The very low‐pressure dehydrogenation of cis‐2‐butene. The activation energy for 1,4‐H₂ elimination

Pyrolysis of cis‐butene‐2 under conditions of very low pressure (VLPP) has been studied in the range of 1100–1300°K. The principal products are butadiene and H₂, obtained in a unimolecular reaction. A competing reaction to form butene‐l accounts for from 10% to 40% of the overall decomposition over the range. Using a «tight» model for the transition state and RRKM theory yields a high‐pressure, unimolecular rate constant for the 1,4‐H₂ elimination of (Formula Presented.) where θ = 2.303RT in kcal/mol. There is some surface reaction of butadiene at these temperatures to yield H₂ + nonvolatile residue. Butene‐l proceeds to decompose irreversibly to allyl + methyl radicals which have been observed directly. Comparison with related reactions leads to the conclusion that orbital symmetry‐forbidden, 1,2‐H₂ elimination from saturated organic compounds will have activation energies too high to observe.