The gas phase pyrolyses of four 3,3-dimethyl-l-alkylcyclopropenes were studied for which the 1-alkyl substituents are ethyl-(6), isopropyl-( 7 ) , 1'-butyl-(8) and 1'-isoamyl-(9). Rate data over a 50°C temperature range were obtained and Arrhenius parameters are reported both for overall reactions and individual pathways for all compounds. Tests confirm all reactions to be unimolecular and homogeneous. For 8 and 9, noteworthy products are cyclopentenes, whose presence demonstrates the involvement of vinylidene intermediates. The substantial product yields of 2,4-alkadienes together with dramatic rate enhancement effects of these 1-alkyl substituents (relative to 1-methyl) are explained by the involvement of vinylcarbene intermediates in this pathway. Minor, transient products from 6, 8 and 9 are probably isomeric cyclopropenes. This suggests further pathways for vinylidene intermediates, whose reactivity pattern is briefly reviewed.
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