Aryl Derivatives of Benzene

Diarylethanes, ArCH CH Ar, can be prepared from 1,2-dichloroethane and arenes. Triphenylmethane, PhgCH, can be obtained by reacting an excess of benzene with trichloromethane under Friedel-Crafts conditions. 

Reactions of diphenylmethane are similar to those of biphenyl, since the benzyl group, CgH CH, is also ortholpara directing, although bromi-nation results in reaction at the methylene group. 

Alkylbenzenes are prepared by the Friedel-Crafts reaction in which benzene is reacted with an alkyl halide in the presence of a Lewis acid such as AlCl. Diarylmethanes are formed in this manner from dichloromethane and triarylmethanes from trichloromethane. 

Alkylbenzenes are attacked at the 2- and 4-positions by electrophiles. The reaction is easier than for benzene. 

Biphenyls can be obtained from the reaction of bromobenzene with sodium and from diazonium salts. 

Dial ylethanes, ArCH CH. Ar, can be prepared from t.2-dichloroethane and arenes. 

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Aryl halide derivatives of benzene and toluene have many uses in chemical synthesis as pesticides and raw materials for pesticides manufacture, solvents, and a diverse variety of other applications. These widespread uses over many decades have resulted in substantial human exposure and environmental contamination. Three example aryl halides are shown in Figure 1.17. Monochlorobenzene is a flammable liquid boiling at 132°C. It is used as a solvent, heat transfer fluid, and synthetic reagent. Used as a solvent, 1,2-dichlorobenzene is employed for degreasing hides and wool. It also serves as a synthetic reagent for dye manufacture. Bromobenzene is a liquid boiling at 156°C that is used as a solvent, motor oil additive, and intermediate for organic synthesis. 

Considering what was said about stabilization energies in our previous discussion of thermochemical mimics, we now turn to aryl halides. There are several conceptual approaches to their thermochemistry one can take. The first is to consider halogenated derivatives of benzene, then of naphthalene, then of the isomeric anthracene and phenan-threne, etc. This approach, perhaps more appropriate for a study of generally substituted aromatic hydrocarbons, is immediately thwarted. Although there are many appropriate derivatives of benzene worthy of discussion, thermochemical data on halogenated naphthalenes are limited to the isomeric 1- and 2-monosubstituted derivatives, and halogenated derivatives of other aromatics remain thermochemically unstudied. 

Nucleophiles containing elements of the second row have also been used in photosubstitution reactions of aryl halides. The selenocyanate ion739, selenourea740 and thiourea741 are capable of replacing halogen atoms in various derivatives of benzene, naphthalene, pyridine and pyrimidine. Both chlorine atoms in 6>rj/zc>-dichlorobenzene are substituted upon irradiation in (MeO)3P at 60 °C for 5 days and a 50% yield of 1,2-bis(phosphino)-benzene is obtained742. 

In derivatives of benzene, a hydrogen atom is replaced by a functional group. The group C6H5- is called a phenyl group, and the general name for substituted derivatives of the phenyl group is aryl. 

Alkyl derivatives of benzene may be prepared by reacting an alkyl halide and an aryl halide with sodium in an inert solvent such as diethyl ether (Scheme 3.5). Although symmetrical by-products are also formed, it is possible to introduce long unbranched side-chains by this route without isomerization occurring. 

With the simple aryl halides such as the mono-chlor derivatives of benzene or its homologues this reaction does not take place. Tf, however, a benzene halide has also substituted in the ring two nitro, sul-phonic acid or carboxyl groups, in the ortho and para positions to the halogen, then treatment of the halide with potassium hydroxide results in replacing the halogen with hydroxyl and the corresponding substituted phenol will be obtained. 

Tetrachlorocyclopropene in the presence of Lewis acids or the trichlorocyclopropenyl cation are important starting materials in cyclopropenone syntheses In this method the cyclopropene or salt is reacted with benzene or derivatives of benzene bearing functional groups such as alkyl, alkoxy, hydroxy, or halogen to yield diaryl-substituted cations. Upon hydrolysis these cations yield the cyclopropenone. In some cases the monoaryl cation (32) can be obtained and converted to the aryl chlorocyclopropenone 33. Alternatively, the monoaryl cation can be reacted with a second aromatic species to give cyclopropenones with different aryl groups (equation 30). 

Consensus on nomenclature had been reached by the 1890s. Aniline was the parent of its derivatives, though sulfonic acids were considered derivatives of benzene, such as aminobenzenesulfonic acid. The prefix amino- was added to naphthalene and its derivatives. Many trivial names came into use, particularly for aminonaphthalenesulfonic acids, found in both academic and industrial research laboratories. Though IUPAC convention now numbers amino aryl compounds according to the parent hydrocarbon, the earlier system of numbering has often been retained, since some names include the positions of substituents at carbon atoms numbered according to the older systems. 

Suitable solvents for the preparation of alkyl derivatives of lithium are hydrocarbons such as benzene, cyclohexane, light petroleum, and, when the alkyllithium is to be brought into immediate further reaction, also diethyl ether or tetrahydrofuran, as in the last two the reaction is appreciably faster. A paper by Gilman and Schwebke39 should be consulted concerning the stability of 72-alkyl derivatives of lithium in mixtures of diethyl ether and tetrahydrofuran. Aryl derivatives of lithium are almost always prepared in diethyl ether since they do not react with ethers. All solvents should, of course, be completely anhydrous they should have been kept over sodium for a considerable time. 

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. 

Alkyl- and aryl-hydrazones of aldehydes and ketones readily peroxidise in solution and rearrange to azo hydroperoxides [1], some of which are explosively unstable [2], Dry samples of the p-bromo- and p-fluoro-hydroperoxybenzylazobenzenes, prepared by oxygenation of benzene solutions of the phenylhydrazones, exploded while on filter paper in the dark, initiated by vibration of the table or tapping the paper. Samples were later stored moist with benzene at —60°C to prevent explosion [3], A series of a-phenylazo hydroperoxides derived from the phenyl-or p-bromophcnyl-hydrazones of acetone, acetophenone or cyclohexanone, and useful for epoxidation of alkenes, are all explosive [4], The stability of several substituted phenylazo hydroperoxides was found to be strongly controlled by novel substituent effects [5],... 

The preceding examples show the possibilities and also the present limitations of PTC. The conversion of the heterocycle to its /V-oxide or complex-ation of benzene with Cr(CO)3 promote reaction of unactivated compounds. But these approaches are based on a multistep procedure. A very promising new method is the one-step reaction of unactivated aryl (and heteroaryl) halides by way of the SRN1 mechanism developed by Bunnett.232,233 This has been applied to heterocyclic molecules mainly by Wolfe and Carver.234 A pyridine derivative serves as an example. 

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