Mono Olefins

Author : George Richard De Mare
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.:/5 (143 download)

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Download or read book Some Reactions of the Triplet State of Mono-olefins written by George Richard De Mare and published by . This book was released on 1965 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: The Hg 6( P^) photosensitized decomposition of cyclohexene follows the pattern established for mono-olefins in general. The products, ethylene, 1,3-butadiene, vinyl- cyclobutane, cyclohexane, various C^-compounds and polymer, can best be accounted for by allylic C-H and C-C cleavage of the vibrationally-excited, triplet-state molecule and by the reactions which the various species formed by such cleavage subsequently undergo. The ring closure and fragmentation reactions of the open chain allylic biradicals, formed by allylic C-C bond cleavage, results in the formation of vinylcyclobutane or cyclohexene and ethylene and 1,3-butadiene, respectively. Vinylcyclopentane is an important product of the sensitized decomposition of cycloheptene showing that ring contraction is a general, characteristic reaction of the cyclic mono-olefins. This favors a process involving internal addition in an open chain biradical to a concerted mechanism for the ring contraction. The products of the photosensitized decompositions of 3-methylcyclohexene, 4-methylcyclohexene and 4-vinyl- cyclohexene, clearly establish the allylic positions as the sites of the C-C bond cleavages which result in the formation of ethylene and 1,3-butadiene in the photosensitization of cyclohexene. The major products of the Hg 6pP^) photosensitized decomposition of methylenecyclobutane are ethylene and allene while hydrogen, methylcyclobutane, isoprene and 1,4-pentadiene are minor products. The primary quenching process yields an excited molecule which decomposes by at least two modes, one of which gives a hydrogen atom and a radical, the other gives 1,4-biradical,2-methylenebutane with a quantum yield of 0.70 to 0.75. The 1,4-biradical,2-methylenebutane is either collisionally stabilized or decomposes to ethylene and allene. Isomerization of the biradical is at most a very minor process. Within experimental error the yield of allene is unaffected by as high as 45% molecular oxygen, indicating the inability of the oxygen to trap either the initially formed triplet methylenecyclobutane molecule or the biradical. The Hg 6( P^) photosensitized decomposition of spiropentane is postulated to proceed by at least three different primary quenching processes. One of these, with a quantum yield of 0.45, is independent of substrate pressure and involves direct quenching into a C-C bond at the quaternary position. The resulting 1-cyclopropyl bismethylene biradical rapidly isomerizes to the isoelectronic 1,4-biradical,2- methylenebutane which is either collisionally stabilized to methylenecyclobutane or decomposes to ethylene and allene. The pressure dependence of the yields of ethylene and allene is similar to that in the methylenecyclobutane system thus supporting the evidence for allylic C-C bond cleavage in that system.