Benzene from acetylene

C15-0097. With appropriate catalysts, it is possible to make benzene from acetylene. The reaction has simple overall stoichiometry and can be studied in kinetics experiments 3 C2 H2(g) Cg Hg(g)... 

Even more than [6 + 4] and [8 + 2] cycloaddition reactions, the [2 + 2 + 2] cycloaddition reactions require a very well preorganized orientation of the three multiple bonds with respect to each other. In most cases, this kind of cycloaddition reaction is catalyzed by transition metal complexes which preorientate and activate the reacting multiple bonds111,324. The rarity of thermal [2 + 2 + 2] cycloadditions, which are symmetry allowed and usually strongly exothermic, is due to unfavorable entropic factors. High temperatures are required to induce a reaction, as was demonstrated by Berthelot, who described the synthesis of benzene from acetylene in 1866325, and Ullman, who described the reaction between nor-bomadiene and maleic anhydride in 1958326. As a consequence of the limiting scope of this chapter, this section only describes those reactions in which two of the participating multiple bonds are within the same molecule. 

Benzene from Acetylene.—Benzene may be prepared directly from one of the unsaturated open chain compounds, viz., from acetylene, CH = CH. When this hydrocarbon is passed through a red hot tube it is simply broken down into its elements. If, however, it is heated more slowly, it polymerizes and benzene is obtained. 

Fig. 1.9 Temperature-programmed desorption (TPD) of (a) hydrogen and (c) benzene from acetylene on clean Pd(lll) compared with the spectra for (b) hydrogen and (d) benzene from ethyUdyne-covered Pd(l 11). (With kind permission from Springer Science and Business Media.)...

Sesselman W, Woratschek B, Ertl G, Kiippers J, Haberland H (1983) Low-temperature formation of benzene from acetylene on a Pd(lll) surface. Surf Sci 130 245... 

Kaltchev M, Molero H, Stacchiola D, Wu G, Blumenfeld A, Tysoe WT (1999) On the reaction pathway for the formation of benzene from acetylene catalyzed by palladium. Catal Lett 60 11... 

Polymerization and GycliZation. Acetylene polymerizes at elevated temperatures and pressures which do not exceed the explosive decomposition point. Beyond this point, acetylene explosively decomposes to carbon and hydrogen. At 600—700°C and atmospheric pressure, benzene and other aromatics are formed from acetylene on heavy-metal catalysts. 

Over the last decade, the chemistry of the carbon-carbon triple bond has experienced a vigorous resurgence [1]. Whereas construction of alkyne-con-taining systems had previously been a laborious process, the advent of new synthetic methodology based on organotransition metal complexes has revolutionized the field [2]. Specifically, palladium-catalyzed cross-coupling reactions between alkyne sp-carbon atoms and sp -carbon atoms of arenes and alkenes have allowed for rapid assembly of relatively complex structures [3]. In particular, the preparation of alkyne-rich macrocycles, the subject of this report, has benefited enormously from these recent advances. For the purpose of this review, we Emit the discussion to cychc systems which contain benzene and acetylene moieties only, henceforth referred to as phenylacetylene and phenyldiacetylene macrocycles (PAMs and PDMs, respectively). Not only have a wide... 

In the cracking of benzene to acetylene over alumina- and silica-supported nickel catalysts it was observed that the selectivity of the reaction, expressed as the ethyne/ ethene ratio, was dramatically affected (from 1 9 to 9 1) by controlling the micro-wave energy input (i. e. 90% selectivity) [83]. 

Figure 5.5 Alkanes, alkenes, aromatics and cyanopolyynes 1, methane 2, ethene 3, propene 4, benzene 5, acetylene 6, propyne 7, butadiyne 8, pentadiyne 9, hexatriyne 10-14, cyanopolyynes (the largest family with the longest molecule to date, 14). (Reproduced from Guillemin et at. 2004 by permission of Elsevier)...

Systems with two condensed selenophene rings were first mentioned by Briscoe et in search of an improved synthesis of selenophene from acetylene and selenium. They obtained at 400° a mixture, including benzene, selenophene, naphthalene, and a compound, b.p. 240°-250°, containing about 20% of selenium, considered to be seleno-phenoselenophene (11). 

Levedahl (106) noted that aliphatics from acetylene to octane all gave hot flames at 600° C. He suggested that some reaction, such as thermal decomposition, which was common to all aliphatics, became important. Benzene showed a very high, inconsistent hot flame limit, while cyclohexane was low and variable. Levedahl believed the cool flame and subsequent reaction, along with compression, served to raise the mixture temperature to the critical value. Acetylene was believed to play a major role in the ignition reaction. 

Density functional theory, 21, 31, 245-246 B3LYP functional, 246 Hartree-Fock-Slater exchange, 246 Kohn-Sham equations, 245 local density approximation, 246 nonlocal corrections, 246 Density matrix, 232 Determinantal wave function, 23 Dewar benzene, 290 from acetylene + cyclobutadiene, 290 interaction diagram, 297 rearrangement to benzene, 290, 296-297 DFT, see Density functional theory... 

Cooking constitutes a source of VOCs in the indoor environment Food stuffs and fuels emit ample amounts of VOCs. For example, up to 54 hydrocarbons were identified from a study of 16 fuels/ stoves combinations that are usually used in urban and rural settings in China (Tsai et al., 2003). The worst stove/fuel VOC emitters include metal stoves with a flue/unprocessed coal powder, metal stoves with a flue/washed coal powder, brick stove with a flue/maize residue while the least emitters ofVOCs are traditional gas stove/coal gas fuel, improved brick stove with a flue/maize residue and metal coal stove with a flue/honeycomb coal briquette. Many of the compounds emitted in substantial amounts are reactive unsaturated compounds such as benzene, ethylene, acetylene, and propene. For example, up to 2856 mg of ethylene was emitted by per kg of coal powder in the metal stove. Similarly, the observation of elevated levels of benzene and toluene in a food-court in South China has been rationalized in terms of emission from liquefied petrol gas (LPG) stoves (Tang et al., 2005). Thus, cooking is an important contributor of precursors of photochemical smog. 

The photoreaction of dimethyl acetylenedicarboxylate at room temperature was also reported by Grovenstein and co-workers [57,58]. From acetylene and benzene, only traces of cyclooctatetraene could be detected [59,60],... 

Fig. 12. Vertical ionisation energies, Fermi (E.) and vacuum levels (V. L.) for gaseous (1), condensed and chemisorbed phases of (a) benzene, (b) acetylene and (c) ethylene, all plotted relative to cr-orbital ionisation potentials (I. P.) for the gas phase. Relaxation shifts are given by the vacuum level shifts while bonding shifts are given by relevant jr-orbital shifts. [Reproduced with permission from J. E. Demuth and D. E. Eastman, Phys. Rev. Letters 32, 1123 (1974)]...

Figure 5. Computed reaction path for the formation of benzene starting from acetylene promoted by a Pd atom supported on a neutral oxygen vacancy at a MgO terrace, Fjt (DFT BP results).

Production of Coke and Other Pyrolysis Products From Acetylene, Butadiene, and Benzene in Various Tubular Reactors... 

Thermal reactions of acetylene, butadiene, and benzene result in the production of coke, liquid products, and various gaseous products at temperatures varying from 4500 to 800°C. The relative ratios of these products and the conversions of the feed hydrocarbon were significantly affected in many cases by the materials of construction and by the past history of the tubular reactor used. Higher conversions of acetylene and benzene occurred in the Incoloy 800 reactor than in either the aluminized Incoloy 800 or the Vycor glass reactor. Butadiene conversions were similar in all reactors. The coke that formed on Incoloy 800 from acetylene catalyzed additional coke formation. Methods are suggested for decreasing the rates of coke production in commercial pyrolysis furnaces. 

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