The thermal isomerizations of cis and trans-l,2,3,4-tetramethylcyclobutene and bicyclo[4.2.0]oct-7-ene have been studied in the gas phase. In the temperature range 157-2Ol0C, cis-tetramethylcyclobutene isomerizes by a homogeneous first-order process to yield cisYtrans-3,4-dimethylhexa-2,4diene and the rate constants fit the Arrhenius equation kl = 1014.10 exp (-37,360IRT) sec-1.In the temperature range 125-167°C the trans tetrametliylcyclobutene isomerizes to trans,trans-3,4dimethylhexa-2,4-diene, but under the conditions employed this product undergoes surface-catalyzed polymerization. The rate of disappearance of the reactant obeyed first-order kinetics and the rate constants obtained fitted the Arrhenius equation, k2 = 10*3.85 exp (-33,59O/RT) sec-1. The isomerization of bicyclo[4.2.0]oct-7-ene yields cycloocta-l,3-diene by a homogeneous first-order process, and in the temperature range 236-285" the rate constants fit the Arrhenius equation, k3 = 1014.13 exp (-43,18O/RT) sec-1.Above 1 mm the rate constants of the three isomerizations are independent of pressure. The results obtained are compared with those for other cyclobutenes with special reference to the mechanism of the transformation.
The thermal isomerizations of 1,3-dirnethylcyclobutcne and 1,4-dimethylcyclobutene have been studied ovcr a temperature range of 50". The isomerizations are first-order and yield trans-2met hylpenta-1,3 diene and trans-3-methylpenta-1,3-diene respectively. The experimental results, which indicate that both isomerizations are unimolecular, fit the Arrhenius equations :k (1,3-dimethylcyclobutene) = 1013.65 exp (-33,W/RT) sec-1, k (1,4-dimethylcyclobutene) = 1013.52 exp (-33,39O/RT) sec-1.
The thermal isomerization of 3,3-dimethylcyclobutene and 1,3,3-trimethylcyclobutene have been investigated in the gas phase in the temperature ranges 150-200°C and 172-213°C respectively. These isomerizations were homogeneous first-order reactions with rate constants independent of pressure over a wide pressure range. The rate constant for the isomerization of 3,3-dimethylcyclobutene fitted the Arrhenius equation, k = 10 13.93 exp (-36,09O/RT) sec-1.Those for the 1,3,3-trimethylcyclobutene fitted the Arrhenius equation, k = 10 13-90 exp (-37,03O/RT) sec-1.The effect of alkyl substitutents on the free energy of activation for the isomerization of cyclobutenes is discussed.This paper reports a continuation of work on the kinetics of the thermal isomerization of cyclobutenes.1-4 It proposes a connection between the rates of isomerization and the position of alkyl substituents in the cyclobutene ring.
EXPERIMENTAL PREPARATION O F MATERIALS~,~-DIMETHYLCYCLOBUTENE. 3,3-Dimethylcyclopropane nitrife was prepared from 2,2-dimethylpropane-1,3-dioI by the method of Nelson et aZ. 5 The nitrile was reduced to the corresponding aldehyde using triethoxy lithium aluminium hydride.6 The 3,3-dimethylcyclopropane carboxaldehyde was converted into its toluene sulphonylhydrazone by treatment with toluenesuplhonylhydrazide and the hydrazone was decomposed in diglyme in the presence of excess sodium methoxide at 140°C. The resulting mixture of hydrocarbon products was
The thermal isomerization of l-ethylcyclobutene has been investigated in the gas phase in the temperature range 151-194°C. The reactant isomerizes to 2-ethylbuta-lY3-diene by a first-order homogeneous reaction. All the evidence indicates that it is a true unimolecular transformation. The rate constants obtained at various temperatures fit the Arrhenius equation,The first-order rate constant is independent of pressure from 1 to 20 mm. The results obtained are compared with those obtained for other cyclobutenes.
The thermal isomerizations of 1-n-propyl, 1-isopropyl, 1 -ally1 and I-cyclopropyl cyclobutenes have been studied in the gas phase at temperatures in the range 145-200°C. The isomerizations are almost certainly unimolecular and were investigated in their high pressure region. Rate constants for the reactions yielded the Arrhenius equations :1-n-propyl, k = 1013-55 exp (-34,546/RT) sec-l ; 1-isopropyl, k = 1013*55 exp (-34,669/RT) sec-' ; 1 -ally1 I k = 1013-48 exp (-34,2O/RT) sec-l ; 1-cyclopropyl, k = 1013*48 exp (-34,14O/RT) sec-I.The effect of alkyl substitution on the free energy of activation for the isomerization of the cyclobutenes is discussed.
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