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 K1,2= 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 ΔH1,2=–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 K3,4 and K5,6 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.