Thermal decomposition of 1-methyl-3-phospholene

Abstract

The homogeneous gas-phase thermal decomposition of the 5-ring 1-methyl-3-phospholene (C₄H₆PCH₃) has been followed from 356.0 to 444.6°C in excess toluene and at pressures of phospholene ranging from 8.3 to 27.4 mmHg (1106–3652 N m^–2). Butadiene, apparently a primary reaction product, partially decomposes to give isomeric butenes. Rate constants calculated on the basis of the observed C₄-hydrocarbon formation satisfy a first-order rate expression and yield the following Arrhenius relationship (with standard errors): log k(s^–1)= 12.10±0.22)–(50.0±0.7)/θ where θ= 4.58 × 10^–3T K in kcal mol^–1; log k(s^–1)=(12.10±0.22)–(209±3)/θ where θ= 19.14 × 10^–3T K in kJ mol^–1.

The observed activation parameters are in agreement with a mechanism involving ring opening to a biradical followed by fragmentation into butadiene, phosphorus and methyl radicals. Methane, hydrogen, phosphorous and a phosphorus-containing polymeric material are the other reaction products. The molecular elimination of hydrogen from the starting material is a possible side reaction. The results suggest a dissociation energy for the secondary phosphorus carbon bond of 68.1±1.3 kcal mol^–1(285±5 kJ mol^–1).