Hydrogenation of Alkyl-Substituted Benzenes

Although the work described in Section 10.2 related predominantly to benzene, many of the studies described used toluene and the xylenes as well, and because the hydrogenation of all of these compounds had many features in common it would have been inappropriate to separate them. However there are some further aspects which merit attention, and which if introduced earlier might have interrupted the flow of the argument and also there are results to be mentioned concerning more highly substituted molecules. Some recapitulation of what has been said before is however inevitable in order to provide a complete picture. 

TABLE 10.4. Ratio of the Adsorption Coefficients for Toluene (Kj) and Benzene on the Noble Metals of Groups 8 to 10 

Pti-x ZXjc supported on carbon or alumina, Kt/Kb is proportional to x, suggesting electron transfer from platinum to zirconium, as predicted by the Engel-Brewer theory, and (2) chemisorption of sulfur on platinum has been shown to decrease electron density of the surface, while carbon has the opposite effect. The ratio Kt/Kb was very large for ruthenium, about 10 for rhodium and about unity for palladium, which may help to explain their different activities in these and other reactions. An extensive kinetic study of the hydrogenation of mixtures of benzene and toluene on NiA zeolite has however revealed a situation of some complexity, and it is not certain that the original simple concept is totally valid. 

It has been known since 1922 ° that the principal product formed by the hydrogenation of dialkylbenzenes is usually the corresponding Z-dialkylcyclohexane, although the amount of the /f-isomer depends upon a number of factors. (1) With the xylenes, it varies with the isomer, and with 

There have been lengthy discussions concerning the significance of these observations, without firm conclusions being reached. ° Epimerisation of the cyclohexane is clearly not a viable general explanation, and the phenomena are obviously related to those seen with alkylcyclohexenes, discussed in Chapter 7. Stereochemistry is presumably decided as the final pair of hydrogen atoms is added, so that Z-addition, as with the cyclohexenes, is not unexpected but the considerable formation of the -isomer may require the temporary desorption and re-adsorption of one of the intermediates, as was suggested previously (Section 7.5.3). 

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The hydrogenation of alkyl-substituted benzenes leads us into areas of stereochemistry that are related to those we have visited in Chapter 7 and the hydrogenation of fused or condensed aromatic ring systems likewise has fascinating stereochemical consequences also akin to those discussed in Section 7.5.3. The reduction of these molecules has mainly been conducted by organic chemists, and mechanistic aspects have scarcely been examined. 

The competihve hydrogenation of alkyl-substituted arenes was also performed with lr(0) nanoparhcles [49]. Using toluene as a standard substrate, several toluene/ benzene and toluene/monoalkylbenzene hydrogenation experiments were conducted in order to determine the selectivity constants of the transition-metal nanoparticles. These selechvity constants can be used to predict the relative reactivity of any other couple of monoalkylbenzenes. A series of initial reaction... 

The characteristic pattern of overtone bands and combination vibrations between 2000 and 1660 cm in the IR spectra of alkyl substituted benzene derivatives as well as the out-of-plane deformation vibrations of the hydrogen atoms in the IR spectra are frequently less reliable as indicators of the substitution pattern, especially in the presence of polar substituents, in which case their position tends to shift, and they often overlap with bands of substituents. 

The aromatic protons of alkyl substituted benzene rings usually appear as a single broad peak near 7.1 ppm or a complex multiplet in the range from 6.9-7.5 ppm for highly branched chains such as the tert-butyl group. Aliphatic groups shield the ortho aromatic hydrogens by a factor of about 0.34 ppm, as evidenced by the aromatic resonance of mesitylene (1,3,5-trimethyl benzene) which appears at 6.69 ppm. 

Hydrogen evolution by such a mechanism has been observed during photolysis of alkyl substituted benzene [352]. Further energy transfer to the dienes results in the formation of trienes, and so on. Evidence for the formation of conjugated sequences of double bonds has been also obtained from fluorescence spectra. 

Reaction of rhenium atoms with alkyl-substituted arenes forms dirhenium- l-arylidene compounds (2 2) (Figure 3). The products require insertion, presumably sequential, into two carbon-hydrogen bonds of the alkyl substituent. These reactions seem highly specific and require only the presence of an alkyl-substituted benzene that possesses a CH2 or CH3 substituent. Thus, co-condensation of rhenium atoms with ethylbenzene gives two isomers (see Figure 3) in which the products arise from insertion into the carbon-hydrogen bonds of the methylene or the methyl group. The product distribution in this reaction is in accord with statistical attack at all available sp3 C-H bonds. 

The hydrogenation of benzoic acid proceeds about equally over rhodium, platinum or Raney nickel. The conditions required are essentially those described for the hydrogenation of alkyl-benzenes (Section 2.5.1.1.1.1,6.1.). Since the esters are hydrogenated more readily than the free acid, an added esterification step is usually worthwhile. In general, hydrogenation of ortho-substituted benzoic acids occurs in a highly diastereoselective fashion giving nearly exclusively the cd-substituted cyclohexanes, e.g., 228 29. 

Aromatics Basic structures have one to six or more benzene rings with some of the carbon-hydrogen bonds replaced by carbon-carbon bonds of alkyl substituents. Generally frequency declines with an increasing number of rings. Alkyl-substituted benzenes with 1,4 alkyl groups have... 

Much of the basic information available on thermochemical aspects of HDA came initially from academic studies16-20 on pure compounds, undertaken to establish some of the basic chemistry of hydrogenations in general. Kistiakowsky et al.16 calorimetrically established that saturation of aromatic rings was exothermic and that the enthalpies of hydrogenation (H355.K) of a number of monoaromatics decreased with increasing alkyl substitution—benzene (49.8 kcal/mole), ethylbenzene (48.9 kcal/mole), o-xylene (47.25 kcal/mole), and 1,3,5-trimethyl-benzene (47.62 kcal/mole). 

Fig. 1. Hydrogenation of the ring of alkyl-substituted aromatic compounds with 5% Rh on AljOs powder as catalyst and 100 ml. glacial acetic acid as solvent. 1) 1 g. catalyst, 0.5 ml. benzene 2) 1 g. catalyst, 0.5 ml. toluene 3) 1 g. catalyst, 1 ml. p-xylene 4) 1 g. catalyst, 1 ml. mesitylene 5) 1 g. catalyst, 0.5 ml. butylbenzene 6) 1 g. catalyst, 500 mg. dibenzyl 7) 2 g. catalyst, 500 mg. durene.

There is ample chemical evidence that the 1-alkenyl substituent on the arene plays a crucial role during the assembly of the metal cluster. For example, no arene cluster complexes are formed with alkyl substituted benzene derivatives. Such complexes, 14, are perfectly stable and could be obtained indirectly through hydrogenation of the corresponding /rs-alkenylbenzene precursors (Table... 

It is also noteworthy that the iridium complex (lb) is a very much poorer catalyst than the rhodium analog. This point and the fact that the more highly alkyl-substituted benzenes are only hydrogenated to a small degree, even by [RhC5Me3Cl2]2, can be explained by the observations that (i) iridium forms more stable complexes of the type [M(CoMe5)-(arene )] than rhodium does and (ii) within a series of alkylbenzenes, the more highly substituted ones form the more stable arene complexes for both Rh and Ir (20,21). Presumably the more stable complexes are reduced less easily. 

Alkyl-substituted benzenes ionize by loss of a rr electron and undergo loss of a hydrogen atom or methyl group to yield the relatively stable tropylium ion (see Section 14.7C). This fragmentation gives a prominent peak (sometimes the base peak) at mlzS> ... 

AU of the results of substitution of various groups for hydrogen on benzene (CeH ) outhned above also apply to alkyl-substituted benzenes. They apply as well to other aromatic systems. In both cases, the symmetry of benzene (CeHe) has been perturbed and thus it is common for more than one product to be obtained. Consider the case of naphthalene (CioHs), the first of the class of polynuclear aromatic hydrocarbons. A samphng of some of the reactions of naphthalene (CioHg) is provided in Table 6.13 and, when comparing the reactions to those of Table 6.12, it is important to note that it is common to find simultaneous formation of more than one substitution product. 

In order to achieve high yields, the reaction usually is conducted by application of high pressure. For laboratory use, the need for high-pressure equipment, together with the toxicity of carbon monoxide, makes that reaction less practicable. The scope of that reaction is limited to benzene, alkyl substituted and certain other electron-rich aromatic compounds. With mono-substituted benzenes, thepara-for-mylated product is formed preferentially. Super-acidic catalysts have been developed, for example generated from trifluoromethanesulfonic acid, hydrogen fluoride and boron trifluoride the application of elevated pressure is then not necessary. 

The hydrogenation of simple alkenes, such as hexene, cyclohexene, cyclo-hexadiene and benzene, has been extensively studied using biphasic, alternative solvent protocols. These hydrocarbon substrates are more difficult to hydrogenate compared to substrates with electron withdrawing groups. Benzene and alkyl substituted aromatic compounds are considerably more difficult to hydrogenate... 

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