Thermal decomposition of 2,3-dihydro-1,4-benzodioxin and 1,2-dimethoxybenzene

Abstract

Rates and mechanisms of decomposition of 2,3-dihydro-1,4-benzodioxin (1) and 1,2-dimethoxybenzene (27) have been investigated in the gas phase near atmospheric pressure between 750 and 900 K in a tubular flow reactor in a large excess of radical trapping agents. The following rate expressions for decomposition have been determined: log k_t/s^–1(1)= 15.7 –(271 kJ mol^–12.303 RT); log k_t/s^–1(27)= 15.7 –(251 kJ mol^–12.303 RT). The main decomposition routes for 1 are the formation of o-benzoquinone (2) and 2-methyl-1,3-benzodioxole (7) through a biradical intermediate. The measured activation energy is 20 kJ mol^–1 above the required C–O bond dissociation energy. Compound 2 rapidly loses CO to form cyclopenta-2,4-dien-1-one (6) which after dimerisation decomposes mainly into 3-phenylprop-2-enal (12) and indenols (14). The main product of the thermolysis of 27 is o-hydroxybenzaldehyde (33). The O-methyl bond is weakened by 16 kJ mol^–1 compared to methoxybenzene as a result of the o-methoxy-substitution.