Rearrangement reactions enthalpies

A second issue that arises in relation to isodesmic reaction enthalpies is why they should exist at all. If all we are doing is rearranging bonds, shouldn t the summed bond energies be the same on either side of the reaction Not really. A negative 6-3IG MP2 enthalpy of 5 kcal mol for the reaction... 

The thermal automerization and rearrangement reactions of PAHs have been widely investigated during the past two decades (for examples see refs. [31 e, g, 62-64]). The main objective was to understand the processes of formation of aromatic hydrocarbons in fuel rich flames and the mechanisms of transformation of the PAHs that have been observed at these elevated temperatures. In most cases, thermally initiated rearrangement reactions in the carbon skeletons of PAHs require high enthalpies of activation resulting in low product selectivities and poor overall yields. Because the expected products are often more effectively prepared by conventional routes, this approach has been used as a synthetic tool only in a few cases, e.g. the synthesis of azulenes [65] and the rearrangement of bifluorenylidenes to benzenoid hydrocarbons [38]. 

Scheme 11.35 Activation enthalpy, activation entropy and reaction enthalpy for the thio-Claisen rearrangement of allyl vinyl sulfide predicted on different levels oftheo7 (1996).

A type II fragmentation corresponds to decomposition of the parent ion via a rearrangement into a stable fragment ion and a stable neutral fragment. Because of the stable products the reaction enthalpy of this process is usually small, as in the case of the... 

Enantiomers, preferential crystallization of 59 Endo selectivity 798 Ene reactions 808, 809 Enones, synthesis of 732 Enthalpies of formation 102, 103 Enynes, synthesis of 956 Enzymatic kinetic resolution 829 Epimerization 399 Episulphides, oxidation of 237 Episulphones 650, 775 Episulphoxides, photolysis of 742 a,/J-Epoxysulphones reactions of 811, 812 rearrangement of 685 synthesis of 612 / ,y-Epoxysulphones 781 y,<5-Epoxysulphones 627, 628 Epoxysulphoxides reactions of 613 rearrangement of 744 synthesis of 327, 612 Erythronolides 831... 

The predictions of the reactivities by the geminal bond participation have been confirmed by the bond model analysis [103-105] of the transition states and the calculations of the enthalpies of activation AH of the Diels-Alder reaction [94], the Cope rearrangement [95], the sigmatropic rearrangement [96], the Alder ene reaction [100], and the aldol reaction [101] as are illustrated by the reactions of the methyl silyl derivatives in Scheme 38 [102], The bond is more electron donating than the bond. A silyl group at the Z-position enhances the reactivity. 

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