Other Arenes

Hydrogenation of benzene and other arenes is more difficult than hydrogenation of alkenes and alkynes Two of the more active catalysts are rhodium and platinum and it IS possible to hydrogenate arenes m the presence of these catalysts at room temperature and modest pressure Benzene consumes three molar equivalents of hydrogen to give cyclohexane... 

One explanation for the structure and stability of benzene and other arenes is based on resonance according to which benzene is regarded as a hybrid of the two Kekule structures... 

Chlorination is carried out m a manner similar to brommation and provides a ready route to chlorobenzene and related aryl chlorides Fluormation and lodmation of benzene and other arenes are rarely performed Fluorine is so reactive that its reaction with ben zene is difficult to control lodmation is very slow and has an unfavorable equilibrium constant Syntheses of aryl fluorides and aryl iodides are normally carried out by way of functional group transformations of arylammes these reactions will be described m Chapter 22... 

Why are some organic compounds colored while others aren t /3-Carotene, the pigment in carrots, is purple-orange, for instance, while cholesterol is colorless. The answer involves both the chemical structures of colored molecules and the way we perceive light. 

Elegant evidence that free electrons can be transferred from an organic donor to a diazonium ion was found by Becker et al. (1975, 1977a see also Becker, 1978). These authors observed that diazonium salts quench the fluorescence of pyrene (and other arenes) at a rate k = 2.5 x 1010 m-1 s-1. The pyrene radical cation and the aryldiazenyl radical would appear to be the likely products of electron transfer. However, pyrene is a weak nucleophile the concentration of its covalent product with the diazonium ion is estimated to lie below 0.019o at equilibrium. If electron transfer were to proceed via this proposed intermediate present in such a low concentration, then the measured rate constant could not be so large. Nevertheless, dynamic fluorescence quenching in the excited state of the electron donor-acceptor complex preferred at equilibrium would fit the facts. Evidence supporting a diffusion-controlled electron transfer (k = 1.8 x 1010 to 2.5 X 1010 s-1) was provided by pulse radiolysis. 

Benzene and other arenes can be oxidized to c -l,2-cyclohexadienediol enan-tiospecifically using a mutant of Pseudomonas putida through microbial techniques (equation 188)310. 

Species functionalized at boron, such as 50-53, have also been prepared from the corresponding MC2B4 closo clusters via the removal of the apex BH unit (decapitation) with I M Kl)A.54 57 The same approach has been employed to link CoC2B3 units to benzene or other arenes 5 As shown in the following subsection, di- and trinuclear complexes such as 53 and 54 are easily converted to multi-triple-decker sandwich systems. 

Menaker We are now doing this in our laboratory. The preliminary results are interesting some tissues are compensated, others aren t. 

Slight decomposition during the reaction indicated by formation of grey-green precipitates was found difficult to avoid with chlorobenzene and some other arenes. This appears to become progressive and reaction should be stopped and solutions filtered when such precipitation is observed. Solvent dibutyl ether (60 itiL) + tetrahydrofuran (5 mL) most of the product from this reaction crystallized from the solution when cooled in ice. 

Reactions of cobalt and nickel atoms with toluene and other arenes yield condensates in which the metal is in a very reactive state (105). However, none of the products of reaction of these condensates with other ligands has contained the arene coordinated to the metal. It seems possible that the condensates contain ditoluenecobalt and ditoluene-nickel, but in these complexes (unlike the chromium or iron complexes) the two arene rings are probably not symmetrically bonded to the metals. 

The XH and 13C NMR spectra of (i76-C6H5SiMe3)Cr(CO)3 have been analyzed in detail and compared with those of other arene-chromium tricarbonyl complexes (48, 125). 

Some of the reagents used in olefin epoxidation can be applied in the direct oxidation of arenes to arene oxides. Benzene oxide, however, like other arene oxides, exists in equilibrium with oxepin, its valence tautomer, and has not been isolated. Existence of benzene oxides as intermediates can be concluded from observations like the NIH shift discussed above.752,753... 

Oxidation of Other Arenes. Aromatic compounds with longer alkyl side chains can be converted to ketones or carboxylic acids. All the previously discussed reagents except Cr02Cl2 usually afford the selective formation of ketones from alkyl-substituted arenes. Oxidation with Cr02Cl2 usually gives a mixture of products. These include compounds oxidized in the P position presumably formed via an alkene intermediate or as a result of the rearrangement of an intermediate epoxide.110,705... 

The selectivity of chlorination reactions carried on in solution is increased markedly in the presence of benzene or alkyl-substituted benzenes because benzene and other arenes form loose complexes with chlorine atoms. This substantially cuts down chlorine-atom reactivity, thereby making the chlorine atoms behave more like bromine atoms. 

Why is it that some objects are chiral but others aren t In general, an object is not chiral if, like the coffee mug, it has a symmetry plane cutting through its middle so that one half of the object is a mirror image of the other half. If you were to cut the mug in half, one half of the mug would be the mirror image of the other half. A hand, however, has no symmetry plane and is therefore chiral. If you were to cut a hand in two, one "half" of the hand would not be a mirror image of the other half (Figure 20.20). 

Parabanic acid (86) has been shown to produce highly electrophilic species in superacidic CF3SO3H, and the resulting electrophile is capable of reacting with and moderately deactivated arenes.40 The superacid-promoted condensation reaction of parabanic acid with benzene (and other arenes) in CF3SO3H provides satisfactory yields of the 5,5-diarylhydantoins (eq 30),... 

Benzene is one of the major chemicals produced by the petroleum industry. More than 1.6 billion gallons are produced each year by cracking and reforming various petroleum fractions. Most of this is used in the production of styrene, which is then polymerized to polystyrene. Other arenes that are made in large amounts include toluene (830 million gallons), cumene, o-xylene, and p-xylene. At one time, benzene was an important solvent in the organic laboratory. Recently, however, its use has been phased out because of its potential adverse health effects. Long exposure to benzene has been shown to lead to bone marrow depression and leukemia. 

The U ring distances are short relative to other ) -arene complexes, ranging from 2.503(9) to 2.660(8) A. In addition, there is a slight distortion in the bound toluene hgand the average C-C distance increases by approximately 0.04A compared to that found in free toluene. Density functional calculations carried out on the molecule suggest that four... 

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