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• 2040-73-5 [18]annulene 

Relevant academic research and scientific papers

Elucidation, by scanning microcalorimetry, of the detailed mechanism of the thermal rearrangement of [18]annulene into benzene and 1,2-benzo-1,3,7-cyclooctatriene. Determination and interpretation of the thermochemical and kinetic parameters of the three consecutive reactions implicated

The thermograms Qr(T) recorded for the thermal rearrangement of [18]annulene 1 have been reinvestigated using an iterative method (CALIT-3 program) based on the numerical integration of the kinetic equations established for the following reaction mechanism: [18]annulene 1 →k1 tetracyclic intermediates 2 →k2 transbicyclo[6.4.0]dodeca-2,4,6,9,11-pentaene 3 + benzene 5; 3 → k3 1,2-benzo-1,3, 7-cyclooctatriene 4. Independent 13C-NMR experiments indicate that three chiral tetracyclic intermediates 2a, 2b, and 2c are implicated in the reaction. They are considered to play the same role in the mechanism and are therefore considered as a single compound 2 in the kinetic equations. A programmed differential microcalorimeter with linearly increasing temperature was used for producing the thermograms. The reaction enthalpies Δr,iH of each individual reaction (: i = 1, 2, or 3 as well as the Arrhenius parameters Ea,i, log Ai, and the activation thermodynamic quantities ΔH?i, ΔS?i, and ΔG?i (at 298.2 K) have been established by the iterative numerical simulation of the thermograms. These results are discussed. The enthalpy of formation of [18]annulene 1, as well as the stabilization and π-bond delocalization energies of this molecule have been deduced: ΔH°f(1, g, 298.2 K) = 123.4 ± 3.7 kcal mol-1; ΔHstab(1) = -37.6 ± 4.5 kcal mol-1; ΔHdeloc(π-bonds in 1) ? -120.5 kcal mol-1. The stabilization enthalpy ΔHstab(1) has been used for discussing the activation enthalpy of the conformational mobility in 1 previously reported. The pathway multiplicities deduced from the detailed mechanisms for each individual reaction are discussed in view of the interpretation of the large activation entropies found. Of special interest is the discussion of the mechanism of the two suprafacial 1,5 hydrogen shifts implied in the reaction 3 → k3 4 and of the high activation entropy observed. In independent measurements, the rate of disappearence of the [18]annulene 1 has been followed by UV spectroscopy in order to obtain the kinetic parameters for the first reaction. The results of these independent measurements are in good agreement with those obtained by the analysis of the thermograms. The reported investigation points to the fact that the thermograms recorded with our calorimetric method constitute very precise data and that their correct analysis allows the elucidation of complex reaction mechanisms.