Enthalpies of rearrangement

E. Taskinen and K. Nummelin, J. Org. Chem., 50, 4833 (1985). These authors performed the isomer equilibration at several temperatures and so could use the experimentally derived equilibrium constant to derive the enthalpy of rearrangement. There was no need for assuming the entropy of isomerization is 0 or just determined by symmetry number corrections. 

The enthalpy of formation of CH3N=CHOCH3 also has to be estimated. Experimental measurements to derive the enthalpy of formation of imidates are as rare as they are for amidines. To do so, we assume the same enthalpy of rearrangement to the corresponding amide as measured for CH3N=C(CH3)OCH3,68 kJ mol1 [62], and add... 

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... 

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 isomerization of 5 to 7 and 8 involves a chain-branching type rearrangement (lOis ll) (Brouwer and Oelderik 1968) and has a free-enthalpy of activation of about 22 kcal mole . This result, combined with the data of Fig, 2, the free-enthalpy of activation of 17 kcal mole for the rearrangement 9-i ll (Brouwer and Hogeveen, 1972), and an estimated difference in free-enthalpy of about 0-8 kcal mole between 10 and 11 constitutes the basis for the free-enthalpy diagram in Fig. 3. 

From Fig. 4 it is seen that the free-enthalpy of activation for the rearrangement of tertiary butyl to secondary butyl cation is 30-4 — 3.9 = 26-5 kcal mole . As the reverse rearrangement has been found by direct observation to have JG cl7-18 kcal rnole" (Saunders et al., 1968), it follows that the difference in stabilization between tertiary and secondary butyl cations is indeed 9 + 1 kcal mole . This value is in excellent agreement with a previous experimental value of 10 + 1 kcal mole (Brouwer and Hogeveen, 1972). 

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. 

These energies relate to bond rearrangement in gaseous molecules, but calculations are often performed for reactions of condensed phases, by combining the enthalpies of vaporization, sublimation, etc. We can calculate a value without further correction if a crude value of AHr is sufficient, or we do not know the enthalpies of phase changes. 

A theoretical study of substituent effects in the thio-Claisen rearrangement (91) -> (92) has been carried out. The study has shown that 2,5-disubstimtion leads to tighter transition states and to a substantial lowering of the enthalpy of... 

Chemisorption denotes the situation in which an actual chemical bond is formed between the molecules and the surface atoms. A molecule undergoing chemisorption may lose its identity as the atoms are rearranged, forming new compounds that better satisfy the valences of the surface atoms. The enthalpy of chemisorption is much greater than that of physical adsorption. The basis of much catalytic activity at surfaces is that chemisorption may organize molecules into forms that can readily undergo reactions. It often is difficult to distinguish between chemisorption and physical sorption, because a chemisorbed layer may have a physically sorbed layer deposited above it. 

In addition, rearranging Eq. (21) and combining with Eq. (19) illustrates that i = k" y. Therefore, a two parameter (one term) model would require that a plot of the enthalpies of adduct formation of one acid versus the enthalpies of adduct formation of another acid for the same series of bases be linear with a zero intercept. The enthalpies of adduct formation for 12 and phenol with a wide series of bases does not give rise to such a plot as can be seen in Fig. 5. These acids have very different C/E ratios and their enthalpies of adduct formation cannot be correlated by a one term model. Furthermore, a one term model could riot incorporate systems in which reversals in donor-acceptor strength are observed 32). However, it is possible to correlate enthalpies of adduct formation for acids with very similar C/E ratios such as hydrogenbonding acids using a one term equation. Correlations restricted to one particular type of acid are, of-course, only a subset of the overall E and C correlation. 

An extensive AMI semiempirical study on a wide range of anomerically substituted acetamides was published in the first major paper describing this reaction. For rearrangement of 221, enthalpies of activation were clearly lowered in the sequence... 

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