REACTIONS OF CYCLIC HYDROCARBONS

During the past few years a very considerable amount of work has been done on the decompositions and isomerizations of cyclic compounds. Perhaps the most important reason for the recent interest in these reactions of cyclic hydrocarbons is that a number of them occur in a reasonably simple manner, so that they are particularly suitable for the evaluation of theories of unimolecular reactions. The simplest of these processes is the conversion of cyclopropane into propene 

This reaction almost certainly occurs in an elementary manner, and it therefore represents one of the simplest of all unimolecular reactions. As will be discussed later, it has been carefully studied over a wide range of temperatures and pressures, and has played an important part in the testing of the theories of unimolecular reactions. 

The reactions that occur when cyclic hydrocarbons are heated are of various types, and may be classified as follows. 

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Later on, Kazanskii and his co-workers, using Raman spectroscopy to analyze the reaction products, systematically studied the contact reactions of cyclic hydrocarbons, especially cyclobutanes and cyclopentanes. The important results that they obtained, none of which was invalidated when more sophisticated analytical tools were available, may be summarized as follows ... 

Reactions of Cyclic Hydrocarbons in the Presence of Metals of Group VIII of the Periodic System... 

REACTIONS OF CYCLIC HYDROCARBONS ON GROUP VIII METALS 785... 

Table I.IS gives total bonding energies in kilocalories per mole for some simple molecules. The B3iyP results are comparable in accuracy to G1 and G2 results. Another comparison was done with a series of cyclic hydrocarbons as the test case. The calculations were done using an isodesmic reaction scheme. The results are given in Table 1.19. Density functional calculations have also been successfully extended to functionalized molecules. ...

The reaction of cyclic conjugated dienes with la in hydrocarbon solvent also gives stereospecific rraMi-l-silyl-S-vinylcyclopentanes as the major products. 

In the case of pentanes, disproportionation to isobutane and hexane is pronounced. This undesirable side reaction can be suppressed by the addition of small amounts of cyclic hydrocarbons or by operation under hydrogen pressure (6). 

The hexanes undergo side reactions even more readily than do the pentanes. Although disproportionation and cracking can be suppressed to some extent by the addition of cyclic hydrocarbons, this treatment is not effective enough to ensure satisfactory catalyst life, and hydrogen at relatively high pressure must be used as the inhibitor. 

The butanes show little tendency to crack or disproportionate (7) thus butane isomerization is fairly straightforward. However, the suppression of side reactions becomes more difficult as the molecular weight increases. With pentanes, disproportionation to isobutane and hexane is pronounced, amounting to as much as 63%. A typical composition of pentane disproportionation products is shown in Table III. Besides lowering the yield of isopentane, such side reactions shorten the life of the catalyst. Adding small amounts of cyclic hydrocarbons (7,15,18)... 

Thermal cyclooligomerizations of olefins and alkynes require severe and often dangerous reaction conditions and the yields of cyclic products are usually very low. Acetylene ean be trimerized to benzene at 500 °C [1] and butadiene (BD) dimerizes at 270 °C and under high pressure to give small amounts of 1,5-cyclo-octadiene [2]. Reppe s discovery in 1940 that acetylene can be cyclotetramerized to cyclooctatetraene (COT) using a nickel catalyst [3] shows that transition metals can act as templates for the synthesis of cyclic hydrocarbons from acetylenic or olefinic building blocks (Scheme 1). 

The thermal disrotatory [n6] electrocyclization of cis-1,3,5-hexatriene systems has been extensively employed for the synthesis of cyclic hydrocarbons. The average enthalpy of activation is in the range of about 120 kJ mol 1 [36]. The incorporation of two of the hexatriene double bonds in phenyl rings (stil-bene, 1) stabilizes the precursor significantly and necessitates temperatures of 1050°C to obtain a 30% yield of phenanthrene (2, see Scheme 2, [37]). An enthalpy of activation of (250 20) kJ mok was estimated for the conversion of 9,9 -bifluorenylidene (3) to benz[e]indeno[l,2,3-hi]acephenanthrylene (4), a reaction that is accompanied by the radical initiated isomerization of 3 to diben-zo[g,p]chrysene (5, Scheme 2, [38]). It is assumed that both reactions 1 —> 2 and 3 —> 4 are initiated by an electrocyclic ring closure forming a 4 a,4 fr-dihydro-phenanthrene (la) intermediate. 

Garin et al. (58) also compared the contact reactions of C6 hydrocarbons over several Pt single crystals, foils, and supported catalysts. They found that isomerization by bond-shift or cyclic mechanisms and hydro-genolysis over crystalline Pt surfaces simulate well data for supported Pt of large Pt cluster size, and that isomerization by bond shift is more important when the surface is stepped. 

This computation shows that the complex process represented by Equation 3 is by far the most important initial mode of reaction. The result is a product that consists almost entirely of cyclic hydrocarbons. At higher conversion levels, more light paraffins are formed, but the processes leading to cyclic species are still predominant. 

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