Kinetics of the gas-phase thermal decompositions of 1-methoxy-1-methylcyclopropane and cis- and trans-1-methoxy-2-methylcyclopropane

Abstract

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= 10^14.76±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: k_cis→trans/s^–1= 10^15.25±0.23 exp (–235.4±2.8 kJ mol^–1/RT), k_trans→cis/s^–1= 10^14.99±0.80 exp (–233.7±9.9 kJ mol^–1/RT), k_{cis→cis-1-methoxybut-1-ene}/s^–1= 10^13.79±0.29 exp (–233.1±3.6 kJ mol^–1/RT), k_{cis→trans-1-methoxybut-1-ene}/s^–1= 10^13.29±0.66 exp (–234.4±8.1 kJ mol^–1/RT), k_{cis→1-methoxy-2-methylpropene}/s^–1= 10^12.4±1.0 exp (–225±12 kJ mol^–1/RT), k_{trans→cis-1-methoxybut-1-ene}/s^–1= 10^14.05±0.62 exp (–243.7±7.8 kJ mol^–1/RT), k_{trans→trans-1-methoxybut-1-ene}/s^–1= 10^13.0±0.9 exp (–233±11 kJ mol^–1/RT), k_{trans→1-methoxy-2-methylpropane}/s^–1= 10^13.5±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.