Thermoneutral isomerization

A Wagner-Meerwein rearrangement can be part of the isomerization of an alkyl halide (Figure 14.4). For example, 1 -bromopropane isomerizes quantitatively to 2-bromopropane under Friedel-Crafts conditions. The [l,2]-shift A — B involved in this reaction again is an H atom shift. In contrast to the thermoneutral isomerization between carbenium ions A and B of Figure 14.3, in the present case an energy gain is associated with the formation of a secondary carbenium ion from a primary carbenium ion. Note, however, that the different stabilities of the carbenium ions are not responsible for the complete isomerization of 1-bromopropane into 2-bromopropane. The position of this isomerization equilibrium is determined by thermodynamic control at the level of the alkyl halides. 2-Bromopropane is more stable than 1-bromopropane and therefore formed exclusively. 

There seems to be agreement that the interconversion of MeHSi=CH2 and Me2Si is nearly thermoneutral, with a barrier of ca 40 kcalmoC1,102. MeHSi=CH2 was found in higher concentration under conditions such that the two species could isomerize in a... 

These condensation reactions often lead to the formation of isomers. Structural isomerism of bis(silyl)hydrazines was first observed in 1964.19-24 In the absence of strong steric or electronic constraints, the bis(silyl)hy-drazines such as bis(trimethylsilyl)hydrazine give in a thermoneutral reaction essentially equal amounts of the N,N- and N,N -isomers at equilibrium.7 23 Wannagat et al. found that the reaction of hydrazine with trimethylchlorosilane at room temperature results only in the formation of N,N -bis(trimethylsilyl)hydrazine, whereas the same reaction in boiling solvents leads to a mixture of N,N- and /V,/V -bis(trimcthylsilyl)hydrazine. Both could be separated by preparative gas chromatography. Their struc-... 

Ab initio quantum-mechanical calculations have also been carried out to estimate theoretically the relative energies of different transition states and intermediates. To keep the calculations at a manageable level the model catalytic cycle shown in Fig. 7.4 has been considered. According to this calculation, conversion of 7.14 to 7.15 is found to be the rate-determining step. This is consistent with the empirical kinetic results. Isomerization of 7.15 to 7.16 is found to be thermodynamically highly favorable. The reductive elimination step, that is, 7.16 to 7.7, is found to have a very low free energy of activation and be thermoneutral, that is, thermodynamically neither strongly favorable nor unfavorable. 

We have eliminated the quantity jS which appears in D(M,y) by setting it equal to unity. If the over-all reaction is thermoneutral, then /S = 2 and a factor of 2 should appear in the denominators of Eqs. (XI.4.5) and (XI.4.6). This arises from the fact that this equation was derived for the unimolecular isomerization equation (XI. 1.9) and we have set the rates of isomerization kx A b and also the rates of deactivation k2 = ki-At these veiy low pressures we have the result that A and B are in equilibrium with each other but not with processes of activation or deactivation, which are much slower. Thus only half of the active species reaches the final state B. If kx were much greater or much smaller than /Itb (which occurs if the reaction is very energetic), then the factor of 2 disappears and j9 = 1 as implied above. 

Let us start with the three isomeric C-monomethylated species (the toluidines or methyl-benzenamines) and accept the recent experimental measurements and analysis17. This source points out the contradictory and incomplete measurements for these species and uses the expression in disarray to describe the literature. From thermoneutral equation 2, we would predict enthalpies of formation of —12, —5 and 55 kJ mol-1 for all three solid, liquid and gaseous toluidines, in reasonable agreement with the experimental values. We note that the o-isomer is slightly more stable than its m- and p-isomers. In comparison, the spread of enthalpy of formation values for the isoelectronic and isosteric xylenes is but 1 kJ mol 1 and for the so-related methylphenols3 (cresols) is but 7 kJ mol 1. As an additional check for thermochemical accuracy, consider the formal methylation reaction 4. For Ar = Ph, the enthalpies of reaction are —36.6 (lq) and —32.2 (g) kJ mol 1. For Ar = C6H4NH2, the ortho, meta and para gas-phase enthalpies of reaction are the... 

Of the three isomeric C-ethylated anilines, there are thermochemical data24 only for the ortho isomer. Equation 2 predicts enthalpies of formation of —29.7 (lq) and 34.4 (g) k.l mol 1. Equation 2 also suggests that the difference between enthalpies of formation of 2-ethylaniline and any of the C,C-dimethylaniIines would be the same as that between ethylbenzene and any of the dimethylbenzenes, namely ca 11-13 kJmol-1. Or said differently, equation 6 is expected to be thermoneutral. The expectation is met for 2,4- and 2,5-dimethylaniline, and nearly so for 2,6-dimethylaniline. 

The simplest species with but one amino group are the C-phenylated anilines. From Reference 4 there are enthalpies of formation of the isomeric 2- and 4-aminobiphenyl, 112.0 6.3 and 81.0 6.3 kJmol-1. However, there is a newer measurement for the 2-isomer of 93.8 1.1 kJmol-152. Still, the 13-31 kJmol-1 enthalpy difference between these two isomers is rather large since the 2-isomer is not expected to be much more strained. The enthalpy of formation difference is less, however, than that found for the corresponding biphenyl carboxylic acids53 of ca 40 kJmol-1. Equation 2 is essentially thermoneutral for ArNH2 = 4-aminobiphenyl while it is quite endothermic for ArNH2 = 2-aminobiphenyl. There is no obvious reason why the 2-derivative should be so different from its 4-isomer, especially since the two phenyl groups in the biphenyl species are not coplan ar. 

We recognize many of these azo species as classical dyes and so expect thermochemical data, if available, to be rather old and for the solids. Our expectations are fulfilled from the nearly century old Reference 134, we find the data for solid 4-aminoazobenzene, 2,4-diaminoazobenzene and 4-dimethylaminoazobenzene. From this early source we find the enthalpy of formation of azobenzene itself of 374 kJmol-1 suggesting that reaction 26 is nearly thermoneutral, an altogether reasonable result if it is assumed that the isomeric 2- and 4-amino azobenzenes have very nearly the same enthalpy of formation. Likewise, reaction 27 is roughly thermoneutral. 

In 1986, Pilcher and his coworkers measured the enthalpies of combustion and of sublimation of all three isomers. The ortho and para isomers are less stable and, most probably, the meta is more stable. For the gas phase, both ortho and para amino substitution is destabilizing relative to meta. However, there are two other sets of measurements. The first consists of a value from early in the last century for the p-isomer, — 168 kJ mol. This value is so discordant from the others, as well as so ancient, that it is easily disqualified. However, such early values are the only ones available for some phenols and other interesting and important compounds. Late in the last century, another thermochemical stud) also reported the enthalpies of formation for all three isomers from measured enthalpies of combustion and of sublimation. From this source it is much more decisive that in the gas phase the o-isomer is the most stable and the p- is the least. The individual enthalpies of formation and of sublimation from the two contemporary sources for the solid phenols differ by 4-17 kJ mol with no apparent explanation for the large isomeric disparities. If there were no interaction between the amino and hydroxy substituents, the exchange reaction 10 would be nearly thermoneutral, and the gas phase enthalpy of formation of the three aminophenols would be —92 kJ mol, a value close... 

The thermolysis of 8, 10, and 12 at 140°-175°C in inert solvents (e.g., benzene) does indeed afford the rearrangement products 9, 11, and 13, respectively, the reactions leading to equilibria that are easily detected by H— NMR spectroscopy 10). Equilibrium is reached rapidly, e.g., within 2 hours at 175°C. Heating an independently synthesized sample of 11 leads to the same equilibrium as 10. All reactions are clean, no traces of side products being observed. The fluxional behavior of 8 and 12 is strictly thermoneutral, i.e., the equilibrium constants have the value K = 1.0 within experimental error. The valence isomerization 10 11 is... 

It was found that these reaction enthalpies too are very nearly thermoneutral, resulting in enthalpy of formation values of —187.4 1.5 and —188.9 1.5 kJ/mol for the liquid m- and prii-ferf-butylbenzenes respectively. No o di-ferf-butylbenzene was observed in this study as an equilibration product. This absence is not surprising since we expect considerable steric destabilization for this species. Equivalently, the isomerization enthalpy of o-di-ferf-butylbenzene is expected to be large, and so it is reasonable to expect it to be directly observable calorimetrically. Since the equilibrium product mixtures from isomerization of 0-, m-, or p-di-ferf-butylbenzene should be identical, the difference between enthalpies of reaction of the 0- and m-isomers (and/or o- and p-isomers) equals the desired difference between the enthalpies of formation of the isomers. This last study has been performed, and the difference of 93 3 kJ/mol was found. The somewhat larger value of 103 7 kj/ mol was found in the same study from the difference between the appearance energies of the gas-phase reactions... 

However, the evidence is very strong (30) that in such reaction systems those phosphoranyl radicals observed by ESR have five-membered dioxa ring systems attached to phosphorus apical-equatorial as in 15 (15) and its mirror form 25. This applies to six-membered ring 1,3-diaza species too (39) The apparent failure of the isomerization of radical 15 to 16 (24) (a thermoneutral process which keeps the odd electron equatorial) to compete with loss of R (R = t-Bu), indicates that the cyclic phosphoranyl radicals are not simple analogs of pentacovalent phosphorus intermediates, at least where permutational properties are concerned. Thus by contrast to the apparent high AG of >11 kcal/mol assigned to process Mi of J.5 (15 24), that for the... 

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