Isomer relative stability

The three isomers of thiazoleacetic acid can be decarboxylated, the order of facility being 2>5>4, though the relative stability depends on each particular compound and the reaction conditions (72-75). This reaction may be used to obtain certain alkylthiazoles (73). Malonic derivatives can also be decarboxylated to give aliphatic thiazole acids (49, 51)... 

Heats of combustion can be used to measure the relative stability of isomeric hydrocarbons They tell us not only which isomer is more stable than another but by how much Consider a group of C His alkanes... 

The heat evolved on burning an alkane increases with the number of car bon atoms The relative stability of isomers may be determined by com paring their respective heats of combustion The more stable of two iso mers has the lower heat of combustion... 

You have seen that measurements of heats of reaction such as heats of combustion can pro vide quantitative information concerning the relative stability of constitutional isomers (Section 2 18) and stereoisomers (Section 3 11) The box in Section 2 18 described how heats of reaction can be manipulated arithmetically to generate heats of formation (AH ) for many molecules The following material shows how two different sources of thermo chemical information heats of formation and bond dissociation energies (see Table 4 3) can reveal whether a particular reaction is exothermic or en dothermic and by how much... 

E-Cyclooctene is also significantly straine4 but less so than -cycloheptene. As the ring size is increased, the amount of strain decreases. The. E-isomers of both cyclononene and cyclodecene are less stable than the corresponding Z-isomers, but for cycloundecene and cyclododecene, the E-isomers are the more stable. Table 3.10 gives data concerning the relative stability of the C7 through C12 cycloalkenes. 

The relative stability of the intermediates determines the position of substitution under kinetically controlled conditions. For naphthalene, the preferred site for electrophilic attack is the 1-position. Two factors can result in substitution at the 2-position. If the electrophile is very bulky, the hydrogen on the adjacent ring may cause a steric preference for attack at C-2. Under conditions of reversible substitution, where relative thermodynamic stability is the controlling factor, 2-substitution is frequently preferred. An example of this behavior is in sulfonation, where low-temperature reaction gives the 1-isomer but at elevated temperatures the 2-isomer is formed. ... 

Enamines derived from 1-azabicycloalkanes, readily accessible by mercuric acetate oxidation of saturated bases (112), have been extensively studied recently (113-115). Since an immonium salt is formed during dehydrogenation, the composition of the liberated enamine mixture shows the relative stability of the various possible isomers. The study of infrared and NMR spectra has shown that the position of the enamine double bond is determined by factors similar to those determining the relative stability of simple olefins. 

The elaborate treatment for the H2O system is only possible beeause of its small size. For larger systems, less rigorous methods must be employed. Let us as a more realistic example consider a determination of the relative stability of the C4H6 isomers shown in Figure 11.15. There are experimental values for the first eight structures, which allows an evaluation of the performance of different methods. This in turn enables an estimate of how much trust should be put on the predicted values for 9, 10 and 11. 

Many aryhydrazones provide two or more isomers when subjected to the conditions of the Fischer indole cyclization. The product ratio and the direction of indolization can also be affected by different reaction conditions (i.e. catalysts and solvents), which is attributed, at least in part, to the relative stabilities of the two possible tautomeric ene-hydrazine intermediates. Generally, strongly acidic conditions favor formation of the least substituted ene-hydrazine, while cyclization carried out in weak acids favors the most substituted ene-hydrazine. Eaton s acid (10% P2O5 in MeSOsH) has been demonstrated to be an effective catalyst for the preparation of 3-unsubstituted indoles from methyl ketones under strongly acidic conditions. Many comprehensive reviews on this topic have appeared. ... 

AHC(S1), pp. 384,385]. For some derivatives of 119 and 120, the relative stability of oxo isomers may be enhanced. Thus, the predominance of the 5,5-diphenyldihydro-4//-l,2,3-triazol-4-one structure 121 in solution has been confirmed by UV and and NMR studies (93CB103). Also, 1-methyl-5-hydroxy-l,2,3-triazole exists mainly in the tautomeric form 120 (R = Me, R = H) [76AHC(S1), p. 385], although the existence of a minor amount of oxo form 122 was postulated to explain the exchange of the 4-position proton in D2O. 

If we were to measure what are called heats of hydrogenation (A/Phydrog)tor the two double-bond isomers and find their difference, we could determine the relative stabilities of cis and Ivans isomers without having to measure an equilibrium position. In fact, the results bear out our expectation. For c/s-2-butene, AEP y rog -120 kj/mol (-28.6 kcal/mol) for the trans isomer, AFP]lvdrfM, = -116 kj/mol (-27.6 kcal/mol). 

P.N. D yachkov, A.A. Levin, Vibronic theory of relative stability isomers in inorganic molecules and complexes, Itogy Nauki i tekhniki, ser. Stroenie molecul i khimicheskaya svyas, 11 (1987) (in Russian). 

The relative stabilities of RSSR and R2S=S for R=OH and SH have been determined by Steudel and co-workers [23, 24]. At the MP2/6-311G //HF/ 6-311G +ZPE level of theory, (HO)2S=S (thiosulfurous acid) is just 13 kj mol less stable than the chain-like isomer HOSSOH (dihydroxydisul-fane) [23]. For R=SH, (HS)2S=S is less stable than the corresponding trisul-fane HSSSH by 132 kJ mor (MP2/6-31G -//HF/4-31G) [24]. 

Geometries, hyperfme structure, and relative stabilities of the different positional isomers of monodeuterated benzene cations have been studied theoretically by density functional theory, using the B3-LYP functional, and experimentally by ESR and ENDOR spectroscopy. A comparison between theoretical and experimental results at 30 K gives acceptable agreement, but further experiments on multiply deuterated species should improve the analysis by making the effects of deuteration larger. 

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