Kinetics of the gas-phase thermal decompositions of 1-methoxy-1-methylcyclopropane and cis- and trans-1-methoxy-2-methylcyclopropane
In the temperature range 665–737 K the thermal decomposition of 1-methoxy-1-methylcyclopropane follows first-order kinetics with a rate constant given by the equation, k/s–1= 1014.76±0.81 exp(–252±10 kJ mol–1/RT). The presence of the 1-methyl substituent destabilises the transition state for reaction. Secondary decomposition of the initially formed isomeric products precludes the determination of their individual rates of formation.
Cis- and trans-1-methoxy-2-methylcyclopropane undergo first-order, reversible, geometric isomerisation in competition with structural isomerisation to give cis- and trans-1-methoxybut-1-ene and 1-methoxy-2-methylpropene in the temperature range 597–689 K: kcis→trans/s–1= 1015.25±0.23 exp (–235.4±2.8 kJ mol–1/RT), ktrans→cis/s–1= 1014.99±0.80 exp (–233.7±9.9 kJ mol–1/RT), kcis→cis-1-methoxybut-1-ene/s–1= 1013.79±0.29 exp (–233.1±3.6 kJ mol–1/RT), kcis→trans-1-methoxybut-1-ene/s–1= 1013.29±0.66 exp (–234.4±8.1 kJ mol–1/RT), kcis→1-methoxy-2-methylpropene/s–1= 1012.4±1.0 exp (–225±12 kJ mol–1/RT), ktrans→cis-1-methoxybut-1-ene/s–1= 1014.05±0.62 exp (–243.7±7.8 kJ mol–1/RT), ktrans→trans-1-methoxybut-1-ene/s–1= 1013.0±0.9 exp (–233±11 kJ mol–1/RT), ktrans→1-methoxy-2-methylpropane/s–1= 1013.5±1.0 exp (–235±13 kJ mol–1/RT).
On the basis of a biradical mechanism the results provide evidence for the formation of distinguishable biradicals on opening the cis- and trans-1-methoxy-2-methylcyclopropane ring. Estimates are made of the relative rates of ring closing, internal rotation and hydrogen-atom transfer of the biradicals.