Substitution of benzene

Nucleophilic substitution of benzene itself is not possible but the halogeno derivatives undergo nucleophilic displacement or elimination reactions (see arynes). Substituents located in the 1,2 positions are called ortho- 1,3 meta- and 1,4 para-. 

So far we ve been concerned only with electrophilic substitution of benzene Two impor tant questions arise when we turn to substitution on rings that already bear at least one substituent... 

Nucleophilic Substitutions of Benzene Derivatives. Benzene itself does not normally react with nucleophiles such as haUde ions, cyanide, hydroxide, or alkoxides (7). However, aromatic rings containing one or more electron-withdrawing groups, usually halogen, react with nucleophiles to give substitution products. An example of this type of reaction is the industrial conversion of chlorobenzene to phenol with sodium hydroxide at 400°C (8). 

Aromatic hydrocarbons, like paraffin hydrocarbons, react by substitution, but by a different reaction mechanism and under milder conditions. Aromatic compounds react by addition only under severe conditions. For example, electrophilic substitution of benzene using nitric acid produces nitrobenzene under normal conditions, while the addition of hydrogen to benzene occurs in presence of catalyst only under high pressure to... 

Most of the subsequent work on this reagent was concerned with the formation of aryl radicals (see review by Cadogan, 1971). However, 2-terf-butyl-A-nitrosoacet-anilide was found to decompose in benzene to give, instead of 2-tert-butylbiphenyl, as expected for a substitution of benzene by a 2-tert-butylphenyl radical, a mixture of isomeric tert-butylphenyl acetates. A careful reexamination (Cadogan and Hib-bert, 1964) suggested that the ratio of 2- and 3-tert-butylphenyl acetates was consistent with the involvement of 2-tert-butylbenzyne, i.e., the product of an ionic dediazoniation, as an intermediate. This was later confirmed by trapping experiments designed to detect aryne intermediates. 

Certain groups attached to an aromatic ring can donate electrons into its delocalized molecular orbitals. Examples of these electron-donating substituents include —NH2 and —OH. Electrophilic substitution of benzene is much faster when an electron-donating substituent is present. For example, the nitration of phenol, C6H5OH, proceeds so quickly that it requires no catalyst. Moreover, when the products are analyzed, the only products are found to be 2-nitrophenol (ortho-nitrophenol, 37) and 4-nitrophenol (pnra-mtrophcnol, 38 . 

We have seen how an —OH group can accelerate a reaction. Are there substituents that can slow down the electrophilic substitution of benzene One way to reduce the electron density in the benzene ring and to make it less attractive to elec-... 

Aromatic rings are much less reactive than their double-bond character would suggest they commonly undergo substitution rather than addition. Electrophilic substitution of benzene with electron-donating substituents is accelerated and takes place at the ortho and para positions preferentially. Electrophilic substitution of benzene with electron-withdrawing substituents takes place at a reduced rate and primarily at the meta positions. 

Peroxide decomposition in aromatic and other unsaturated solvents homolytic aroniMic substitution and olefin polymerization Decomposition of peroxides in aromatic solvents leads to attack on the aromatic nucleus by radicals and hence to substitution products (for a recent summary, see Williams, 1970). In the substitution of benzene and related substrates by phenyl radicals, for example, cyclohexadienyl... 

Scheme 18 Orbital phase properties for the electrophilic substitutions of benzenes with an electron-donating group...

A series of four fluorene-phenylene vinylene copolymers 320-323 clearly demonstrates the effect of the exact position of CN groups in the vinylene fragment on the emission of the materials (Scheme 2.48) [408], Substitution of benzene rings in copolymers 320 and 321 by thiophene results in red-shifted PL and EL, where copolymers 322 and 323 exhibit pure red emission with chromaticity values very close to the standard red (CIE x = 0.66, y = 0.34), although no PLQY values were reported. The ITO/PEDOT/322/Ca/Al device showed a very... 

The same procedure can be applied to all 13 isotopic substitutions of benzene, 9 of which are shown in Figure 6.5. The calculated frequencies of the fundamental vibrations of benzene and some of its substituted form are shown in Table 6.3. 

Some substituted guanidines have been obtained [457] by reaction of amines with the disulphide H2N(HN )C S S C( NH)NH2. Papers on the structure and p/fa s [458], and the synthesis [458, 459] of acylguanidines have been published. Reaction of guanidine with alkyl-, alkenyl-, and benzyl-halides, followed by distillation under basic conditions, is reported to give useful yields of amines [460]. A novel electrophilic substitution of benzene to give A -methyl-A -phenyl-guanidine amongst other products has been published [461 ]. 

Since benzenesulfonyl peroxide was used as an initiator in polymerization reactions, it was thought that a free radical aromatic substitution of benzene by the benzenesulfonoxy radical takes place. A detailed study by Dannley and Knipple reveals that attachment of the sulfonoxy group derived from a bis(arylsulfonyl) peroxide to the aromatic ring occurs by electrophilic aromatic substitution (equation 5) °. 

Viscosity measurements were performed in THF solution, after substitution of benzene by THF, in order to prevent a possible aggregation of species in hydrocarbon solvents. 

Furans substituted with electron releasing groups usually undergo polymerization with mineral acids due to facile protonation at the 2-position. Aluminum chloride also causes resinification of furan but benzo[6 ]furan and compounds with electron withdrawing groups are, however, more stable. The reversion to type so characteristic of the electrophilic substitution of benzene is by no means prevalent in the chemistry of furan and benzo[6]furan and many apparent electrophilic substitutions in reality proceed by addition. 

Study of isomer distribution in substitution of benzene rings already carrying one substituent presents some potential pitfalls. Inspection of product ratios for ortho, meta, and para substitution, as in investigation of electrophilic substitution (Section 7.4, p. 392), might be expected to give misleading results because of the side reactions that occur in radical substitution. The isomeric substituted cyclo-hexadienyl radicals first formed by radical attack partition between the simple substitution route and other pathways (Equation 9.102). In order for the... 

Clearly, as already summarized in Figure 4, benzene and allyl radical are quite special in having a combination of strong o-resistance which overcomes a moderately distortive Tr-component. On the contrary, in cyclobutadiene the fine balance is tipped in favor of the r-distortivity. However, this balance is fragile and can be tampered with. Even substituted cyclobutadienes have been considered at times to be effectively square.144-146 As we shall see later, manipulation of the o-resistance by proper substitution of benzene enables its r-distortivity to become manifest. Heilbronner s expression13 of the fragility of the jt o-balance for benzene and allyl radical is perhaps a most appropriate one If by an act of God all the spf—spf o-bond force constants were reduced to half... 

Relative Rates and Product Distributions for Substitution of Benzene and Toluene ... 

Novel techniques for the study of fast reactions were employed to study the bromination of AT.AT-dimethylaniline and its derivatives by Bell and Ramsden (1958). The second-order rate constant for the bromination of N,AT-dimethylaniline in acid solution was approximately 109 1. mole-1 sec-1. An estimate of the rate of bromination relative to the rate of substitution of benzene indicates that the methylated aniline is 1019 times more reactive (Robertson et al., 1953). The large influence of the p-dimethylamino substituent has discouraged extended quantitative study. Nevertheless, Eabom and his associates (Eabom, 1956 Eaborn and Pande, 1961a Eabom and Waters, 1960) assessed the influence of the group in several displacement reactions. Eaborn points out, however, that the resulting partial rate factors are only approximate. 

In the oxidative aromatic substitution of benzene with the nitrosonium cation (NO+), the benzene complex with symmetry 12 has been calculated as a local minimum at the B3LYP/6-31G(d) level of theory with an energy of 48 kcal mol-1 above that of the 7t-complex 13 <1999PCA4261> therefore, the former should not be relevant for the nitrosation mechanism as was previously proposed (Figure 3) <1985RJOC842>. 

Figure 3 Geometries of the local minimum 12 and of the (C6H5- -N0)+ 7r-complex 13 in the oxidative aromatic substitution of benzene with NO+ at the B3LYP/6-31 G(d) level.

It is thus probably impossible at present to interpret unambiguously the magnitude of p, and this difficulty is compounded by the paucity of data. Some p factors for electrophilic substitutions of benzene and thiophene are gathered in Table 6.14 only the acetylation, detritiation, and protio-... 

The sigma complex for meta substitution has its positive charge spread over three 2° carbons this intermediate is similar in energy to the intermediate for substitution of benzene. Therefore, meta substitution of toluene does not show the large rate enhancement seen with ortho and para substitution. 

Oftentimes one can use a quite resource-efficient method to obtain excellent geometries. Sequential ethynyP or cyano substitution of benzene leads to a systematic increase in the mean bond length within the ring. As seen in Table 2.3, quite excellent reproduction of this trend, and the bond distances themselves, can be had with even B3LYP/6-31G. ... 

In some cases w-substituted biphenyls (41) are also obtained. From these experiments relative rate constants were measured for the overall substitution of benzene derivatives by 3 b. These data are collected in Table 8. 

Table 8. Relative rate constants for overall substitution of benzene derivatives by. 36 2)...

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