Acetylene from photolysis

Stable, isolable metallacycles are also obtained from reaction of complexes that serve as sources of the CpCo fragment (e.g. CpCo(PPh3)2) and alkynes. Upon carbonylation diese typically give high yields of cobalt-complexed cyclopentadienones. Direct reaction of CpCo(CO)2 with alkynes is similarly useful. The cycloaddition of di(t-butoxy)acetylene upon photolysis with CpCo(CO)2 is an example (Scheme 5). In all these systems the final complexes lack coordinated CO, and therefore amine oxides are not suitable reagents for liberating the stable cyclopentadienones. Tetra(t-butoxy)cyclopentadienone is accessible on a preparative scale via controlled electrochemical oxidation. Other oxidants such as Cr have been used as well in other systems. 

In a study by Okabe and McNesby the photolysis of ethylene was studied at 1236,1470 and 1849 A, with emphasis on the mechanism of hydrogen elimination. The products found agreed with the results of Sauer and Dorfman, and the above mechanism was confirmed. The examination of the isotopic distribution of hydrogen and acetylene from the photolysis of CH2CD2 permitted a comparison of the relative importance of the following reactions... 

Although research on the FVT of the thiophene anhydride 229 has been primarily directed at obtaining evidence for the intermediacy of the aryne 4, it has also yielded some information on the chemistry of this species. The most studied reaction of thiophyne (4) thus far is with dienes 146 to give thianaphthenes 293, presumably via the Diels-Alder adducts 390. The ubiquity of this reaction, as discussed above, appears to make it virtually diagnostic for the presence of the aryne (4). Thus the formation of thianaphthene (293) when benzene is the diene has been interpreted to involve a retro-Diels-Alder loss of acetylene from the initially formed thiophyne-ben-zene adduct (390c). Conversely the absence of thianaphthene (293) in the photolysis of the mercury compound 286 in benzene (Section III.3.A.a) strongly suggests that the aryne 4 is not produced in this reaction. 

Systems of the type [CpM(CO)3R] (M=Group VI metal) have attracted a considerable amount of attention studies have been concerned with both their thermal and photochemical reactions. Scheme 19 shows the principal products obtained from photolysis of the species where R represents methyl or benzyl the primary process is evidently CO dissociation. The photochemical reactions of [CpM(CO)sMe] with ethylene (M = W), acetylene (M = Mo or W), and trimethylphosphine (M = Cr, Mo, or W) have been investigated the results are summarized in Scheme 20. The unusual structure (12), is proposed for the... 

The observation of ketene and acetylene formation in photolyses at 248 nm provides strong evidence for the occurrence of process (I) the observed HCO formation confirms the occurrence of process (II) in photolyses at this wavelength. CO formation could arise from processes (II) through (V) or as a secondary product from photolysis of primary product ketene. Although process (I) was considered as a possible source of acetylene, the authors suggest that it is likely formed from the photodecomposition of the initial product, Z-3,4-diformyl-cyclobutene, formed in (VI). This cyclobutene product was suggested as a primary product in the studies of Klotz et al. (1995, 1999). 

Irradiation of ethyleneimine (341,342) with light of short wavelength ia the gas phase has been carried out direcdy and with sensitization (343—349). Photolysis products found were hydrogen, nitrogen, ethylene, ammonium, saturated hydrocarbons (methane, ethane, propane, / -butane), and the dimer of the ethyleneimino radical. The nature and the amount of the reaction products is highly dependent on the conditions used. For example, the photoproducts identified ia a fast flow photoreactor iacluded hydrocyanic acid and acetonitrile (345), ia addition to those found ia a steady state system. The reaction of hydrogen radicals with ethyleneimine results ia the formation of hydrocyanic acid ia addition to methane (350). Important processes ia the photolysis of ethyleneimine are nitrene extmsion and homolysis of the N—H bond, as suggested and simulated by ab initio SCF calculations (351). The occurrence of ethyleneimine as an iatermediate ia the photolytic formation of hydrocyanic acid from acetylene and ammonia ia the atmosphere of the planet Jupiter has been postulated (352), but is disputed (353). 

Electronic excitation from atom-transfer reactions appears to be relatively uncommon, with most such reactions producing chemiluminescence from vibrationaHy excited ground states (188—191). Examples include reactions of oxygen atoms with carbon disulfide (190), acetylene (191), or methylene (190), all of which produce emission from vibrationaHy excited carbon monoxide. When such reactions are carried out at very low pressure (13 mPa (lO " torr)), energy transfer is diminished, as with molecular beam experiments, so that the distribution of vibrational and rotational energies in the products can be discerned (189). Laser emission at 5 p.m has been obtained from the reaction of methylene and oxygen initiated by flash photolysis of a mixture of SO2, 2 2 6 (1 )-... 

Photochemistry. Vinyl chloride is subject to photodissociation. Photexcitation at 193 nm results in the elimination of HCl molecules and Cl atoms in an approximately 1.1 1 ratio (69). Both vinyUdene ( B2) [2143-69-3] and acetylene have been observed as photolysis products (70), as have H2 molecules (71) and H atoms [12385-13-6] (72). HCl and vinyUdene appear to be formed via a concerted 1,1 elimination from excited vinyl chloride (70). An adiabatic recoil mechanism seems likely for Cl atom elimination (73). As expected from the relative stabiUties of the 1- and 2-chlorovinyl radicals [50663-45-1 and 57095-76-8], H atoms are preferentially produced by detachment from the P carbon (72). Finally, a migration mechanism appears to play a significant role in H2 elimination (71). 

The photolysis of chlorodiazirine was investigated in several cases. From chloromethyl-diazirine (232) vinyl chloride was formed as the stable primary product of stabilization of chloromethylcarbene, with acetylene and hydrogen chloride as secondary products. Some 1,1-dichloroethane was assumed to have been formed through a linear diazo compound by reaction with HCl. Added HBr yielded 1-bromo-l-chloroethane (76MI5Q800). 

Tile following photochemical conversions also involve 1,2-dithietes as intermediates whose chemical trapping was reported in most cases. Tlie formation of the dithiin 249 from 250 may best be explained by the formation of the dithiete dimer 251 and the loss of S2 (73ZC424).Tlie formation of 252 and 253 from 254 (78NJC331) should be compared with the sulfuration of the acetylene 182 with elemental sulfur (93BCJ623).Tlie photolysis of 255 provides a rare example when the ejection of a nitrile was employed for the generation of a 1,2-dithiete (73ZC431). 

To monitor a possible influence of molecular products of photolysis of ethylene (acetylene, ethane, and butane) on the sensor, a second sensor was positioned at a distance of 50 cm from the photolysis zone. The second sensor was designed to introduce corrections into the readings of the movable sensor. The specified distance was chosen so that atoms and radicals produced in the lower part of the vessel could not reach the... 

Methylene cyclopropene (5), the simplest triafulvene, is predicted to be of very low stability. From different MO calculations5 it has been estimated to possess only minor resonance stabilization ranging to 1 j3. Its high index of free valency4 at the exocyclic carbon atom causes an extreme tendency to polymerize, a process favored additionally by release of strain. Thus it is not surprising that only one attempt to prepare this elusive C4H4-hydrocarbon can be found in the literature. Photolysis and flash vacuum pyrolysis of cis-l-methylene-cyclopropene-2,3-dicarboxylic anhydride (58), however, did not yield methylene cyclopropene, but only vinyl acetylene as its (formal) product of isomerization in addition to small amounts of acetylene and methyl acetylene65 ... 

The compound 251 decarbonylates on photolysis to bis(4-hydroxyaryl) acetylene 253, which is easily oxidized to the quinonoid cumulene 254. This is also obtained by thermal decarbonylation of the product of oxidation of cyclopropenone 251, the diquinocyclopropanone 252. Likewise, the blue derivative of 3-radialene 256 (a phenylogue of triketo cyclopropane) is formed from tris-(4-hydroxyaryl) cyclopropenium cation 255 by oxidation34. ... 

Photolysis of H3NBH3 with 121.5 nm radiation yields imidoborane, HBNH, which has been of theoretical interest Spectral shifts observed for several isotopic species containing °B, N, and D show clearly that the spectrum is due to HNBH which is isoelectronic with HBO, HCN and HCCH. From the spectrum of the isolated species two of the and one of the tr-type vibration frequencies for a linear molecule have been obtained. The location of the missing S (B-H stretch) frequency has been calculated. A comparison of observed and calculated frequencies for HBNH is given in Table 7. Another isolated product observed in these experiments is identified as HNB. This radical may be generated by photodissociation of HNBH subsequent to its formation. In this respect the photolysis mechanism would be similar to the formation of C2H from acetylene. 

Ab initio and RRKM calculations indicate that the reactions of C, CH, and (H2C ) with acetylene occur with no barrier." Laser flash photolysis of the cyclopropanes (69) and (70) was used to generate the corresponding dihalocarbenes. The absolute rate constant for the formation of a pyridine ylide from Br2C was (4-11) x 10 lmoP s. The rates of additions of these carbenes to alkenes were measured by competition with pyridine ylide formation and the reactivity of BrClC was found to resemble that of Br2C rather than CI2C . 

Pyridine and some of its derivatives have been photolyzed under various conditions in the quest for Dewar pyridines and azaprismanes, amongst other products. This quest has proven successful (76MI20503). Irradiation of pyridine itself in n-butane at -15 °C produces Dewar pyridine that can be observed spectrophotometrically and intercepted by sodium borohydride and water (Scheme 219). When pyridine is photolyzed in a matrix, hydrogen cyanide and acetylene are formed (equation 184). The same products have been obtained from the vapour phase photolysis of pyridine. On vapour phase photolysis, alkylpyridines isomerize for example, 2-picoline gives a mixture of 3- and 4-picolines. Azaprismanes (288) have been suggested as the intermediates in this process (equation 185). 

Several polysilabicyclosilirenes (79) have been obtained in the reactions of polysil-acyclooctynes (78) with dimesitylsilylene generated from the photolysis of 2,2-dimesityl-l,l,l,3,3,3-hexamethyltrisilane (Eq. 8). Thermolysis of bis(silacyclo-propane) (80) in the presence of bis(trimethylsilyl)acetylene at 60 °C affords bis-(silacyclopropene) (81) in 61% yield (Scheme 14.41). ... 

In 1972 Berry and co-worker detected 3,4-pyridyne (101) by MS. Trapping experiments also provided evidence for the existence of this intermediate, although the chemistry of 101 differs considerably from that of o-benzyne. Thus, neither anthracene nor dimethylfulvene form Diels-Alder adducts with 101. Nam and Leroi were able to generate 101 in nitrogen matrices at 13 K and characterized it by IR spectroscopy. Irradiation of 3,4-pyridinedicarboxylic anhydride (103) with 1 > 340 nm results in formation of 101, which upon short wavelength photolysis (k > 210 nm) fragments to buta-l,3-diyne (104) and HCN, and to acetylene (105) and cyanoacetylene (106, Scheme 16.24). The assignment of an intense... 

There is no experimental evidence of any predissociation process from excited pyridine in the usual range 2600-2800 A. Investigations have not revealed any stable decomposition products from pyridine excited by wavelengths shorter than 2600 A but HCN and acetylene are formed by flash photolysis.88... 

The ground state C2H is a major primary product of the acetylene and lialoacetylene photolysis. Okabe (773) has observed the production of an electronically excited C2H that fluoresces in the region 4000 to above 5500 A m the vacuum ultraviolet photolysis of acetylene and bromoacetylene. The lifetime of this fluorescence is about 6 /rsec and the fluorescence is quenched icadily by C2H2, H2, N2, and Ar [Becker et al (81)]. On a theoretical basis, Sliih et al. (872a) speculate that the fluorescence arises from a transition X - X2I. 

The ground state C1(Xl L ) is a primary product of acetylene photolysis. The r/ ll state is formed from the photolysis of bromoacetylene in the vacuum ultraviolet. It is also formed in flame and discharges through carbon containing compounds. The Swan system is a major feature of emission spectrum from the heads of comets. 

Thus, the main photolysis products are C2H4 and C2H6. Acetylene, a minor observed product, may be formed from the photolysis of ethylene and ethane. 

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