Compound relative stability

Strain energies in some alicyclic compounds Relative stabilities of cw- and cycloalkenes... 

MarkownikofT s rule The rule states that in the addition of hydrogen halides to an ethyl-enic double bond, the halogen attaches itself to the carbon atom united to the smaller number of hydrogen atoms. The rule may generally be relied on to predict the major product of such an addition and may be easily understood by considering the relative stabilities of the alternative carbenium ions produced by protonation of the alkene in some cases some of the alternative compound is formed. The rule usually breaks down for hydrogen bromide addition reactions if traces of peroxides are present (anti-MarkownikofT addition). 

For the transition metals it is often impossible to reach a noble gas structure except in covalent compounds (see effective atomic number rule) and it is found that relative stability is given by having the sub-shells (d or f) filled, half-filled or empty. 

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)... 

In each of the following groups of compounds identify the one with the largest heat of combustion and the one with the smallest In which cases can a comparison of heats of combus tion be used to assess relative stability" ... 

For the other broad category of reaction conditions, the reaction proceeds under conditions of thermodynamic control. This can result from several factors. Aldol condensations can be effected for many compounds using less than a stoichiometric amount of base. Under these conditions, the aldol reaction is reversible, and the product ratio will be determined by the relative stability of the various possible products. Conditions of thermodynamic control also permit equilibration among all the enolates of the nucleophile. The conditions that permit equilibration include higher reaction temperatures, protic solvents, and the use of less tightly coordinating cations. 

Table 8.6. Relative Stabilization of Carbonyl Compounds by Substituent Groups"...

The relative stability of the anions derived from cyclopropene and cyclopentadiene by deprotonation is just the reverse of the situation for the cations. Cyclopentadiene is one of the most acidic hydrocarbons known, with a of 16.0. The plCs of triphenylcyclo-propene and trimethylcyclopropene have been estimated as 50 and 62, respectively, from electrochemical cycles. The unsubstituted compound would be expected to fall somewhere in between and thus must be about 40 powers of 10 less acidic than cyclopentadiene. MP2/6-31(d,p) and B3LYP calculations indicate a small destabilization, relative to the cyclopropyl anion. Thus, the six-7c-electron cyclopentadienide ion is enormously stabilized relative to the four-7c-electron cyclopropenide ion, in agreement with the Hixckel rule. 

The C-C distance in CaC2 is close to that in ethyne (120.5 pm) and it has been suggested that the observed increase in the lanthanoid and actin-oid carbides results from a partial localization of the supernumerary electron in the antibonding orbital of the ethynide ion [C=C] (see p. 932). The effect is noticeably less in the sesquicarbides than in the dicarbides. The compounds EuC2 and YbC2 differ in their lattice parameters and hydrolysis behaviour from the other LnC2 and this may be related to the relative stability of Eu and Yb (p. 1237). 

The nature of the substituent group X plays an important role in determining the relative stability of the various tautomeric forms. In aliphatic systems the tendency of C—XH to become CH— C 1=X increases markedly in the order X = CHz < NH < O, and certainly hydroxy compounds show less inclination to exist as such than do amino compounds. The position of S in this series is not completely... 

The apparently high relative stability of KF-4KNb03 can be related to the steric similarity between potassium and fluorine ions, which is hardly the case with other alkali metals. Table 10 presents a general list of compounds that can be obtained by the interaction of Nb02F with alkali metal carbonates. 

It is hard to generalize about the chemical reactivities of a group of elements since reactivities depend upon two factors (A) the relative stability of the specific compounds formed compared with the reactants used up, and, (B) the rate at which the reaction occurs. In special cases there are other complications. For example, chromium metal (familiar in the form of chrome plate) is highly reactive toward oxygen. Still, a highly polished piece of chromium holds... 

Through a display of a series of electrocyclization reactions, the Nicolaou group demonstrated the biomimetic , one-step synthesis of the endiandric acids involving the cascade of reactions proposed by Black. The polyunsaturated compounds 37 and 38 (Scheme 7) were designed for their relative stability and potential to serve as... 

It is apparent, from the above short survey, that kinetic studies have been restricted to the decomposition of a relatively few coordination compounds and some are largely qualitative or semi-quantitative in character. Estimations of thermal stabilities, or sometimes the relative stabilities within sequences of related salts, are often made for consideration within a wider context of the structures and/or properties of coordination compounds. However, it cannot be expected that the uncritical acceptance of such parameters as the decomposition temperature, the activation energy, and/or the reaction enthalpy will necessarily give information of fundamental significance. There is always uncertainty in the reliability of kinetic information obtained from non-isothermal measurements. Concepts derived from studies of homogeneous reactions of coordination compounds have often been transferred, sometimes without examination of possible implications, to the interpretation of heterogeneous behaviour. Important characteristic features of heterogeneous rate processes, such as the influence of defects and other types of imperfection, have not been accorded sufficient attention. 

Diazocarbonyl compounds are especially useful in these reactions because of their ease of formation, relative stability, and controlled reactivity in catalytic reactions [ 1,11 ]. As outlined in Scheme 1, a wide diversity of methodologies are available for this synthesis, with access dependent on the nature of Z. Vinyl- and aryldiazoacetates are accessible by other pathways [2]. The order of reactivity toward diazo decomposition has diazoketones and diazoacetates much more reactive than diazoacetoacetates or diazomalonates. However, the influence of electronic effects on reactivities is more pronounced with phenyl- and vinyl-diazoacetates than with diazoacetoacetates and, especially, diazoacetates [12]. 

Many complex ions, such as NH4+, N(CH3)4+, PtCle", Cr(H20)3+++, etc., are roughly spherical in shape, so that they may be treated as a first approximation as spherical. Crystal radii can then be derived for them from measured inter-atomic distances although, in general, on account of the lack of complete spherical symmetry radii obtained for a given ion from crystals with different structures may show some variation. Moreover, our treatment of the relative stabilities of different structures may also be applied to complex ion crystals thus the compounds K2SnCle, Ni(NH3)3Cl2 and [N(CH3)4]2PtCl3, for example, have the fluorite structure, with the monatomic ions replaced by complex ions and, as shown in Table XVII, their radius ratios fulfil the fluorite requirement. Doubtless in many cases, however, the crystal structure is determined by the shapes of the complex ions. 

Furthermore, gallium compounds can serve as model systems for aluminum congeners. Cationic gallium alkyls are of interest in synthesis and catalytic applications involving polar substituents because of the relative stability of the Ga—R bond toward hydrolysis and electrophilic cleavage compared to the otherwise superior Al-R species [11]. 

A major goal was to investigate the solid state structures of such compounds by single crystal X-ray diffraction. It was found that Lewis acid-base adducts R3M—ER3 show general structural trends, which allow estimations on the relative stability of the adducts. The experimental results were confirmed by computational calculations, giving even deeper insights into the structural parameters and the thermodynamic stability of simple Lewis acid-base adducts. In addition, their thermodynamic stability in solution was investigated by temperature-dependent NMR spectroscopy. 

Most of the known borides are compounds of the rare-earth metals. In these metals magnetic criteria are used to decide how many electrons from each rare-earth atom contribute to the bonding (usually three), and this metallic valence is also reflected in the value of the metallic radius, r, (metallic radii for 12 coordination). Similar behavior appears in the borides of the rare-earth metals and r, becomes a useful indicator for the properties and the relative stabilities of these compounds (Fig. 1). The use of r, as a correlation parameter in discussing the higher borides of other metals is consistent with the observed distribution of these compounds among the five structural types pointed out above the borides of the actinides metals, U, Pu and Am lead to complications that require special comment. 

There is no ready explanation for the relative instability of the alkyl-substituted cyclopentadienylthallium(I) compounds. The change in the hydrocarbon substrate, which results from introduction of an alkyl group, is likely to be small and insufficient to account for the dramatic stability differences [in the case of 9-R-substituted fluorenes, for example, where R = H, CHj, C2H5, or terf-butyl, the maximum difference in p. units between any two compounds is only 1.6 (9)]. The change in orbital overlap caused by introduction of an alkyl group is also likely to be small the l,T-dialkylferrocenes, for example, are stable compounds 137). At present, the only plausible explanation appears to be that the relative stabilities of the crystal lattices vary markedly 169). 

---