Photochemistry, acetylene excited state

The electronic structure of alkynes is related to that of alkenes, and the photochemistry of the two classes of compound reflects this similarity. Because the photochemistry of alkenes has received greater attention and has already been described in systematic form - it is not unexpected that the present account should point out the ways in which alkyne photochemistry parallels, or is markedly different from, that of alkenes. There is a considerable difference, however, in the range of compounds which has been studied in each class. Reports of photochemical reactions of alkynes very often refer to mono- or disubstituted acetylenes in which the substituents are alkyl, aryl or alkoxycarbonyl. There have been studies on diyne and enyne systems, but as yet there has emerged nothing in alkyne chemistry to match the wealth of photochemistry reported for dienes and polyenes. This reflects in part the greater tendency of the compounds containing the C=C bond to undergo photopolymerization rather than any other reaction on irradiation. Within this limitation there is a wide variety of reactions open to the excited states of alkynes, and quite a number of the processes have synthetic application or potential. 

From spectroscopic data it seems likely that the excited states involved in alkyne photochemistry are either (tt, — ) states, in which an electron from a bonding tt molecular orbital has been promoted to an antibonding - molecular orbital, or Rydberg stales, in which a tt electron has been promoted to an extended a-type orbital covering more than one nucleus. The spectra of acetylene, propyne and but-l-yne all show features characteristic of both types of electronic transition. In... 

Photochemistry.— The photochemical addition of dimethyl acetylene-dicarboxylate, methyl propiolate, or methyl phenylpropiolate to benzo[h]-thiophen and its 2- and 3-methyl and 2,3-dimethyl derivatives leads to cyclobutene derivatives of unexpected structures. From benzo[ >]-thiophen and dimethyl acetylenedicarboxylate (376) is obtained. Methyl propiolate adds in a direction opposite to that of methyl phenylpropiolate, suggesting that the excited state of benzo[h]thiophen is highly polarized. The mechanism of the addition is discussed, and several mechanistic alternatives are suggested. Only with diphenylacetylene, which reacts slowly, can the normal addition product (377) be obtained. The... 

Photochemistry. The Hg(3P,) sensitized photolysis ofC2H2 has produced benzene, hydrogen, and polymer [Shida et al. (872)]. Since the Hg(3Fi) state does not have sufficient energy required to dissociate the H—C2H bond, the products must be produced by reactions of an electronically excited C2H2. The photolysis of acetylene at 1849 A has produced hydrogen, ethylene, vinylacetylene, diacetylene, benzene, and solid polymers [Tsukada and Shida (979), Zelikoff and Aschenbrand (1082)]. 

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