Benzene derivatives substituents

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. 

If, on the other hand, the encounter pair were an oriented structure, positional selectivity could be retained for a different reason and in a different quantitative sense. Thus, a monosubstituted benzene derivative in which the substituent was sufficiently powerfully activating would react with the electrophile to give three different encounter pairs two of these would more readily proceed to the substitution products than to the starting materials, whilst the third might more readily break up than go to products. In the limit the first two would be giving substitution at the encounter rate and, in the absence of steric effects, products in the statistical ratio whilst the third would not. If we consider particular cases, there is nothing in the rather inadequate data available to discourage the view that, for example, in the cases of toluene or phenol, which in sulphuric acid are nitrated at or near the encounter rate, the... 

TABLE 9.3 The nitration of benzene derivatives containing positively charged substituents ... 

In view of the extremely wide variety of groups which occur as substituents in benzene derivatives and for which substituent constants have been calculated, it is surprising that almost none of them... 

Charton has recently examined substituent effects in the ortho position in benzene derivatives and in the a-position in pyridines, quinolines, and isoquinolines. He concludes that, in benzene derivatives, the effects in the ortho position are proportional to the effects in the para position op). However, he finds that effects of a-sub-stituents on reactions involving the sp lone pair of the nitrogen atoms in pyridine, quinoline, and isoquinoline are approximately proportional to CT -values, or possibly to inductive effects (Taft s a ). He also notes that the effects of substituents on proton-deuterium exchange in the ortho position of substituted benzenes are comparable to the effects of the same substituents in the a-position of the heterocycles. 

The problems encountered in any attempt to treat the transmission of the effects of one substituent in a disubstituted heterocycle through the heterocyclic nucleus to a reaction site in the other substituent (i.e. the side-chain) are enormous, and it is consequently not surprising that relatively little work has been done in this area. First, while in benzene derivatives there are three positions, i.e. three relations between substituent and reacting side-chain to be considered, the number of complexities is much greater in heterocycles. Thus, e.g., in pyridine alone, after elimination of the orientations involving a vicinal relationship between substituent R and the side-chain Y to which no Hammett-type relation is likely to be applicable, the following cases should be considered ... 

W-OCH3 group should lower the basicity of naphthoic acids (both a and j8) and the nucleophilic reactivity of aza-benzene derivatives. In terms of a, the substituent effect for alkoxy groups wiU be negative at all positions. The w-SCHg group behaves similarly. 

Studies on substituent effects are scanty. For the five-membered monocyclic nuclei, the relative positions have been compared to the classic ortho, meta, and para positions of benzene derivatives according to formulae 60 and 61. This comparison is based on the assumption... 

The accelerative effect of London forces of attraction between a nucleophile and nearby substituents has been investigated in quinoline and benzene derivatives by Bunnett and co-workers, ii7b, 307 In 2-, 4-, and 6-arylsulfonyl-3-nitrochlorobenzene, Loudon and Shulman found that arylmercaptide ion, presumably through this effect, displaced the arylsulfonyl group while methoxide or ammonia displaced the nitro or chloro group. 

The scheme shows that the influence of substituents on CH acidity in ethynylpy-razoles is not additive as compared with benzene derivatives (84IZV923), and its value depends, for each substituent, on the nature of other groups in different positions of the azole. 

Benzene derivatives. Tbe nomenclature is a combination of the lUPAC system and traditional names. Many of the derivatives are named by the substituent group appearing as the prefbt. These may be considered a subclass of the aliphatic-aromatic hydrocarbon family, which contains both aliphatic and aromatic units in its structures. Thus, alkylbenzenes are made up of a benzene ring and alkane units alkenylbenzenes are Composed of a benzene ring and alkene units and alkynylbenzenes comprise a benzene ring and alkyne units. Examples of alkylbenzenes include... 

Treatment of 1,2,4-triazines 91a-91e with the electron-deficient die-nophile dimethyl acetylenedicarboxylate gave products, depending on the substituents [77LA( 10) 1718]. Pyrrolo-[2, -/][ ,2,4]triazines 92 were obtained via [4 + 2]-cycloaddition [77LA(9)1413, 77LA( 10)1718] with 91, but interaction with 91b in the absence of solvent gave, in addition to 92, the pyrido[2,l-/][l,2,4]triazine 93 and [l,3]oxazino[2,3-/][l,2,4]-triazine 94. In case of 91a pyridine and benzene derivatives were also formed in addition to 92 (Scheme 23). 

Monosubstituted benzene derivatives containing an electrophilic substituent. 

Disubstituted benzene derivatives containing electrophilic substituents C6H3... 

In Scheme 7-1 kx and kn refer to the rate constants for a benzene derivative, in our case the benzenediazonium ion, bearing a substituent X in the 3- or 4-position, and the corresponding unsubstituted benzene derivative respectively. The term p is Hammett s reaction constant for the reaction, and o is Hammett s substituent constant which is, at least in principle, independent of the nature of the reaction but different for the 3- and 4-positions. A plot of log kx (or log kx - log kH) versus o should give a straight line. Its slope (positive or negative) corresponds to the reaction constant p. Equilibrium relationships are treated analogously. 

Before we close this section we make reference to an extended form of the Hammett equation in which the substituent constant and the reaction constant are separated into contributions from the field effect (F) and the mesomeric effect (R). This procedure was suggested by Taft in 1957 for 4-substituted benzene derivatives. It is called a dual substituent parameter (DSP) equation (Scheme 7-2). 

For the introduction of fluorine into aromatic and heteroaromatic compounds the photolytic fluoro-de-diazoniation sometimes has advantages compared with the corresponding thermal dediazoniation (Balz-Schiemann reaction, see Sec. 10.4). For aromatic substrates the reaction was studied by Rutherford et al. (1961), Christie and Paulath (1965), Petterson et al. (1971), and Becker and Israel (1979). Hexafluorophos-phates sometimes give better yields than tetrafluoroborates (Rutherford et al., 1961). In analogy to Balz-Schiemann reactions in solution (Fukuhara et al., 1987), photolytic fluoro-de-diazoniations of benzene derivatives with electron-withdrawing substituents give lower yields. 

Of considerable interest is a reaction of monoarylazo derivatives of B10Hio containing electron-withdrawing substituents in the benzene ring (e.g., 2,4,6-Br3, 4-N02), which was subsequently found by Leyden and Hawthorne (1973, 1975). If the azo coupling system mentioned above (acetonitrile, - 35 °C) is allowed to come to room temperature and is treated with excess sodium propionate, the 1-diazonio-B10H9 anion and the corresponding benzene derivative are formed in considerable yield (52% with the 2,4,6-tribromo derivative as shown in Scheme 12-85). 

Recently, a kinetic study has been made of the substitution of diazotised sulphanilic acid in the 2 position of 4-substituted phenols under first-order conditions (phenol in excess) in aqueous buffer solutions at 0 °C131a. A rough Hammett correlation existed between reaction rates and am values, with p about -3.8 however, the point for the methoxy substituent deviated by two orders of magnitude and no explanation was available for this. The unexpectedly low p-factor was attributed to the high reactivities of the aromatic substrates, so that the transition state would be nearer to the ground state than for reaction of monosubstituted benzene derivatives. 

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. 

The symbol k or K is the rate or equilibrium constant, respectively, for a side-chain reaction of a meta- or para-substituted benzene derivative, and k° or K° denotes the statistical quantity (intercept term) approximating to k or K for the parent or unsubstituted compound. The substituent constant a measures the polar (electronic) effect of replacing H by a given substituent (in the meta- or para-position) and is, in principle, independent of the nature of the reaction. The reaction constant p depends on the nature of... 

Charton, M., Prog. Phys. Org. Chem., 8, 235 (1971) has reported extensively on correlations of rate data for ortho substituted benzene derivatives using the dual substituent parameter treatment in the form with an additional (intercept) parameter, and in our opinion, too limited substituent data sets. For these and related reasons which we have discussed, we question the significance of many of Charton s correlations. 

Nametkin and co-workers hrst reported the alkylation of benzene derivatives with allylchlorosilanes in the presence of aluminum chloride as catalyst. " 2-(Aryl)propylsilanes were obtained from the alkylation of substituted benzenes (Ph—X X = H, CL Br) with allylsilanes such as allyldichlorosilane and allyltrichlo-rosilane.The yields ranged from 34 to 66% depending upon the substituents on the benzene ring, but information concerning reaction rates and product isomer distribution was not reported. 

Substituent effects of the benzene ring in the alkylation of benzene derivatives with I were studied by comparison with benzene it.self. These results are summarized in Table HI. 

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