a values of the parent phenols. Because of the strong acidities involved and the choice of the appropriate acidity functions, the pATa values are not as accurate as those measured under milder conditions (pH 2-12). There is no simple correlation between the pX a values and the a substituent constants. This is not surprising, since the a constant reflects the electron distribution in the molecule while the value depends on the electron distribution in the radical, which is different from that in the parent molecule. There appears to be some correlation between the effect of substituents on the pATj values and their effect on the spin density distribution in the radical, but not all the substituents... 

The NMR shifts of phenol give us an indication of the electron distribution in the it system. 

Another factor influencing the electron distribution may be the contributions to the overall resonance hybrid of phenol made by structures 2—4. Notice that the effect of these structures is to withdraw electrons from the hydroxyl group and to make the oxygen positive ... 

By introducing reasonable values (about 2 for nitrogen, 4 for oxygen) for the electron affinity parameter relative to carbon, 8, and for the induced electron affinity for adjacent atoms (32/8i = Vio), we have shown that the calculated permanent charge distributions for pyridine, toluene, phenyltrimethylammonium ion, nitrobenzene, benzoic acid, benzaldehyde, acetophenone, benzo-nitrile, furan, thiophene, pyrrole, aniline, and phenol can be satisfactorily correlated qualitatively with the observed positions and rates of substitution. For naphthalene and the halogen benzenes this calculation does not lead to results... 

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