Activation energy cyclo

The methylene triplet adds to ethylene symmetrically through the triplet biradical directly (B). The central methylene group (formed from ethylene) is bent downwards by this process (Fig. 10). At this stage rotation or direct ring closure can occur, with loss of stereochemistry following bond formation to yield cyclopropane. The cyclo-addition of the triplet requires only a small activation energy of about 5 kcal/mole 52). 

K[Ru(0)(PDTA)].3Hj0 and Ru(0)(HEDTA) (PDTA=(propylenediaminetetra-acetate) -) are made by oxidation of K[Ru "Cl(PDTA.H)] or K[Ru" Cl(EDTA.H)] with PhIO electronic and ESR spectra were recorded. Rates and activation energies for epoxidation by stoich. Ru(0)(PDTA)] or Ru(0)(HEDTA)/water-dioxane of cyclo-alkanes were measured, as were those for oxidation of cyclohexane to cyclohexanol and cyclohexanone [632],... 

Since the generation of radicals from cyclo-S or -Sg requires an activation energy of more than 29 kcalmol the radical mechanism may account for the thermally or photochemically initiated processes, but it cannot prevail for the transformations that occur at ambient temperature. Hypervalent thiosulfoxides... 

The (high-pressure) pre-exponential factor for the ring-opening of cyclo-butene into butadiene is 1013 4 s 1, and the activation energy is 137.6 kJ/mol. Using the RRK theory calculate the unimolecular rate constant for the reaction at an excitation energy of 200 kJ/mol. 

These kinetic parameters take into account the formation of a five-membered ring intermediate, the H-abstraction reaction of two H-atoms of allyl type and the formation of the resonantly stabilized l-hexen-3-yl radical. These facts explain why the reverse isomerization reaction requires greater activation energy. As clearly shown in Fig. 6, there is a new class of important reactions, i.e. ring decomposition (e.g., cyclo-hexyl to form hexenyl radical) and the reverse cyclo-addition reaction. The activation energies of ring decomposition to form primary radicals are 31,500 and 28,000 kcal/kmol respectively for the... 

The activation energy of the cyclo-elimination has been estimated at 3 kcal mole-1 below Tg [88]. Since the isobutene yield decreases in the presence of naphthalene, benzophenone and oxygen, the reaction is thought to proceed through the excited triplet state of the ester side group [88]. 

More recent views on the theory of (2 + 2)-cycloaddition, in particular with respect to the question whether the two novel o-bonds are formed via a concerted or a stepwise mechanism, have been presented by Epiotis.28 He predicts that, if in the reaction of two n -electron systems one of the reactants has an electron-donating and the other an electron-accepting character, the activation energy of the concerted non-allowed 1 2 + n2s]-cycloaddition will be lowered, so that such a reaction may occur in a concerted manner under relatively mild conditions. As this condition is satisfied in most of the reported thermal (2 + 2)-cyclo-addition reactions of heterocyclic compounds, care must be taken in drawing any conclusions as regards the reaction pathways followed. 

While dehydrating secondary alcohols over MgS04, a considerable constancy of activation energy is also observed. The alcohols cyclo-hexanol, cyclopentanol, 2-pentanol, and 2-propanol were investigated the percent of their dehydration at 370° C were as follows 17.7 17.8 18.0 18.8 and e=15.0 14.4 15.2 and 14.8 kcal/mole, respectively. This also testifies to an equal orientation of the molecules of the index group CHCO toward the catalyst (181). 

The units used for rate coefficients will be molar and seconds. Activation energies will be given in kcal/mole. The symbol 0 represents / rin 10 where R is 1.987 X 10 kcal/mole-K. For reactants which give more than one set of products, the reaction channels are split into a, b, c, etc. Thus the overall reaction 2 has two channels, reactions 2a and 2b, with 2 = 2a + 2b- Prefix abbreviations are n, normal i, iso c, cyclo s, secondary t, tertiary. 

Olefin rotation occurs in the complexes mer-tricarbonyl(T7 -olefin)(T7 -norbor-nadiene) tungsten (3) with Arrhenius activation energies of 58 and 54.8 kJ moP for complexes, L = T/ -(Z)-cyclo-octene and 17 -ethene, respectively. " Restricted... 

According to a qualitative orbital consideration, the vi/i orbital of the ji-system of nitrone can interact with the formal nitrogen nonbonding electron pair of cyclo adducts. The anomeric effect (classic and kinetic) that can become operational in the transition state of cycloaddition can stabilize the transition state. On the other side, the unoccupied x /3 orbital of the jt-system of nitrone through its interaction with the unoccupied o orbital of Cl-O bond can decrease the activation energy, and with that increase the electrophilic character of nitrone (Fig. 8.37). 

In contrast to the photoinduced electron transfer reactions of quadricyclane, its direct 185-nm photolysis in the liquid phase leads to NBD and to the formation of two side-products, namely 1,3,5-cycloheptatriene and 6-methylfulvene (Scheme 10). On the other hand, selective excitation of the CH bonds of gaseous Q by absorption of 598.8 nm laser photons produces a vibrationally hot [NBD]" , which fragments into cyclo-pentadiene, acetylene and 1,3,5-cycloheptatriene (Scheme 10). These CH overtone transitions involve excitation energies greater than the activation energy for the Q NBD isomerization to account for the hot molecule effect. The photoproducts 6-methylfulvene and 1,3,5-cycloheptatriene are also formed in trace amounts in the direct 254-nm (< 5%) and triplet-sensitized (<0.5%) photolysis of Q, which limits the usefulness of the NBDi Q interconversion as an efficient solar-energy storage system . 

The genesis of the fumaric acid formed in the Diels-Alder reaction of 2-methylfuran with maleic acid has been re-investigated. Kinetic measurements indicate that fumaric acid is formed in a very minor side reaction, its rate of formation being very much less than the rates of cycloaddition between the furan and maleic acid and the cyclo-reversion of the exo-cis- and cndo-cis-Diels-Alder adducts. It therefore appears that the fumaric acid is obtained as a product of a very minor side reaction, a non-concerted reverse cycloaddition pathway, that is superimposed on the lower activation-energy concerted pathway. 

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