Thermal unimolecular isomerization of 1,4‐dimethylbicyclo[2.2.0]hexane, 1,4‐dimethylbicyclo[2.1.1]hexane, and 1,4‐dimethylbicyclo[2.1.0]pentane

Abstract

The thermal isomerization of the title compounds was studied in the vapor phase. Over the temperature range from 445.1 to 477.5°K, 1,4‐dimethylbicyclo[2.2.0]hexane underwent a homogeneous unimolecular reaction to 2,5‐dimethyl‐1,5‐hexadiene, the rate constants being represented by the equation: k = 1.86 × 1011 exp (−31000 ± 1800/RT) sec−1. Over the temperature range from 630.0 to 662.2°K, 1,4‐dimethylbicyclo[2.1.1]‐hexane also underwent a unimolecular isomerization to the same product, the rate constants being given by the equation: k = 8.91 × 1014 exp (−56000 ± 900/RT) sec−1. The pyrolysis of 1,4‐dimethylbicyclo[2.1.0]pentane gave 1,3‐dimethylcyclopentene‐1 and 2,4‐dimethyl‐1,4‐pentadiene in the ratio of 9:1. The former reaction was influenced by surface effects but the latter was not. The rate constants for the formation of 2,4‐dimethyl‐1,4‐pentadiene fitted the equation: k = 1.66 × 1017 exp (−57400 ± 3100/RT) sec−1. The effect of the two methyl groups at the bridgehead positions in these molecules in influencing the rate of decomposition is discussed in terms of the non‐bonded repulsive forces between the substituents. Copyright © 1969 John Wiley & Sons, Inc.