Thermal Behaviour of Selected C₁₀H₁₆ Monoterpenes

Abstract

The presented work investigates the thermal behaviour of selected monoterpenes under various reactor temperatures and residence times (carrier gas, reactor inserts). In addition to the analysis of the liquid products by capillary GC and GC‐MS, chemical derivatisation techniques (Diels–Alder reaction, hydrogenation) were used to identify the liquid‐phase products. A thermal conversion of β‐pinene (1), myrcene (2) and limonene (4) in a reaction network is presented and the experimental evidence for the formation of pyrolysis products by a biradical pathway is discussed. The reaction network was modified based on the identification of additional C10H16 terpene isomers. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Thermal Isomerization of (+)‐cis‐ and (−)‐trans‐Pinane Leading to (−)‐β‐Citronellene and (+)‐Isocitronellene

“…This contact was not detected in the case of 1 b, whereby an interaction of H À C 7 and H À C 7' with H À C 10 was observed. Therefore, the conjectures concerning an interaction between H 10 and H 9 in case of 1 a could not be confirmed. Nevertheless, the results allow for the conclusion that the trans geometry is advantageous for stabilization of the molecule due to attractive interactions with other protons (HÀC 7 , HÀC 7' , HÀC 10 ), similar to the diaxial attraction in cis-1,3-disubstituted cyclohexanes.…”

“…Scheme 4 illustrates the groundstate conformation of both pinane isomers. If the conjectures would be correct, a NOE effect is to be expected between H À C 10 and H À C 9 or H À C 2 and H À C 9 for 1 a and 1 b, respectively.…”

“…Experiments were carried out in an electrically heated quartz reactor described previously by using oxygen-free nitrogen as the carrier gas. [10] Chromatograms depicted in Figure 1 resulted from experiments with the pure compounds listed on the right-hand side. Figure 1 reveals that 1 a and 1 b differ from each other with respect to the selectivity of the products formed.…”

“…Without the presence of functional groups that can stabilize a free radical by conjugation (e.g. C=C in a-or b-pinene [4,8,10] or C=O in nopinone, [4,30] ) the lifetime of the formed biradical is very short and the barrier of activation is high. Therefore, a decision as to which of the two routes (concerted or stepwise) leads to the detected acyclic main products requires further investigations focused on both kinetic experiments [31] and ab initio calculations [32] of the presumptive transition states.…”

“…Both yield limonene and y-limonene, the latter as an additional monocyclic reaction product from the thermal isomerization of b-pinene. [4,8,10,33] According to the formation of 2 from 1 a and 1 b, the initial formation of a 1,4-carbon-centered biradical (BR1; cf. Scheme 3) by bond breakage between C 1 and C 6 is necessary for the side-reaction pathway leading to 6-8. p-Menthenes 6-8 are formed by [1,5]Hshifts, in which the hydrogen migration from C 9 to C 2 yields those isomers with the exocyclic double bond.…”