Benzene formation

Cyclotrimerisation at 140K takes place almost exclusively in the disordered areas between the domains of (3 x 3) R3Q° but simultaneously with the formation of this phase. Benzene formation stops when the (3 x 3) R30° phase is close to completely covering the surface. No further reaction occurs on further exposure to acetylene or on heating to 200 K, establishing that the acetylene... 

The effect of microwave irradiation on the catalytic hydrogenation, dehydrogenation, and hydrogenolysis of cydohexene was studied by Wolf et al. [81]. Optimum conditions for benzene formation were a hydrogen flow, N-CaNi5 catalyst, atmospheric pressure, and 70 s irradiation time. Cydohexane was the main product when the irradiation time was 20 s, or in a batch/static system. 

There are several examples of the concerted mechanism. However, no report of an insertion of a carbon—carbon triple bond into a metallacyclopentadiene had appeared prior to discovery of this reaction. At low temperatures, during the reaction of zirconacyclopentadienes with DMAD, the formation of trienes (79) is observed upon hydrolysis. This clearly indicates that the benzene formation involves the insertion (addition) reaction of DMAD. As shown in Eq. 2.50, the alkenyl copper moiety adds to the carbon—carbon triple bond of DMAD and elimination of Cu metal leads to the benzene derivatives 72. Indeed, a copper mirror is observed on the wall of the reaction vessel. 

Eq. 2.50. Reaction mechanism of benzene formation from zirconacyclopentadienes and DMAD in the presence of CuCI. 

Benzene formation from three different alkynes... 

Transition metal mediated or catalyzed benzene formation reactions have been reported using various metals. However, the use of three different alkynes is difficult [38], In many cases, a mixture of several benzene derivatives is obtained. In 1974, Wakatsuki and Yamazaki used three different alkynes with Co complexes [27b], but isomers were formed and a tedious chromatographic separation was necessary. The first selective coupling of three different alkynes in high yields was reported in 1995 using a combination of unsymmetrical zirconacydopentadienes and CuCl, as shown in Eq. 2.52 [7k]. 

Eq. 2.52. Benzene formation from three different alkynes using CuCl via a zirconacyclopentadiene. 

R alkyl, R = alkyl Eq 2.56. Benzene formation from zirco-82 50-55% nacyclopentadienes catalyzed by Ni. 

Pentacene is a useful compound in materials science, in particular for use in solar batteries, superconductors, and laser emission materials, etc. Unfortunately, however, pentacene is not soluble in organic solvents. As shown in Eq. 2.57, a combination of the aforementioned benzene formation reaction and alkynylation of phthalate leads to alkyl-substituted pentacenes, which are very soluble in various organic solvents... 

Ring closure of triazinyl diester 276 was reported by Moderhack etal. <2002M1165>. This compound was treated with hydrazonoyl chloride in the presence of triethylamine in refluxing benzene. Formation of the intermediate 277 was anticipated, which, upon intramolecular cyclization, afforded the fused triazole 278 in medium yield (42%). Syntheses of other related compounds via modified synthetic routes have also been described. 

Side reactions are exchange of organic groups, followed by homo coupling, P-hydrogen elimination with alkene formation, isomerisation followed by coupling, or benzene formation and alkene liberation. Examples are shown in Figure 13.22. 

The entropy factor should also be considered since cyclization results in a more ordered structure. The C5 cyclization of n-hexane involves an entropy decrease of about 15-17 entropy units (e.u.). The corresponding values for cyclohexane and benzene formation are about 25 and 38-45 e.u., respectively. These values are comparable with calculated values of adsorption entropy (29). Thus, adsorption of a molecule to be cyclized may supply a considerable part of the entropy change in other words, adsorption should take place in a geometry favorable for cyclization. This is one of the main roles of the catalyst. 

Stepwise cyclohexane dehydrogenation revealed the possible importance of unsaturated intermediates in benzene formation 48). 

That diolefins play a role in benzene formation has also been shown over over a nickel-on-alumina catalyst. Product composition from 1-heptene as a function of the catalyst amount is shown in Fig. 3. This points also to diene intermediates 50). The same was found with carrier-free nickel and platinum 51). 

The components of the starting mixture are in rapid adsorption-desorption interaction with the surface. For example, a part of adsorbed -hexane desorbs as -hexane another part reacts to give benzene. If benzene formation involves an n-hexene surface intermediate, this hexene—the concentration of which may be eventually so small that it does not appear in the gas phase—interacts with the inactive hexene in the starting material and increases its specific radioactivity. 

There is a very significant difference between the rate of aromatization of trans- and c/i-hexatriene (Table III), which shows that geometrical isomerization prior to cyclization may be rate limiting. Since this occurs via half-hydrogenated species (60), it is promoted by the presence of hydrogen, and so is benzene formation. It should be noted that cyelohexane and cyclohexene are produced from cw-triene. The hydrogenation of cyclohexadiene may explain their formation here and in other cases of stepwise Cg dehydro-cyclization. 

Benzene formation from all isohexanes had a similar energy of activation value. With platinum this was nearly twice as high as that of n-hexane aromatization (62) with palladium black, however, nearly the same values were found for -hexane and isohexanes (97a). This indicates a common rate-determining step for aromatization with skeletal rearrangement. This is not the formation and/or transformation of the C5 ring. We attribute benzene formation to bond shift type isomerization preceding aromatization. It requires one step for methylpentanes and two steps for dimethyl-butanes this is why the latter react with a lower rate, but with the same energy of activation. 

Special attention has been paid to acid-catalyzed ring expansion. Sterba and Haensel (J19) reported that the rate of benzene formation from methyl-cyclopentane increases with increasing fluorine content of the catalyst (up to 1.0% F with 0.3% Pt on alumina). At the same time, increasing platinum content also promoted this reaction (up to 0.075% Pt with 0.77% F on alumina). This indicates the remarkable cooperative action of a dual function catalyst (119, p. 11). 

These results can be summarized as follows (1) the cobalt-mediated pyridine formation and alkyne cyclotrimerization depend on the square of the alkyne concentration and are independent of the nitrile concentration (2) a common cobaltacydopentadiene intermediate is responsible for both the pyridine and the benzene formation and may be regarded as a key intermediate for both hetero- and carbocyclic pathways. 

Table 4 shows that benzene is formed at almost identical rates from cyclohexene, hexadiene, methylcyclohexene and cyclohexadiene react under low partial pressure over Ga-HZSM-5. which suggests that over these catalysts benzene is formed from the same intermediate. In contrast over H-ZSM-5, under identical experimental conditions, the rate of benzene formation from the hydrocarbons cited was one to two orders of magnitude lower. These results prove again that gallium plays a decisive role in aromatization. Over H-ZSM-5 the major hydrocarbon formed is methylcyclopentene from cyclohexene (ring contraction)... 

Fig. 10. Selectivities in hexane conversions versus temperature for benzene formation (Be), hydrogenolysis (Hy), methylcyclopentane formation (MCP), isomerization (ISOM), and dehydrocyclization (Dehy) (9 wt. % Pt on inert Si02).

Similar arguments can be made for the effects of temperature on the reforming of a methylcyclopentane-hexane mixture at 2620 kPa, as shown in Fig. 17. Higher temperatures favor the benzene formation. 

With respect to catalyst contact time, the effects of temperature and pressure on the yields are shown in Figs. 18, 19, and 20. Activity (as measured by the C5- gas make) is a strong function of temperature, as shown in Figs. 18 and 19. Again, the higher-temperature operation favors benzene formation. KINPTR s prediction of activity as a function of pressure is shown in Fig. 20. Lower-pressure operation favors the yield of benzene. 

The spectra of the reacted 1,2-EB, DPMS and Al(acac)3 is shown in Fig. 6. The C-13 chemical shifts are shown in Table 6. There is a signal at 128.3 ppm that does not appear in the unreacted silanol material. This peak is due to benzene 51). This indicates phenyl cleavage of the silanol under the reactive conditions. The 1,2-EB and DPMS without Al(acac)3 did not show any peak at 128.3 ppm which indicates that the aluminium species is responsible for phenyl cleavage and benzene formation. 

A survey of the studies of metal-catalyzed acetylene cyclotrimerization on single-crystal, supported, and bimetallic catalysts is available.539 A new study with size-selected Pdn clusters found that clusters as small as Pd7 are able to induce benzene formation at 157°C.540... 

Dealkylation may also become the predominant reaction with bismuth molybdate. This was shown by van der Wiele [347] for the progressive reduction of the catalyst with toluene pulses. While initially the product spectrum is similar to that obtained in presence of air, a shift to benzene formation occurs at increased reduction. 

Another mechanism for benzene formation and parallel combustion is proposed by Germain and Laugier [129]. They suggest that toluene is yr-adsorbed on a surface cation via the nucleus, and then looses two ben-zylic H-atoms to form an o,a(a)-yr-adsorbed carbenoid complex, viz. 

Rotation of the nucleus (yr-bond rupture) is necessary to introduce surface oxygen into the methyl group. Therefore a weak yr-bond favours aldehyde formation, while a strong bond causes C—C bond rupture and benzene formation or oxidation of the nucleus. 

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