Acetylene, and methylacetylene

Scheme 5 is a summary of the experimental frequencies of the v(CH), v(C = Q, and v(OH) modes for the 1 1 7i-bonded acetylene and methylacetylene complexes. Notice that the shift of v(C = C) and v(OH) bands is always negative with respect to the free hydrocarbon molecules (v(C = C) = 1974 and 2142cm for acetylene and methylacetylene, respectively) or Bronsted acid group, respectively. The shift of the v(OH) is a normal consequence of hydrogen-bonding and has been abundantly documented for many bases adsorbed molecularly in zeolites with proton affinities in the range of 420—840kJmol (20,21,156-161). Furthermore, the v(C = C)... 

A variety of copolymers of acetylene and other alkynes have been prepared. Copolymers of acetylene and methylacetylene (86), a soluble polyene, were more tractable than polyacetylene itself, but as the fraction of methylacetylene in the copolymer increased, both the tractability and the final conductivity of the iodine-doped polymer decreased. At an acetylene mole fraction of 0.5, the copolymer was insoluble, and the final conductivity had fallen from greater than 100 to 1 S/cm. Some plasticization remained, however, and the polymer could be stretched to elongation ratios as high as 7. Copolymers of acetylene and phenylacetylene containing 26% acetylene were soluble in methylene chloride (24). Like methylacetylene copolymers. 

A comparison with the CN reactions with acetylene and methylacetylene is in order. In all cases, the CN radical attacks a rr orbital of the alkyne on the carbon side and the cisjtrans doublet radical intermediates are formed. Whereas the acetylenic carbon atoms in acetylene and dimethylacetylene are chemically equivalent, methylacetylene is asymmetric and is characterized... 

Property data from the literature (1-55,84-93) are given in Table 3-1. Critical constants have been determined experimentally for acetylene and methylacetylene (1-7). Critical pressure and volume are estimated for 1 butyne and 2 butyne (5). Additional property data such as acentric factor, enthalpy of formation, lower explosion limit in air and solubility in water are also available. The DIPPR (Design Institute for Physical Property Research) project (5) and recent data compilations by Yaws and co-workers (44-55) were consulted extensively in preparing the tabulation. 

In terms of acid-base concepts the acidic protons on acetylene and methylacetylene react with basic oxygen sites on the surface. The detailed mechanism of this interaction is still an open question. When the protons are replaced by two methyl groups, the acidic nature of the acetylene is destroyed and the pi-electrons of the carbon-carbon triple bond then behave as a base and interact with Bronsted acid or Lewis acid sites. 

The first papers on the synthesis of statistical copolymers of acetylene and butadiene appeared in the 1970s [99,100]. Random copolymers were prepared by passing a gas mixture (23% acetylene, 77% butadiene) through a solution of nickel naphthenate in combination with diethylaluminum chloride in toluene [101]. The purified polymer is either a yellow viscous liquid or a waxy polymer. Chien and Wnek [98] obtained a statistical copolymer of acetylene and methylacetylene. For this purpose a concentrated solution of Ti(OBu)4-AlEt3 in toluene was stirred in a vessel until the walls were covered by a layer of a catalyst. Then the mixture of acetylene and methylacetylene was introduced and the copolymer film was formed immediately both on the walls of the vessel and on the catalyst surface. Doping with I2 or AsFs made a copolymer film conducting (up to 40 S/cm) for the sample with a PA content of 55% (doped with AsFj). 

Hydrogenation of acetylene and methylacetylene can be represented by the following equations ... 

Phillips chromium catalyst has been primarily used as ethylene polymerization catalyst in a large industrial scale. However, the same catalyst could cyclotrimerize acetylene and methylacetylene into benzene and tri-methylbenzene (TMB) rather than polyacetylene and polymethylacetylene, respectively (McDaniel, 2010). The mechanism has never been studied up to now, which also own particular academic interests in this field. Hogan and coworkers first reported alkyne cyclotrimerization catalyzed by the Phillips chromium catalyst only a few years later after they invented the Phillips chromium catalyst (Clark et al., 1959). In their report, the acetylene was found primarily cyclotrimerized into benzene, while a ratio of 0.18 of... 

TMB to 1,2,4-TMB was obtained for the methylacetylene cyclo-trimerization. A decade ago, Zecchina et al. revisited this catalytic system and found that 1,3,5-TMB is the only product of the cyclotrimerization of methylacetylene (Zecchina et al., 2003). Recendy, we performed a theoretical investigation on the mechanism of acetylene and methylacetylene cyclotrimerization catalyzed by the PhiUips chromium catalyst (Liu et al., 2012, 2013). 

Zecchina A, Bertarione S, Damin A, et al Temperature resolved FTIR spectroscopy of Cr /Si02 catalysts acetylene and methylacetylene oligomerisation, Phys Chem Chem Phys 5(20) 4414-4417, 2003. 

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