Thermochemical Considerations

4) An approximate working order of selected M-L BDEs can be constructed from the 

The remainder of this chapter presents the established mechanisms of ligand substitution reactions of complexes with 16,17, and 18 valence electrons. These are the first mechanistic data presented in this text. Keep in mind that a mechanism is a theory deduced from the available experimental data. - Logically, a mechanism can only be proven to be invalid and is, therefore, impossible to prove to be true the accepted mechanism explains the available data and allows additional predictions that can be tested experimentally. Modifications and refinements of the mechanisms presented in this chapter and in the rest of the text are likely to be made in the future. 

These data show that, apart from the elements C, N and O and the B-elements of the last row of the Periodic Table (Hg, Tl, Pb and Bi), the reaction between an element and methyl chloride to yield the chloride and methyl of the element is exothermic, and therefore might be expected to provide a method of synthesizing the methyls. In all the examples listed where a negative value of AH is found, such reactions have been observed, and many are important synthetic routes. The mechanisms of the reactions and the conditions under which they occur, however, vary greatly with different elements. Kinetic as well as thermodynamic factors are thus important in controlling the reactions. 

In the B-elements the strength of the M—C bonds falls sharply on going from the elements of the second long Period (e.g. Sn, Sb) to the elements of the third long Period (e.g. Pb, Bi). The alkyls of Hg, Tl, Pb and Bi are ail strongly endothermic compounds, and their unfavourable heats of formation are not outweighed by the formation of the exothermic halides when the elements are treated with alkyl halides. 

Calculated heats of reaction of methyl halides with metals suggest that CH3F should react more exothermically than the other halides. In practice, however, methyl fluoride reacts by far the most reluctantly. Although the detailed mechanism of attack of a methyl halide on a metal will vary with the metal concerned, the activation energy will almost certainly be related 

The reaction between metals and organic halides is often suitable for the synthesis of organometallic compounds of the most electropositive elements. On a laboratory scale, the derivatives of Li, Mg and Al, which are very important in synthetic work, are usually prepared in this way. Some illustrations of the method are given briefly here more detailed discussion of the preparation and properties of the compounds involved appears in Chapter 3. 

Organolithium compounds may be prepared by the action of lithium metal on alkyl or aryl halides, in ether or hydrocarbon solvents, 

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Since Ex, E4 and E6 are not known, only thermochemical considerations concerning Si-H and Si-Si bonds are available to form a basis for choosing between mechanisms (A) and (B). Niki and Mains69 and also Gunning et a/.70 proposed that the enthalpy change for the step... 

Perhaps the most interesting of the inner-sphere pathways are those which result in the net transfer of an oxygen atom. The factors governing the viability of this pathway are still speculative. For cobalt-02 adducts, thermochemical considerations suggest that oxygen atom transfer should be accompanied by electron transfer in the reverse direction. Similarly, this pathway should be enhanced for MO2 complexes in which the metal... 

This criterion was originally established for the fragmentation of alkanes by Stevenson [18] and was later demonstrated to be generally valid. [19,20] The rule can be rationalized on the basis of some ion thermochemical considerations (Fig. 6.4). Assuming no reverse activation barrier, the difference in thermodynamic stability as expressed in terms of the difference of heats of formation of the respective products determines the preferred dissociation pathway ... 

Introduction 198 Experimental techniques 200 Ion cyclotron resonance spectrometry 201 Flowing afterglow 203 High pressure mass spectrometry 204 General features of gas-phase ion-molecule reactions 204 Gas-phase SN2 reactions involving negative ions 206 Thermochemical considerations 206 General aspects of gas-phase SN2 reactions 207 Stereochemistry 209... 

Thermochemical Considerations.—The heats of formation of the three anhydrous chlorides of vanadium have been determined by combustion in a bomb calorimeter and are found to be as follows —6... 

From thermochemical considerations only, light of wavelength less than 8420 A. could possibly bring about ozone formation from oxygen. However, of the probable mechanisms, the one requiring the least energy is... 

The mode of the reaction can be predicted on the basis of simple thermochemical considerations on the stability of the fragments [176-179]. As one may expect, (3-alkoxy-, (3,(3-dialkoxy- and P-aminophenethyl derivatives are likewise cleaved via the radical cation [180-184]. Another extensively investigated class is that of 1,2-diarylpinacols, aminoalcohols and diamines [185-188], particularly efficient when electron donating substituents are present on the aromatic ring [189,190] (Sch. 9). 

The preceding introduction might lead one to believe that this chapter could simply be divided into two basic parts—thermochemical considerations and kinetic considerations—which would cover all the relevant subject matter. However, in the last decade, first-principles (ab initio) computations have become commonplace and their results have often confirmed predictions based on thermochemical approaches, sometimes even surpassing them in accuracy. Hence, there is a need to encompass both thermochemical and ab initio treatments. We group the latter under the heading structural energetics and explore this topic further in Section 2.2. We also talk about the relevant thermodynamic and kinetic factors for specific systems in their respective chapters. For now, we discuss thermodynamics and kinetics in the most general terms. 

Thermochemical considerations also show that the first order reaction would be about 14 kcal mole endothermic for electrons trapped in ice. In contrast the second order reaction (14) is highly exothermic (ca. 90 kcal mole" ) and the rate constant in water at 25°C is very high (ifc= 1 X 10 M" sec Gordon et al., 1963). 

Examples of thermochemical considerations of cupric enolates include the study of the binding of Cu + with kojic acid (16), a cyclic a-ketoenol. Comparison was made between the divalent cations of U02 +, Cu +, Zn +, Ni +, Co +, Cd +, Ca + where these metals are listed in decreasing order of binding constants over 6 powers of 10. In this case carbon-bonded metal seems most unreasonable because it would ruin the chelation as well as any aromaticity in the pyrone ring. It is admittedly an assumption that pyrones are aromatic. There are no one-ring pyrones for which there are enthalpy of formation data for gas phase species, as opposed to the benzoannelated compounds coumarin (I7)i07a, I07b chromone (is) " " "and xanthone (19) . Plausible, but unstable, Cu(II) enolates eliminate copper and form the 1,4-dicarbonyl compounds as shown in equation 8. 

C-Trifluoromethylation of titanium enolates by CF3I was recently compared to that of lithium enolates. Given that CF3 derivatives do not undergo substitution reactions by either Sn 1 or Sn2 reactions, a chain reaction involving CF3 and/or CF3U has been suggested. To understand this, thermochemical considerations include the strengths of Ti—I and Li—I bonds, and barriers for loss of Ti(III) and Li(0) from ketyl radicals. 

A group of researchers in Budapest continued the line of Yoneda [16-21] but avoided the combinatorial explosion of the number of products by the preliminary definition of acceptable reaction products. Thus, the species to be included in the mechanism were fixed a priori, and the program provided the list of reactions. They used the matrix technique of Yoneda for the representation of reactions and species structures, but the number of generated reactions was limited by applying certain restrictions. The most important restriction was that bimolecular reactions were considered only with a maximum of three products. The number of generated reactions was kept low based on reaction complexity and thermochemical considerations. The mechanism obtained was reduced by qualitative and quantitative comparisons with experimental results, including contributions of elementary reactions to measured rates. The method proposed 538 reactions for the liquid phase oxidation of ethylbenzene. The reaction-complexity investigation approved only 272 reactions and the reaction heats were feasible in the cases of 168 reactions. This mechanism was reduced to a 31-step final mechanism. 

Molecular-orbital, spectroscopic and thermochemical considerations support the assignment of the band in the visible to an electronic transition between a lone pair on the nitroso nitrogen and an anti-bonding ji orbital, and that of the near-ultraviolet band to a 7t—71 transition in a nitroso-nitro molecule with a jT-only N—N bond. 

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