Stability of compounds

The relation between heat of formation and stability is not a simple one. A compound will be unstable at low temperatures if its heat of formation is negative if the compound is formed from one element that is solid at room temperature and a second element that is gaseous, the pressure of the latter will be higher than one atmosphere if the heat of formation per mol gas formed is lower than TAS, or 300 X 35 cal = 10 kcal. A halide, therefore, will be unstable at room temperature for it will decompose into its compounds when V (heat of formation per equivalent) is smaller than X 10 kcal, because two equivalents must decompose to form one mol of halogen (for an oxide this limit is J X 10 kcal and for a nitride i X 10 kcal). Thus, in spite of a heat of formation as low as 2 kcal per equivalent, a nitride will be perfectly stable at room temperature. 

It is due to the decrease of the heat of formation that, at the end of the periods, the highest halides, oxides, sulphides and nitrides become unstable. In the second period no normal halides are formed after carbon. In the third period the fluorides extend to SFe, in the fifth group to IF7. The normal oxides, too, extend to higher valencies in the third period, where are S03 and C1207, than in the second, where N2Os is the highest oxide. 

Compounds like carbides and silicides, that on decomposition do not form a gaseous phase, may be stable up to very high temperatures, although their heat of formation is very low CSi, with a heat of formation of 27 per mol, or 6-7 kcal per equivalent, 

The heat of formation of SbCl5 is 105 kcal per equivalent it would therefore require 42 kcal per mol Cl2 to decompose the compound into antimony and chlorine. This reaction can occur only at high temperatures, i.e. above 1000°K, and thus, if only the decomposition reaction is considered, the compound can be said to be very stable. There is, however, another reaction that requires much less energy, viz. the decomposition into a compound of lower valency. The reaction 

2CrCl4 2CrCl3 + Cl2 + 24 kcal 2CrCl3 - 2CrCl2 + Cl2 - 70 kcal CrCl2 - Cr + Cl2 - 97 kcal 

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The oxidation state -1-4 is predominantly covalent and the stability of compounds with this oxidation state generally decreases with increasing atomic size (Figure 8.1). It is the most stable oxidation state for silicon, germanium and tin, but for lead the oxidation state +4 is found to be less stable than oxidation state +2 and hence lead(IV) compounds have oxidising properties (for example, see p. 194). 

Steric requirements also affect the stability of compounds therefore, for the complexes Pt(PR3)4, the PEt3 complex (cone angle 132°) loses lmol phosphine in vacuo at 50°C, but the PMe3 complex (cone angle 118°) is unaffected. 

Investigating thermal stabilities of compounds 23-28 revealed that when heated to 80 C the compounds containing PPh,i i.e., 23,25, and 27, were less stable than their PCyy bearing analogues (24, 26, and 28, respectively). The least stable was (PPhy)2Cl2Ru(3-phenylindenylid-l-ene) (23) which decomposed after 2 h at 80 C (Table IX, entry 1) and the most stable was (lPr)(PPhy)Cl2Ru(3-phenylindenylid-1 -ene) (27) (Table IX, entry 4) which showed decomposition after 42 h at the same temperature. It can be concluded that the presence of the nucleophilic carbene... 

So the stability of compounds with the metal in high oxidation numbers is thought to be possible because this formal oxidation number in fact does not reflect the real... 

The trend toward decreased stability of compounds with increased lipophilicity and MW is obvious. This analysis can provide information about the roles of these two properties in contributing to clearance for this series. Again, the important question for hit triage decision making is how the hypothesized structure-clearance trends correlate to the property trends for other relevant biological properties. 

The most frequently encountered hydrolysis reaction in drug instability is that of the ester, but curtain esters can be stable for many years when properly formulated. Substituents can have a dramatic effect on reaction rates. For example, the tert-butyl ester of acetic acid is about 120 times more stable than the methyl ester, which, in turn, is approximately 60 times more stable than the vinyl analog [16]. Structure-reactivity relationships are dealt with in the discipline of physical organic chemistry. Substituent groups may exert electronic (inductive and resonance), steric, and/or hydrogen-bonding effects that can drastically affect the stability of compounds. A detailed treatment of substituent effects can be found in a review by Hansch et al. [17] and in the classical reference text by Hammett [18]. 

Chemistry can be viewed as a balance between thermodynamic and kinetic factors which dictate the course of chemical reactions and the stability of compounds. Chemists seeking to achieve particular goals, manipulate these factors using chemical or physical means. The papers in this symposium on "High Energy Processes in Organometallic Chemistry" describe recent attempts to apply mainly physical means to get around the thermodynamic and kinetic constraints of conventional organometallic chemistry. 

Electronic transitions like insulator-metal transitions, magnetic order-disorder transitions, spin transitions and Schottky-type transitions (due to crystal field splitting in the ground state in/element-containing compounds) profoundly influence the phase stability of compounds. A short description of the main characteristics of these transitions will be given below, together with references to more thorough treatments. 

Responses to the CSB industry survey50 indicate that most companies consult a variety of information sources as a first step in compiling data on reactive hazards. However, respondents prefer literature sources and expert opinion over computerized tools such as CHETAH, The Chemical Reactivity Worksheet, or Bretherick s Database of Reactive Chemical Hazards. Such programs can be used to predict the thermal stability of compounds, reaction mixtures, or potential chemical incompatibilities. In some cases, they provide an efficient means of identifying reactive hazards without having to conduct chemical testing. Survey responses showed that five of nine companies consider computer-based tools not valuable. Only two of the surveyed companies use The Chemical Reactivity Worksheet.51... 

It is obvious from the definition of standard enthalpy of formation that these quantities do not represent the absolute enthalpic stability of compounds. They merely reflect their enthalpic stability relative to that of the chemical elements in standard reference states (to which AfH° = 0 has been arbitrarily assigned). It is thus unreasonable to state that a given substance is more stable than another just because it has a lower standard enthalpy of formation. We can only use AfH° values to make such direct comparisons when we are assessing the relative stability of isomers. 

This part includes a discussion of the main experimental methods that have been used to study the energetics of chemical reactions and the thermodynamic stability of compounds in the condensed phase (solid, liquid, and solution). The only exception is the reference to flame combustion calorimetry in section 7.3. Although this method was designed to measure the enthalpies of combustion of substances in the gaseous phase, it has very strong affinities with the other combustion calorimetric methods presented in the same chapter. 

A general presentation and discussion of the origin of structure of crystalline solids and of the structural stability of compounds and solid solutions was given by Villars (1995) and Pettifor (1995). For an introduction to the electronic structure of extended systems, see Hoffmann (1987, 1988). In this chapter a brief sampling of some useful semi-empirical correlations and, respectively, of methods of classifying (predicting) phase and structure formation will be summarized. 

The very high thermal stability of compounds with the elements representative of the 15th to 16th groups such as the antimonides, tellurides, etc. may be underlined. 

Polynitroaliphatic compounds have not found widespread use as either commercial or military explosives. This is perhaps surprising considering the high chemical and thermal stability of compounds containing internal em-dinitroaliphatic functionality. In fact, many polynitroaliphatic compounds are powerful explosives, for example, the explosive power of... 

Chapter 6 therefore deals in detail with this issue, including the latest attempts to obtain a resolution for a long-standing controversy between the values obtained by thermochemical and first-principle routes for so-called lattice stabilities . This chapter also examines (i) the role of the pressure variable on lattice stability, (ii) the prediction of the values of interaction coefficients for solid phases, (iii) the relative stability of compounds of the same stoichiometry but different crystal structures and (iv) the relative merits of empirical and first-principles routes. 

It is important that you consider the stability of compounds under the reaction conditions and not at normal temperature and pressure. As we have illustrated in this chapter, a particular method may be chosen because the desired product is stable only at raised pressures or because it decomposes if the reaction temperature is too high. 

We will now see what the effect of resonance is on the stability of compounds. There are four states for the molecule N205 with octet structures only... 

Conversely, the stability of compounds of oxidation state +11 increases dramatically as the atomic number of the element increases. Carbenes, CX2, and silylenes, SiX2, are well established as transient reaction intermediates, and structural data have been obtained in several cases either at high temperatures or by their generation in low-temperature matrices. However, only for germanium, tin and lead are compounds in this oxidation state stable under ordinary conditions. Compounds with the Group IV element in oxidation state +111, the formal oxidation state of the radical species, R3M, are also usually considered as unstable transients. However, when R is very bulky, these metal-centred radicals, such as for example Sn[CH(SiMe3)2]3, have extremely long, perhaps indefinite, lifetimes in solution. 

Methylene carbon has not the capacity of a heteroatom for acceptance of a tautomeric proton, which fact explains the excellent stability of compounds like (107) (73JA7925), which exhibit no tendency to isomerize to the tautomeric thiophene (108). 

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