Para- substituents

The equation does not hold without exceptions even for meta and para substituents, especially when resonance interactions from substituents are possible. 

When we examine the cyclohexadienyl cation intermediates involved m the nitra tion of (trifluoromethyl)benzene we And that those leading to ortho and para substitu tion are strongly destabilized... 

Phenols with bulky ortho- and para-substituents, eg, phenoHc antioxidants, do not undergo this reaction however, they scavenge radicals generated by thermolysis of diacyl peroxides and other peroxides. Diacyl peroxides react with potassium superoxide, KO2, forming singlet oxygen (207). 

It has been demonstrated that the P -selectivity is due to the para-substituents of these dmgs (96). In contrast, (-)-erythro-isoetharine (127), a bronchodilator, is 80 times more selective for receptors than for P ceptors. Isoetharine (97) contains an a-alkyl substituent, thus... 

Li2C03, Mel, DMF, 55°, 18 h, 54-90%. This method selectively protects phenols with pK < 8 as a result of electron-withdrawing ortho- or para-substituents. 

The Hammett equation in the form of Eq. (4.14) or Eq. (4.15) is free of complications due to steric effects, since it is applied only to meta and para substituents. The geometry of the benzene ring ensures that groups in these positions cannot interact stoically with the site of reaction. Tables of a values for many substituents have been collected some values are given in Table 4.5, but substituent constants are available for a much wider range of... 

The ortho effect may consist of several components. The normal electronic effect may receive contributions from inductive and resonance factors, just as with tneta and para substituents. There may also be a proximity or field electronic effect that operates directly between the substituent and the reaction site. In addition there may exist a true steric effect, as a result of the space-filling nature of the substituent (itself ultimately an electronic effect). Finally it is possible that non-covalent interactions, such as hydrogen bonding or charge transfer, may take place. The role of the solvent in both the initial state and the transition state may be different in the presence of ortho substitution. Many attempts have been made to separate these several effects. For example. Farthing and Nam defined an ortho substituent constant in the usual way by = log (K/K ) for the ionization of benzoic acids, postulating that includes both electronic and steric components. They assumed that the electronic portion of the ortho effect is identical to the para effect, writing CTe = o-p, and that the steric component is equal to the difference between the total effect and the electronic effect, or cts = cr — cte- They then used a multiple LFER to correlate data for orrAo-substituted reactants. 

Two groups of workers have demonstrated conclusively that the concept of three sets of substituent constants, ct, a, and ct+, is inadequate and that actually substantially continuous ranges of such constants are necessary for practically all para substituents, as well as for certain substituents in the meta position. Although their arguments, as well as their data and conclusions, are unassailable, it remains surprising that the naive approach using three selected sets is so useful and successful during the past decade we have seen and examined... 

Calculated from para substituent constants only. 

It is well-known that para substituents on the phenyl groups of H2TPP have no influence on the tautomerism rates in the ground state (see Section III,A,1). In the case of PHB, there seems to be only a small substituent effect on <1>phb (the quantum efficiency for hole burning) through modification of the relative energy of Ti (93CM366). 

Equation 35 is very similar to Eq. 33, although the correlation is not as sharp. The most striking difference between Eqs. 35 and 36 is that the latter does not contain a term in E. This means that para substituents have no effect on hydrolysis through... 

The same conclusion was reached in a kinetic study of solvent effects in reactions of benzenediazonium tetrafluoroborate with substituted phenols. As expected due to the difference in solvation, the effects of para substituents are smaller in protic than in dipolar aprotic solvents. Alkyl substitution of phenol in the 2-position was found to increase the coupling rate, again as would be expected for electron-releasing substituents. However, this rate increase was larger in protic than in dipolar aprotic solvents, since in the former case the anion solvation is much stronger to begin with, and therefore steric hindrance to solvation will have a larger effect (Hashida et al., 1975 c). 

The relatively small dependence of the rate on solvent is demonstrated in Table 5. The rate coefficients are a little greater in the more polar solvents but the overall effects are small. This is taken to indicate that there is only little polar character to the transition state. Another probe to determine the polarity of the transition state is the effect of substituents on the rate. The results obtained for the ortho rearrangement by varying the para substituents are shown in Table 6131 for the conversion... 

The Hammett equation is the best-known example of a linear free-energy relationship (LFER), that is, an equation which implies a linear relationship between free energies of reaction or activation for two related processes48. It describes the influence of polar meta-or para-substituents on reactivity for side-chain reactions of benzene derivatives. 

Jaffe (1953)52 showed that while many rate or equilibrium data conform well to the Hammett equation (as indicated by the correlation coefficient), many such data are outside the scope of the equation in its original form and mode of application. Deviations are commonly shown by para-substituents with considerable + Rot — R effect53. Hammett himself found that p-NOz (+ R) showed deviations in the correlation of reactions of anilines or phenols. The deviations were systematic in that a a value of ca. 1.27 seemed to apply, compared with 0.78 based on the ionization of p-nitrobenzoic acid. Other examples were soon discovered and it became conventional to treat them similarly in terms of a duality of substituent constants . 

When a values based on the ionization of benzoic acid are used, deviations may occur with + R para-substituents for reactions involving — R electron-rich reaction centers, and with — R para-substituents for reactions involving + R electron-poor reaction centers. The explanation of these deviations is in terms of cross-conjugation , i.e. conjugation involving substituent and reaction center. 

An example of a reaction series in which large deviations are shown by — R para-substituents is provided by the rate constants for the solvolysis of substituted t-cumyl chlorides, ArCMe2Cl54. This reaction follows an SN1 mechanism, with intermediate formation of the cation ArCMe2 +. A —R para-substituent such as OMe may stabilize the activated complex, which resembles the carbocation-chloride ion pair, through delocalization involving structure 21. Such delocalization will clearly be more pronounced than in the species involved in the ionization of p-methoxybenzoic acid, which has a reaction center of feeble + R type (22). The effective a value for p-OMe in the solvolysis of t-cumyl chloride is thus — 0.78, compared with the value of — 0.27 based on the ionization of benzoic acids. 

The special substituent constants for + R para-substituents are denoted by a, and those for — R para-substituents are denoted by a+ 54. They are based respectively on the reaction series discussed above. Selected values are given in Table 1. Characteristic a or a+ values are sometimes distinguished for meta-substituents also, but only for a minority of substituents which show very marked + R or — R effects do these differ significantly from ordinary a values. The range of applicability of the Hammett equation is greatly extended by means of a and cr+, notably to nucleophilic (by a ) and to electrophilic (by cr+) aromatic substitution. 

For any given reaction series the equation is applied to meta- and para-substituents separately, and so values of p, and pR characteristic both of reaction and of substituent position are obtained. The various crR-type scales are linearly related to each other only approximately. In any given application the scale which gives the best correlations must be found65. ... 

What was claimed79 to be a value of ap° of 0.76 for S02Ph was given by Bodor and Kovendi95 on the basis of a kinetic study of the effect of para- substituents on the isonitrosation of o-nitroethylbenzene. However, there is no obvious reason why ap° should be greater than ap for this substituent. 

Recent studies have found enhanced substituent solvation assisted resonance effects in dipolar non-hydrogen bonding solvents131. For several +R substituents acidities of phenols in DMSO are well correlated with their gas-phase acidities. The substituents include m- and p-SOMe, m- and p-S02Me, m-S02CF3 and m-N02. But there is very considerable enhancement of the effect of p-S02CF3, p-N02 and various other para-substituents in DMSO solution. 

The electronic influence on the donor ability of the monomers as well as the acceptor behavior of the corresponding cation has been characterized by variation of the para-substituents of the styrenes. More detailed assertions can be expected when one varies the structure directly on the double bond by substitution at a- and P-position. A suprisingly good correlation appears between the energetic level of the HOMO and energy of theCT-absorption of the complexes with TCNE (ECT = 20.8 5.55x(HOMO) r = 0.80 n = 12) for styrenes, both with and without a methyl substituent at a- and p-position ... 

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