Thermochemical kinetics: does it still give insights?

Abstract

This tutorial review revisits the subject of the seminal book written by Sidney Benson in 1968. A short summary of the nature of the subject is presented, including its place in the wider world of quantitative chemistry. A number of themes are selected to illustrate its previous and continuing usefulness in evaluating numerical values of important quantities, and probing ideas of reaction mechanism. These include strain enthalpies for biradical combination, chain reactions, why some reactions don't occur and the involvement of carbenes in hydrocarbon rearrangements.

An effort to generalize the thermal isomerization of 6,6-dimethylbicyclo[3.1.1]heptanes and 6,6-dimethylbicyclo[3.1.1]heptenes: Comparative pyrolysis of pinane, α-pinene, and β-pinene

“…If the substituents offer no possibility for mesomeric delocalization (1) both routes are possible. This behavior is in agreement with studies dealing with the thermal decomposition of substituted cyclobutanes [27][28][29][30][31]34]. Whereas the pyrolysis of vinyl-substituted cyclobutanes yield only one set of fragmentation products [31], two sets of products are found for the thermolysis of fourmembered rings that have no sp 2 -carbon atoms attached to the ring [29][30][31]34].…”

“…4) listed in Table 2 indicate that all reactions initially passing through a ''normal'' transition state being typical for the fragmentation of four-membered ring systems (cyclobutane, oxetane) [27][28][29][30][31]. Hence a concerted mechanism involving a 4p antiaromatic transition state is thermally forbidden according to the Woodward-Hoffmann rules [32,33], those reactions have to proceed via a stepwise fragmentation mechanism including a biradical intermediate, which is illustrated in Scheme 2.…”

Kinetic and mechanistic aspects of myrcene production via thermal-induced β-pinene rearrangement

“…The negative values in activation entropy found for the formation of 3 and 4 (k 3 , k 4 ; Table 5) are typical for tight TS typical for reactions with a cyclic six-membered TS [48,49]. Therefore, their formation via concerted [1,n]H-shift reactions seems to be confirmed (Scheme 5).…”

“…The equality for all values in case of k 1 and k 2 points out the predominance of these reactions. Log 10 A around 14.0 and DS # ranging from À10 to 10 J K À1 mol À1 are clear indicators for a normal transition state (TS) the reaction passes through [48,49]. Those TS are characterized by rotational and torsional degrees of freedom similar to those of the initial molecules.…”

N-Protonated Isomers as Gateways to Peptide Ion Fragmentation

“…Ample computational studies, particularly by Paizs and coworkers, have demonstrated successfully that the computational theory of (pseudo-thermal) peptide fragmentation is the same as for any other thermal reaction in organic chemistry [1]. Because of the larger size of peptides, computations are more difficult and expensive than for small organic compounds, but conventional thermochemical kinetics still applies [17]. Indeed, thermochemistry and kinetics have been used as the basis of a heavily parameterized method for predicting peptide ion fragmentation [18].…”