Gas-phase pyrolyses of 1,1-dichlorobutane and 1,1-dichloropropane

Abstract

The gas-phase pyrolysis of 1,1-dichlorobutane has been studied at initial pressures of 5–150 Torr between 416 and 460°C. Analysis by g.l.c. shows that the amount of dichlorobutane removed is equal to the pressure increase as would be expected for the elimination (i). The pressure increase was used to determine the first-order rate PrⁿCHCl₂→cis- and trans-EtCHCHCl + HCl (i) constant for this reaction which is expressed by equation (ii), where the error limits are the 95% confidence limits. log₁₀(k/s^–1)=(13·68 ± 0·27)–(53,872 ± 715)/4·576T(ii)

The observed cis:trans-ratio for the 1-chlorobutene was 1·00 ± 0·02 at these temperatures compared with an equilibrium value of 1·60 ± 0·08 which was found independently. No cis–trans-isomerisation occurred under these conditions.

The gas-phase pyrolysis of 1,1-dichloropropane was reinvestigated at pressures of 9–185 Torr between 389·2 and 456·5°C. The observed cis:trans-ratio of the 1-chloropropene produced was 1·20 ± 0·02 under these conditions and cis–trans-isomerisation is negligible. The comparable equilibrium cis:trans-ratio at these temperatures is 1·90 ± 0·08. The pressure change obeys first-order kinetics and the rate constant is expressed by equation (iii) where the error limits are the 95% confidence limits. log₁₀(k/s^–1)=(14·32 ± 0·30)–(56,679 ± 821)/4·576T(iii)

Both pyrolyses have also been briefly studied in packed reaction vessels. The resulting cis:trans-ratios observed then favour the cis-product olefins.