Benzene and its Alkyl Derivatives

Aromatic hydrocarbons are readily protonated in strongly acidic media, e.g. sulfuric and chlorosulfonic acid (Equation 1). 

The protonated form of the hydrocarbon is considerably less reactive towards electrophilic substitution than the unprotonated substrate. 

The early work on the sulfonation and chlorosulfonation of aromatic compounds has been extensively documented in Suter s book and for aromatic hydrocarbons and their halogen derivatives by Suter and Weston. More recently, the chlorosulfonation of aromatic and heteroaromatic systems was reviewed by 

Bassin, Cremlyn and Swinboume. Details of kinetic and mechanistic studies on sulfonation and chlorosulfonation of aromatic hydrocarbons and other derivatives have been described in Chapter 2. Chlorosulfonic acid is a very active sulfonating agent and when benzene is added to an excess of the reagent at room temperature, benzenesulfonyl chloride is formed ( 70%) with only a trace of diphenyl sulfone. On the other hand, when an excess of benzene is present, the major products are diphenyl sulfone and benzenesulfonic acid.  

The reaction of toluene 1 with chlorosulfonic acid (two equivalents) at 10 °C gives mainly a mixture of the o- and /7-sulfonyl chlorides (4 and 2 X = Cl). Harding was able to obtain toluene-/ -sulfonyl chloride (2 X = Cl) in high yields (95%) by reaction of excess chlorosulfonic acid on toluene at 75-80 °C low temperatures favoured o-sulfonation (see also Chapter 2 p 18). 

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In photoassociating solvents (e.g., benzene and its alkyl derivatives) the transfer of electronic excitation energy from solvent to fluorescent solute is characterized37,63 by rate constants kt which are approximately twice those computed from the less approximate form of Eq. (7) ... 

Not only benzene and its alkyl derivatives can be used as the aromatic component but also naphthalene [341], phenol [340,341] and thiophene [341,342]. Low-molecular weight alkenes, Cj to C5, cyclohexene and dodecene have served as alkylating agents. 

Ungvary G, Tatrai E. 1985. On the embryotoxic effects of benzene and its alkyl derivatives in mice, rats and rabbits. Arch Toxicol Suppl 8 425-430. 

The term BTEX concerns benzene and its alkyl derivatives such as toluene, ethylbenzene and xylenes. These compounds are semivolatile and, therefore, the solution must be prepared with great attention in order to minimise evaporation loss. 

Benzene and its alkyl derivatives do not ordinarily form cation radicals in sulfuric acid. / -Xylene was once thought to give xylene cation radical in solutions of persulfuric-sulfuric acid, but is now known to give the semiquinone cation radical instead (8) (Brivati et... 

Benzene and its alkyl derivatives are classified as aromatic hydrocarbons, or arenes. 

Benzene and its Alkylated Derivatives.—Interest in direct synthesis of six-membered carbocyclic compounds, lacking over the years, has been stimulated by the current interest in cycloaddition reactions. A useful synthesis of 1,2,4-triphenylbenzene (1) by the Diels-Alder reaction of 2,3,5-tri-phenylcyclopentadienone (2) with dimethyl acetylenedicarboxylate has been reported (Scheme 1). 

The object of these studies has been the determination of the degree of association in thiazole and its alkyl derivatives. Various solvents have been used cyclohexane (154), carbon tetrachloride (155, 156), benzene and nitrobenzene (157). 

The dipole moment varies according to the solvent it is ca 5.14 x 10 ° Cm (ca 1.55 D) when pure and ca 6.0 x 10 ° Cm (ca 1.8 D) in a nonpolar solvent, such as benzene or cyclohexane (14,15). In solvents to which it can hydrogen bond, the dipole moment may be much higher. The dipole is directed toward the ring from a positive nitrogen atom, whereas the saturated nonaromatic analogue pyrroHdine [123-75-1] has a dipole moment of 5.24 X 10 ° C-m (1.57 D) and is oppositely directed. Pyrrole and its alkyl derivatives are TT-electron rich and form colored charge-transfer complexes with acceptor molecules, eg, iodine and tetracyanoethylene (16). 

The hydrogenation of benzene and its alkyl-substituted derivatives takes place stepwise (Scheme 11.5). On the basis of instrumental and isotope exchange studies, the involvement of tt-adsorbed (8, 9) and cr-adsorbed (10) species was suggested 96-98... 

Fig. 38. Absorption spectra of nitrobenzene and its alkyl derivatives Z-nitro-benzene, II-o-nitrotoluene, III-o-nitrocumene, IV-nitromesitylene, V-o-nitro-tert-butylbenzene, Vl-m-nitro-tert-butylbenzene, Vll-m-nitrotoluene, VIII-p-nitro-tert-butylbenzene, IX-p-nitrocumene, X-p-nitrotoluene (Brown and...

Electronic spectra of samples of tetramethylcyclobutadiene prepared from different precursors differ among themselves, again indicating intermolecular interactions within the matrix [34]. Thus if pyridazine or phthalic derivatives are also present the products are coloured whereas when o-di(hydroxymethyl)benzene or 3,4-benzo-dihydrofuran are the coproducts the mixtures are colourless [34]. Cyclobutadiene and its alkyl derivatives either absorb at about 300-305 nm or have no bands above 300 nm, consistent with rectangular singlet structures [34-37]. 

As in the alkylation reaction, the reactive intermediate in Friedel-Crafts acylation can be a dissociated acylium ion or a complex of the acid chloride and Lewis acyl.49 Recent mechanistic studies have indicated that with benzene and slightly deactivated derivatives, it is the protonated acylium ion that is the kinetically dominant electrophile.50... 

Alkyl halides, alcohols, epoxides, carboxylic acid and its derivatives, and benzene and its derivatives. 

Oxidation to Quinones. Direct oxidation of arenes to quinones can be accom-plished by a number of reagents. Very little is known, however, about the mechanism of these oxidations. Benzene exhibits very low reactivity, and its alkyl-substituted derivatives undergo benzylic oxidation. Electrochemical methods appear to be promising in the production of p-benzoquinone.797 In contrast, polynuclear aromatic compounds are readily converted to the corresponding quinones. 

The range of preparatively useful electrophilic substitution reactions is often limited by the acid sensitivity of the substrates. Whereas thiophene can be successfully sulfonated in 95% sulfuric acid at room temperature, such strongly acidic conditions cannot be used for the sulfonation of furan or pyrrole. Attempts to nitrate thiophene, furan or pyrrole under conditions used to nitrate benzene and its derivatives invariably result in failure. In the case of sulfonation and nitration milder reagents can be employed, i.e. the pyridine-sulfur trioxide complex and acetyl nitrate, respectively. Attempts to carry out the Friedel-Crafts alkylation of furan are often unsuccessful because the catalysts required cause polymerization. 

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