The gas-phase unimolecular isomerization of 1 ,trans-3,trans-5-heptatriene has been followed in the temperature range 247-332°C and at pressures ranging from 1.6 to 22 torr. Gas-liquid partition chromatography has been used to analyze the product mixture. 1 ,cis-3,trans-5-heptatriene is apparently the primary reaction product and undergoes fast cyclization to 5-methyl-l,3-cyclohexadiene which is isomerized further via intramolecular 1,5-hydrogen transfer, to 1and 3-methyl-l,3cyclohexadienes. The reaction is apparently homogeneous and clean under the reaction conditions used. The only observable side reaction contributing measurably to the overall conversion at lower temperatures and with higher pressures is the Diels-Alder addition reaction.The observed rate constants satisfy the fist-order rate law yielding for the high pressure limit the Arrhenius relationship (with standard errors) : log k (s-I) = (12.28f0.19)-(42.37f0.49)/8 where 8 equals 4.58 x T (K). These rate parameters are in agreement with those expected for a 3,Pbiradical transition state with the planes of the two radical ends at 90" angle to each other. Compared with literature data on 2-olefins and taking the differences in n-bond-energies into account, the observed activation energy implies that the full amount of two " monoallylic " stabilization energies is developed in the biradical transition state. The results indicate that the geometrical isomerization about the 5,Gx-bond is kinetically not a competitive process.

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The composition of equilibrated mixtures of the geometrical isomers of 1.3.5-heptatriene was measured in the temperature range 141 -8-297-3 "C. Hydrocarbon pressures varied from 7.4 to 125.6 Torr and catalyst (NO) pressures from 10 to 545 Torr. For the equilibrium 1 ,trans-3,cis-5-heptatriene (= tc) -, 1 ~rans-3,trans-5-heptatriene (= tt) the observed equilibrium constants result in log K1,, = log (tt/tc) = (0.03 f 0.02) + (0.84 f 0.04)/8, where 8 = 4.58 x 1 0-3T K, leading to AS1,, = 0.1 2 f 0.09 cal mol-1 K-l and AHL2 = -0.84 f 0.04 kcal mol-l. Assigning an entropy difference of 0.6 cal mol-l K-l for SmO(tt) -S" , ( ct ) and for Sam(tt) -S',,(cc) and from measurements of K3,4 and Ks,s at one temperature, we obtain -AH>m(tt) -AH",m(ct)= -1.6 f 0-2 kcal mol-land AH', (tt) -AH' , (cc) = -2.0 f 0.1 kcal mol-l. The results are compared with prevrous data for the geometrical isomer pairs of pentene, heptene, and pentadiene and with predictions, based on the concept of direct additivity of S"-, Hf-, and COP-increments, reported in the literature.

Die thermische Isomerisierung des Triens (I) bei 247‐332°C in der Gasphase zwischen 1,6 und 22 Torr ergibt zunächst das cis‐trans‐Trien (II), das thermisch instabil ist und sich zum Cyclohexadien (III) cyclisiert.

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