Isomerization enthalpy

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. 

Reversing the CH=N group position in 477-5,6-benzoxazin-4-one should result in the same isomerization enthalpy as from HC(NMe)OMe to MeCHNOMe. The enthalpy of formation of the latter species was calculated to be —7.1 kJmoR from the high-level quantum chemical calculations of Reference 4. The enthalpy of formation of the former species is unknown. However, if we accept the gas phase isomerization enthalpy of dimethylamides to methyl imidates (69.6 13.4 kJmoR ) in equation 52 from Reference 75,... 

Now where does the discrepancy lie An acyclic paradigm was used for the oxi-mate/imidate interconversion. Had we used the plausibly aromatic paradigm of isoxa-zole/oxazole, the isomerization enthalpy would have been 95 kJ moR where the requisite enthalpies of formation are from References 76 and 1, respectively. This difference is meaningfully distinct from that of the acyclic paradigm but still does not particularly ameliorate the difference. Is it possible that the N—O bond in aryl and alkyl oxime ethers are profoundly different Could it be that we have neglected any antiaromaticity in the 8 n... 

The formal isomerization enthalpies of methyl n-butyl ether to methyl terf-butyl ether and of di-n-butyl ether to n-butyl ferf-butyl ether are about —24kJmoU (Iq) and —26.5 kJmol (g), respectively. From these, and the experimental enthalpy of formation of di-ferf-butyl peroxide, the enthalpies of formation of di-n-butyl peroxide are ca —333 kJmol (Iq) and —288 kJmol (g), wildly divergent from the reported values. 

The photorearrangement of pyrazoles to imidazoles is probably analogous, proceeding via iminoylazirines (isomerization enthalpy =42 kJ mol ) (82AHC(30)239) indazoles similarly rearrange to benzimidazoles (67HCA2244). 3-Pyrazolin-5-ones (65) are photochemically converted into imidazo-lones (66) and open-chain products (67) (70AHC(ll)l). The 1,2- and 1,4-disubstituted imidazoles are interconverted photochemically. 

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. 

There are four pairs of 1- and 2-alkenes with branching in a substituent group for which we can examine double bond migration enthalpies49 4-methyl-l- and 4-methyl-2-pentene 5-methyl-1- and 5-methy 1-2-pentene 4-methyl-l- and 4-methyl-2-hexene and 4,4-dimethyl-1- and 4,4-dimethyl-2-pentene. Each of the 2-enes exists in cis and tram forms. The first two pairs, which are homologous, have Ci to trans-C2 isomerization enthalpies of ca —13 (for two measurements of trans-4-methyl-2-pentene) and —14.4 kJmol-1, and Ci to cis-C2 isomerization enthalpies of —9.6 and —10.4 kJmol-1. The third pair, not homologous with any other, has a Ci to trans-C2 isomerization enthalpy of... 

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. 

By our definition of cyclic styrenes , it is also logical to include 1-phenylcyclohexene (37) for which an enthalpy of formation has been measured and is given in our archive. In addition, its (Z)-/(E)-isomerization enthalpy is also available from experiment (cf Reference 55). Indeed, it may be argued that discussion of these species belongs in the current section of the chapter. Perhaps, but now we note that we do not consider the reported enthalpy of formation of the conventional isomer to be correct. From the preferred liquid-phase value of the enthalpy of formation of the olefin of interest, —16.8 6.7 kJmoU1, and of the related saturated species, phenylcyclohexane,... 

As a comparison of the enthalpies of formation of isomeric aldehydes and ketones shows, the disubstituted carbonyl compounds (ketones) are more stable than the monosubstituted carbonyl compounds (aldehydes), analogous to the stability order for the corresponding 1,1-disubstituted ethenes and the 1-n-alkenes. For the three aldehyde/methyl ketone isomeric pairs (nc = 3-5), initially it seems that the enthalpies of isomerization are fairly constant. However, if the interpolated value for pentanal is used, the trend is clearly that of more negative enthalpy of isomerization with increasing c(g) —31.7, —33.9, —34.5 kJmol-1. The non-constant enthalpies are expected because the slopes, ag, in Table 4 are quite different for aldehydes and ketones. The greater contribution to the isomerization enthalpy differences comes from the methyl ketone which is more stabilized than the aldehyde by an additional —CH2— group. The enthalpies of isomerization of the corresponding alkenes are much less exothermic (—6.1 0.8 kJmol-1) than those of the carbonyl compounds. 

The only available aldehyde data are for isomerization of butanal to 2-methylpropanal for which AHisom(g) = —11.0 1.7 kJmol-1. The corresponding alkene isomerization enthalpy of 1-pentene to 3-methyl-1-butene is —6.2 0.9 kJmol-1. Hydrogenating all... 

The rate constants, thermodynamic parameters of activation, equilibrium constant, and the isomerization enthalpy for conversion of cholest-5-en-3-one to cholest-4-en-3-one catalysed by EtONa in absolute ethanol were determined by classic and multivariate kinetic methodologies. The multivariate modelling kinetic treatment allowed the concentrations of the species involved to be calculated, revealed the 3,5-dienolate to be a highly reactive intermediate, and was able to discriminate among several applicable mechanisms, thereby supporting the one comprising two reversible steps.18... 

It is an assumption that the n bond eneigy, thus defined, is equal to the internal rotation barrier. For ethylene, the thermochemical n bond energy and the barrier to internal rotation are within experimental error of one another. Note that the isomerization enthalpy is strictly valid only if the DSSE(H3CCH) is determined from H3CCH3. 

In this study, we explored the stability of the n-bromopropane trans and gauche isomers by calculating the isomerization enthalpy at the levels of theory of interest. The results are summarized in Table 11.1. The values vary from —0.12 to... 

We find —23.3 0.5 kJ moR from direct measurement of isomerization enthalpy, from... 

As part of a study of the isomerizations, pyrazole to imidazole and 1-methyl- to 4-methyl-imidazole, isomerization enthalpies have been determined from the experimental heats of formation. For the former rearrangement A/7 was calculated as —42.7 kJ mol" (cf. experimental value, —40.2 kJ mol ), whereas 4-methylimidazole was calculated to be more stable by 27.6 kJ mol than its 1-methyl isomer <86JOC1105>. In view of the known thermal rearrangement of 1- to 2-methylimidazole it would be of interest to apply similar measurements of isomerization enthalpy to the species in this transformation. 

The most common photochemical rearrangement in the azoles is the pyrazole imidazole isomerization (isomerization enthalpy 42 kJ mol" ). Any reverse reaction should be similar to the isomerization of oxazole to isoxazole (via an azirine intermediate) oxidation reactions (see Section 3.02.5.3.11), while rearrangements of A-nitro-, A-acyl-, and C- and A-alkenylimidazoles are discussed under reactions of the appropriate functional group (see Section 3.02.7). 

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