P-substituted benzenes

A point in case is provided by the bromination of various monosubstituted benzene derivatives it was realized that substituents with atoms carrying free electron pairs bonded directly to the benzene ring (OH, NH2, etc) gave 0- and p-substituted benzene derivatives. Furthermore, in all cases except of the halogen atoms the reaction rates were higher than with unsubstituted benzene. On the other hand, substituents with double bonds in conjugation with the benzene ring (NO2, CHO, etc.) decreased reaction rates and provided m-substituted benzene derivatives. 

Data for other p-substituted benzene side-chain reactions are fitted by eq. (1) using the oj and Or values of Table I with widely varying precision measures. However, precision of fit comparable to that achieved for the eight basis set reactions of Table II is obtained (only) with recognizable analogs of the para BA type. Other reaction types are fitted generally with values of / SD/RMS greater by factors of two or more than the i>% level achieved by the para BA type (cf. subsequent Tables VII, IX, XII, XIV). 

Despite establishing such linear relationships for a wide range of reactions of m- and p-substituted benzene derivatives, we still lack any simple form of this quantitative relationship that can actually be used to investigate new situations here again, it was Hammett who supplied the answer. 

Quite early on (p. 361) in this discussion of linear free energy relationships consideration was restricted to the side-chain reactions of m- and p-substituted benzene derivatives. The reactions of o-substituted benzene derivatives, and indeed of aliphatic compounds, were excluded because of the operation of steric and other effects, which led to non-linear, or even to apparently random, plots. 

The semiempirical AMI MO method has been used to calculate heats of formation of a series of m- and p-substituted benzene and toluene derivatives ArY and ArCHaY, and their phenyl or benzyl cations, anions, and radicals heterolytic and homolytic bond dissociation energies (BDEs) and electron transfer energies for the ions have also been calculated and the relationship A//het = A//et-I-AWhomo has been confirmed (it being noted that A//homo is insensitive to ring substituents). The linear relationship found between and the appropriate HOMO or LUMO... 

Phenols, and other simple p-substituted benzene compounds. 

Systematic Hammett o/p correlation with m- and p-substituted benzenes ... 

Another instance of eUminating two water molecules coordinated to Ln(III) and thereby inducing a marked luminescence increase has been found using the pendant p-substituted benzene sulfonamides 71-74 (81). 

Para-xylene may be oxidized to terephthalic acid by means of nitric acid. Liquid-phase oxidation of m- and p-xylene is complicated by the Increased resistance to oxidation of the second methyl group after the first has been oxidized to the carboxyl group. As a consequence of experience with this difficulty, development has been toward oxidation in, two steps, a flrst to the toluic acid stage and a second to the dicarboxylic acid. Esterification of the first carboxyl group results in much easier oxidation of the coond methyl to a carboxyl group. Other p-substituted benzenes such as p-diisopropylbenzene are oxidized by air in the presence of a cobalt catalyst to terephthalic acid. Use is made of this in a recent new approach which permits the use of catalyzed air oxidation of p-xylene and results in formation of dimethyl terephthalate. A four-step process has attained commercial importance air oxidation of p-xylene to toluic acid using oil- Oluble catalysts of cobalt or manganese, esterification with methanol to methyl p-toluate, a second air oxidation to monomethyl terephthalate, and Anally esterification with methanol to dimethyl terephthalate. 

The order of susceptibility towards substituent effects is much less in the acetylene series than in the olefins, but its direction is similar. A similar statement may be made about the direction, rates, and kinetics of addition of sulphenyl halides. While the simple acetylenes, such as sym-diphenylacetylene and 3-hexyne, required catalytic quantities of aluminium chloride before undergoing addition by 2,4-dinitrobenzene sulphenyl chloride , the rates of reaction of acetylene itself with some p-substituted benzene sulphenyl chlorides alone have been measured. The effect of the p-substituents... 

This enables us to introduce a concept of polar effects in homological reactions, seen in the kinetics of radical polymerisation and homolytic arylation of p-substituted benzenes. Combination of heterogroups with different electronegativity in the PSF structure - strong electron-acceptor sulfonyl (the Hammet constant = + 0.7) and electron-donor ether (o = -0.32) groups, as well as weak electron-donor isopropylydene group (o = -0.197), causes alternation of electron density on aromatic carbon atoms, which is naturally displayed in values of chemical shears of NMR (see Figure 7.3). 

Table 2.14 Copper-Free" Sonogashira Reactions of p-Substituted Benzenes ...

A Few Dipole Moments of p-Substituted Benzene Derivatives According to... 

Further evidence to exclude the triplet radical pathway includes the use of cyclopropyl substrates, which serve as a radical clock. In all cases, the reaction proceeds with no indication of ring fragmentation. The nature of the transition state of the C—H insertion step has been analyzed, via a Hammett study of the intermo-lecular C—H amination with p-substituted benzenes. A negative q value of 0.73 is obtained for the intermolecular reaction with trichloroethylsulfamate [71]. Such data indicate that there is a small, but significant, preference for electron-rich substrates, thus the resonance does contribute to the stabilization of a partial positive charge at the insertion carbon in the transition state. A kinetic isotope value of 1.9 is observed for competitive intramolecular C—H amination with a deuterated substrate (Eq. (5.21)). 

A negative q value of —0.47 is obtained from a Hammett study of the intermo-lecular C—H amination with p-substituted benzenes and trichloroethyl-N-tosylox-ycarbamate. A kinetic isotope value of 5 is observed for a competitive C— H amination between a deuterated and undeuterated substrate. These data are consistent with a transition state for the C—H amination similar to the one shown previously (Figure 5.3). 

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