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Thermal Rearrangements, XX [1] The Kinetics of Thermal Isomerisation of 4‐Methyl‐1,2,5‐hexatriene in the Gas Phase
Author(s) -
Hopf Henning,
Wachholz Gerhard,
Walsh Robin
Publication year - 1992
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19921250326
Subject(s) - chemistry , isomerization , cyclopentene , arrhenius equation , diradical , methylene , reaction rate constant , sigmatropic reaction , methyl group , kinetics , gas phase , computational chemistry , photochemistry , medicinal chemistry , singlet state , organic chemistry , activation energy , group (periodic table) , excited state , physics , quantum mechanics , nuclear physics , catalysis
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.