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Thermal Rearrangements, XX[1] The Kinetics of Thermal Isomerisation of 4-Methyl-1,2,5-hexatriene in the Gas Phase

✍ Scribed by Hopf, Henning ;Wachholz, Gerhard ;Walsh, Robin

Publisher
Wiley (John Wiley & Sons)
Year
1992
Tongue
English
Weight
931 KB
Volume
125
Category
Article
ISSN
0009-2940

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✦ Synopsis

4‐Methyl‐1,2,5‐hexatriene (13) has been prepared and subjected to gas‐phase pyrolysis. Above 220°C, 13 rearranges reversibly to both trans‐5‐hepten‐1‐yne (14) and its cis‐isomer 15. Formation of 14 is faster than that of 15. These isomerisations are accompanied by a slower irreversible rearrangement to 1‐methyl‐4‐methylene‐1‐cyclopentene (16) and 1‐methyl‐3‐methylene‐1‐cyclopentene (17). Rate constants and Arrhenius parameters have been obtained for all these reactions which were shown to be homogeneous and unimolecular. For the processes 13 ⇄ 14 and 13 ⇄ 15 the data are consistent with [3,3] Cope‐type sigmatropic rearrangements. For the process 13 → 16 + 17, a rate‐determining formation of the 1‐methyl‐3‐methylenecyclopentane‐2,5‐diyl diradical is proposed. The rearrangements form a consistent pattern with other known examples. – The equilibrium data are shown to be in good agreement with thermodynamic estimates based on Group Additivity. The additivity scheme of Benson required extension to provide values for two new group contributions.

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