Hexatrienes thermochemistry

The thermochemistry of totally cumulated trienes, i.e. species with the C=C=C=C substructure, is very limited. Indeed, the sole examples we know are those reported by Roth, namely (Z)- and ( )-2,3,4-hexatrienes MeCH=C=C=CHMe, species 17 and 18. Their enthalpies of formation are identical to within experimental error, 265 kJ mol-1. This equality is altogether reasonable given the small Me—Me interaction across the 4-carbon, linear, cumulene chain in contradistinction to the 4.3 kJ mol-1 difference that is found for the isomeric (Z)-and (E)-2-butenes with their significantly smaller Me...Me distance. Are cumulated trienes unstable relative to cumulated dienes much as cumulated dienes are unstable relative to simple olefins Briefly regressing to cumulated dienes, this assertion is corroborated by the finding that species 3, i.e. 1,3-dimethylallene, has an enthalpy of decarbonization 18 of 144.5 kJmol-1 (reaction 12)... 

The first nonintroductory section of the text starts with the observation made early in this century that cyclopropanes have significant olefinic character. That is, corresponding ethylene and cyclopropane derivatives have significant similarities. There are literature comparisons of the thermochemistry of direct counterparts such as the parent species (1 and 2, X = H), propene and methylcyclopropane with X = Me, and of methyl acrylate and methyl cyclopropanecarboxylate 8 with X = COOMe. But the chemistry of substituted eth-ylenes is far richer than just that of vinyl compounds. One can retrieve enthalpy of formation data for cumulenes ( cumulenated olefins) such as allene (3) and both cis- and trans-2,3,4-hexatriene (4a and 4b), and for conjugated olefins such as 1,3-butadiene (5) and both (Z)- and (E)-1,3,5-hexatriene (6a and 6b). For the cyclopropane chemist it is natural to... 

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See also in sourсe #XX -- [ , ]

See also in sourсe #XX -- [ , ]

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