Benzene and its reactions with electrophiles

Benzene is a planar symmetrical hexagon with six trigonal (sp2) carbon atoms, each having one hydrogen atom in the plane of the ring. All the bond lengths are 1.39 A (compare C-C 1.47 A and C=C 1.33 A). All the 13C shifts are the same (Sc 128.5 p.p.m.). 

The special stability of benzene (aromaticity) comes from the six tc electrons in three molecular orbitals made up by the overlap of the six atomic p orbitals on the carbon atoms. The energy levels of these orbitals are arranged so that there is exceptional stability in the molecule (a notional 140 kjmor1 over a molecule with three conjugated double bonds), and the shift of the six identical hydrogen atoms in the NMR spectrum (5h 7.2 p.p.m.) is evidence of a ring current in the delocalized tc system. 

This section revises material from Chapters 4,8, and 11 where more details Gan be found. 

Benzene is symmetrical and the circle in the middle best represents this. However, it is impossible to draw mechanisms on that representation so we shall usually use the Kekule form with three double bonds. This does 

When we move away from benzene itself to discuss molecules such as phenol, the bond lengths are no longer exactly the same. However, it is still all right to use either representation, depending on the purpose of the drawing. With some aromatic compounds, such as naphthalene, it 

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Analogously, in the presence of silica-supported palladium catalysts, benzene is oxidized under ambient conditions to give phenol, benzoquinone, hydroquinone and catechol [37b]. Palladium chloride, used for the catalyst preparation, is believed to be converted into metallic palladium. The synthesis of phenol from benzene and molecular oxygen via direct activation of a C-H bond by the catalytic system Pd(OAc)2-phenanthroline in the presence of carbon monoxide has been described [38]. The proposed mechanism includes the electrophilic attack of benzene by an active palladium-containing species to to produce a a-phenyl complex of palladium(ll). Subsequent activation of dioxygen by the Pd-phen-CO complex to form a Pd-OPh complex and its reaction with acetic acid yields phenol. The oxidation of propenoidic phenols by molecular oxygen is catalyzed by [A,A"-bis(salicylidene)ethane-l,2-diaminato]cobalt(ll)[Co(salen)] [39]. 

By analogy with benzene, hiran undergoes reactions with electrophilic reagents, often with substitution. However, it can also react by addition and/or ring-opening depending on reagent and reaction conditions. 

The industrial synthesis of thiophene is based on butane and its reaction with sulfur or carbon disulfide. In comparison with benzene, thiophene can be more easily substituted electrophilically in the 2- or 5- position this reaction is used to produce a wide range of pharmaceutical products. An example of this is the antihistamine thenalidine, which is obtained from thiophene by chloromethylation to 2-thienylmethyl chloride followed by reaction with 4-anilino-l-methylpiperidine. 

In the preceding section, benzene reacted with cations to form substituted benzene derivatives. The cations of interest include Br+, C1+, the nitronium ion, and sulfur trioxide or sulfuric acid, which react as electrophiles. In principle, benzene may react with any cation, including carbocations, once that cation is formed. Carbocations are generated by several different methods they react with nucleophiles, as described for reactions of alkenes with acids such as HX (Chapter 10, Section 10.2) and for S l reactions (Chapter 11, Section 11.4). If benzene reacts with a carbocation, a new carbon-carbon bond is formed, and electrophilic aromatic substitution will give an arene. The reaction of benzene and its derivatives with carbocations is generically called the Friedel-Crafts reaction, after the work of French chemist Charles Friedel (France 1832-1899) and his American protege, James M. Crafts (1839-1917). The reaction takes two fundamental forms Friedel-Crafts alkylation and Friedel-Crafts acylation. Both variations will be discussed, beginning with the alkylation reaction. 

When benzene is alkylated, it reacts with a haloalkane in the presence of aluminium chloride as a catalyst. In the example shown, chloromethane is used and the electrophile CH is generated from its reaction with aluminium chloride ... 

Since the early days of organic chemistry, nitration has been considered to be an important reaction and has been widely used. As early as 1825 Faraday discovered benzene and recorded its reaction with nitric acid. Shortly after, the use of nitric acid sulfuric acid mixtures to effect nitration was reported and was soon quoted in a patent. Nitration figured prominently in the development of ideas of theoretical organic chemistry in the early part of the twentieth century and, as the most widely applicable and most widely used example of electrophilic substitution, it played an important role in the consideration of aromatic stability and reactivity. In 1910 the first report of orientation and deactivation in aromatic electrophilic substitution was published (10MI1). 

About (i) and (2) there can be no dispute, but (3) must be rejected. The implication that the nitronium ion, effectively freed from a close association with another entity, is not the nitrating agent in those reactions of benzene and its homologues, under conditions in which substantial intermolecular selectivity is observed, conflicts with previous evidence ( 3.2). Thus, in nitration in organic solvents and in aqueous nitric acids, the observation of kinetically zeroth-order nitration, and the effect of added nitrate on this rate, is compelling evidence for the operation of the nitronium ion. The nitric acidium ion is not the electrophile under these conditions, and it is difficult to envisage how a species in which the water is loosened but not yet completely eliminated could be formed in a slow step independent of the aromatic and be capable of a separate existence. It is implicit that this species should be appreciably different from the nitronium ion in its electrophilic properties. There is no support to be found for the participation of the aromatic in the formation of the electrophile. 

The hydrated electron is obviously a nucleophile and its reactions are affected by substituents correspondingly. The hydroxyl radical is expected to behave as an electrophile and this behaviour was, indeed, demonstrated with aromatic compounds. The low reactivity of O toward aromatic and olefinic ir-systems suggests that this species behaves as a nucleophile because of its charge. The behaviour of hydrogen atoms is not easily predictable the effect of substitution in benzene demonstrated a slight electrophilicity. 

Reactions are achieved with benzene and its derivatives (including those deactivated for electrophilic attack), polycyclic aromatics, heteroaromatics and transition-metal complexes of aromatic ligands. 

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