Enthalpies of isomerization

The goal of this project is to determine the enthalpies of formation of cis- and trans-2-butene and to calculate the enthalpy of isomerization between them. 

What is the enthalpy of isomerization of propene (CLiH ) to cyclopropane at 298 K by a semi empirical calculation ... 

Oxepin, 4-ethoxycarbonyl-2,3,6,7-tetrahydro-synthesis, 7, 578 Oxepin, 2-methyl-enthalpy of isomerization, 7, 555 Oxepin, 2,3,4,5-tetrahydro-reduction, 7, 563 synthesis, 7, 578 Oxepin, 2,3,4,7-tetrahydro-synthesis, 7, 578 Oxepin, 2,3,6,7-tetrahydro-oxidation, 7, 563 reduction, 7, 563 Oxepin-2,6-dicarboxylic acid stability, 7, 565 Oxepinium ions synthesis, 7, 559 Oxepins, 7, 547-592 antiaromaticity, 4, 535 applications, 7, 590-591 aromatization, 7, 566 bond lengths and angles, 7, 550, 551 cycloaddition reactions, 7, 27, 569 deoxygenation, 7, 570 dipole moment, 7, 553 disubstituted synthesis, 7, 584... 

Y. Harel, A. W. Adamson, C. Kutal, P. A. Grutsch, K. Yasufuku. Photocalorimetry. 6. Enthalpies of Isomerization of Norbornadiene and of Substituted Norbomadienes to Corresponding Quadricyclenes. J. Phys. Chem. 1987, 91, 901-904. 

The enthalpy of isomerization of the primary n-butyl lithium to ec-butyl lithium is ca - -2 kJmoP. From the two different enthalpies of formation of isopropyl lithium and the previously derived liquid-phase enthalpy of formation of n-propyl lithium, the isomerization enthalpy is either ca -hi5 klmoL or nearly zero. Considering both this result and the one earlier that also compares the two different enthalpy of formation values for isopropyl lithium, the value from Reference 7, —57.7 klmop, seems more plausible. 

If the enthalpy of formation of 4-lithiobutyl methyl ether is interpolated between the values for the lithiopropyl and the lithiopentyl ethers to be —285 kJ moP, then the enthalpy of isomerization to the less stable 3-lithiobutyl methyl ether is - -10 klmoP, which is about half that of isomerization of n-butyl lithium to 5ec-butyl lithium (-1-21.3 kJmol ). However, a linear interpolation assumes the same strain energy for the 6-membered 4-lithiobutyl ether as for the above 5- and 7-membered cu-lithioalkyl methyl ethers. If it is less strained, then the isomerization enthalpy would be larger. How much of the isomerization enthalpy difference is due to other differences, such as intramolecular complexation and/or aggregation among the various species, is not known. Unfortunately, there is no enthalpy of formation measurement for the delithiated 7-methoxynorbornane. 

Since we absolutely do not believe the enthalpy of formation of n-PrOOH, let us derive a plausible value for it from the isomerization of /-PrOOH, assuming that whatever its provenance it is the more accurate of the two. From the difference between the enthalpies of formation of w-propyl and isopropyl alcohol, the derived enthalpies of isomerization are — 17.7 kJmor (g) and —15.5 kJmoH (Iq). Using these as the approximate enthalpies of isomerization of the related hydroperoxides, the derived enthalpies of formation of n-PrOOH are thus —179 kJmoU (g) and —227 kJmoU (Iq) from the liquid enthalpy of formation of /-PrOOH that was estimated above. 

The enthalpy of isomerization of hquid 1-hexanol to either 2- or 3-hexanol is ca 15 kJ mol , and the enthalpy of isomerization of hquid 1 -heptanol to 2-, 3- or 4-heptanol is ca 13 kJ mol . From the experimental enthalpy of formation of 1 -hexyl hydroperoxide, the calculated enthalpy of formation of 2- and 3-hexyl hydroperoxide is ca —315 kJ moU , which is about 5-10 kJmoU more negative than their experimental values. As noted earlier, the measured enthalpies of formation of 2- and 3-heptyl hydroperoxide are about the same as that of 1-heptyl hydroperoxide, while that of 4-heptyl hydroperoxide is actually less negative than for its primary isomer. Using instead the above-derived enthalpy of formation of 1-heptyl hydroperoxide of —325 kJmoU , the enthalpy of formation of the secondary isomers would be ca —338 kJ moU. This value is very close to the experimental enthalpy of formation of 4-heptyl hydroperoxide, but 8 kJmoU less negative than the experimental values for 2- and 3-heptyl hydroperoxide. These latter enthalpies of formation are too negative compared to the experimental values for 2- and 3-hexyl hydroperoxide, with a methylene increment of ca 36 kJ mol . The derived values are more plausible. 

Kinetic data on the oxepin-benzene oxide equilibration have been obtained from the temperature-dependent NMR studies. Low values were observed for the enthalpy of isomerization of oxepin (7.1 kJ mol-1) and 2-methyloxepin (1.7 kJ mol-1) to the corresponding benzene oxides (67AG(E)385). The relatively small increase in entropy associated with oxepin formation (5-11 J K 1 mol-1) is as anticipated for a boat conformation in a rapid state of ring inversion. Thermal racemization studies of chrysene 1,2- and 3,4-oxides have allowed accurate thermodynamic parameters for the oxepin-arene oxide equilibration process in the PAH series to be obtained (81CC838). The results obtained from racemization of the 1,2- (Ea 103.7 kJ mol-1, AS 3.7 JK-1 mol-1 and 3,4- (Ea 105.3 kJmoF1, AS 0.7 J K"1 mol ) arene oxides of chrysene are as anticipated for the intermediacy of the oxepins (31) and (32) respectively. 

There are also no experimental thermochemical studies113 of prismane (57), per se, but measurements of the enthalpy of isomerization of its hexamethyl derivative to hexam-ethylbenzene have been reported114. These experimentally measured isomerization enthalpies differ by some 40 kJ moT1. There are also measurements of the isomerization enthalpy for hexakis(trifluoromethyl)prismane to the corresponding benzene115. 

I, 2-diazocine they synthesized (see Section 11,A,2) had structure 20 (R = R1 = R = H), not 21, and consisted almost exclusively of monocyclic, as opposed to diazabicyclo[4.2.0]octatriene (see 86, Section II,C,3), valence tautomers. This latter observation is also supported by estimated enthalpies of isomerization to bicyclic tautomers, which are less favorable than in the carbocyclic series (79JOC1264). 

Heats or free energies of formation can be used to compare directly the energies of isomeric carbocations. Such a comparison is similar to the more familiar comparisons of energies of isomeric olefins, such as cis- and trans-butene. It depends on the energies of formation of isomeric molecules or ions being based on the same combination of elements. Energies of isomerization can also be measured directly, and Bittener, Arnett, and Saunders have measured the enthalpy of isomerization of secondary to tertiary butyl cations in CH2C12 as solvent.39... 

Complementary to enthalpies of hydrogenation for studying the energetics of alkenes are enthalpies of isomerization. These may be directly determined experimentally in favourable cases or derived from other enthalpies of reaction. The most accessible and general-purpose derivation is from enthalpies of formation of the alkenes of interest, as... 

Just as for the enthalpy of hydrogenation in equation 7, the enthalpies of isomerization between alkene members of two homologous series will be constant and equal to the y-intercept ( A//jsom ) only when m = 1. For homologous series with non-parallel slopes... 

It would be helpful if there were more experimentally determined enthalpies of isomerization to provide an independent comparison with those derived from equation 11. The most recent ones are from References 30 and 31. However, as in the analysis of the enthalpies of hydrogenation, it is only because we can derive enthalpies of formation from enthalpies of combustion, isomerization and hydrogenation that we have a large enough database of compounds to confidently attempt an analysis. The same selected alkene enthalpies of formation are used for the analyses in this section as were used in the preceding section. 

Comparing the effect of the double-bond migration within the identical cr-frameworks of isomeric isoalkyl-1-alkenes and the 2-methyl-1-alkenes, we find that the individual enthalpies of isomerization for Cg-Cs are —10.4, —10.9 and —11.3 kJmol-1. These mono- to disubstituted isomerization enthalpies are very similar to those for isomerization of the 1-n-alkenes to the tram -2-n-alkenes, homologous series which also possess a common structural skeleton. 

Several alkene isomers vary structurally in the position of a methyl branch on their parent 1-alkene chain. Generally, moving the methyl group from C3 to positions further from the double bond results in an exothermic enthalpy of isomerization. That is, the isoalkyl-1-alkenes are the most stable isomers and the 3-methyl-1-alkenes are (presumably) the least stable. Because of the problematic 5-methyl-1-hexene data and the lack of data for 3-methyl-1-heptene, nothing more quantitative can be said other than each methyl re-positioning down the chain results in about 1-2 kJmol-1 stabilization. A similar change in branching position from 3-methyl-n-alkanes to 2-methyl-n-alkanes releases about 3 kJmol-1. 

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