Thermodynamic data from equilibrium studies of the nitric oxide-catalysed isomerization of 1,3,5-heptatriene in the gas phase

Abstract

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) [graphic omitted] 1,trans-3,trans-5-heptatriene (= tt) the observed equilibrium constants result in log K_1,2= log (tt/tc)=(0·03 ± 0·02)+(0·84 ± 0·04)/θ, where θ= 4·58 × 10^–3T K, leading to ΔS°_1,2= 0·12 ± 0·09 cal mol^–1 K^–1 and ΔH_1,2=–0·84 ± 0·04 kcal mol^–1. Assigning an entropy difference of 0·6 cal mol^–1 K^–1 for S°₃₀₀(tt)–S°₃₀₀(ct) and for S°₃₀₀(tt)–S°₃₀₀(cc) and from measurements of K_3,4 and K_5,6 at one temperature, we obtain –ΔH°_ƒ300(tt)–ΔH°_ƒ300(ct)=–1·6 ± 0·2 kcal mol^–1 and ΔH°_ƒ(tt)–ΔH°_ƒ(cc)=–2·0 ± 0·1 kcal mol^–1. The results are compared with previous data for the geometrical isomer pairs of pentene, heptene, and pentadiene and with predictions, based on the concept of direct additivity of S°-, ΔH°_ƒ–, and C°_p- increments, reported in the literature.