Enthalpy of addition

Very different enthalpies of addition are found for the acylnitrone PhC(0)CHN(0)Ph. Instead of the above exothermicity of ca 82 kJmoH for the addition of PhCHN(0)Ph and its substituted derivatives to a maleimide, the acylated nitrone adds to a variety of norbomenes with an average exothermicity of ca 120 kJmoL, some 38 kJmoH more favorable. This suggests about 40 kJmol destabilization arises from the presence of the adjacent acyl and nitrone groups. No enthalpy of formation data are available for any acylnitrone. To simulate the mutual destabilizing effect of the acyl and nitrone... 

A number of thermodynamic studies of Ni[R-dtp)2 adducts with amines have been described H2,i43,is6,is8) Daktemieks and Graddon ) have measiued the enthalpies of addition of pyridine, 4-picoline, 2,2 -dipyridyl and 2,9-dime-thyl-l,10-phenanthroline to Ni[ethyl-dtp]2 and pyridine to Nilpropyl-dtpJj. The sums of the stepwise enthalpies for pyridine addition to the ethyl and... 

Thermodynamic studies of the enthalpy of addition of aldehydes and acyl chlorides to [Rh(P Pr3)2Cl]2 and the hypothetical monomer Rh(PPr3)2Cl gave unsaturated five-coordinate Rh(III) compounds according to equations (16) and (17) and afforded the values shown in Table 17. 

Table 17 Enthalpy of addition of XY to [(P Pr3)2RhCl]2 (—AHrxn) and the hypothetical monomer (P Pr3)2RhCl (—AHnim) in solution (kcalmoE )...

Figure 7.1. Comparison of enthalpies of addition of various ligands to W(CO)3(PCy3)2 (binding strength increases going downward). Reprinted with permission from Gonzalez, A. A., et at. ACS Symposium Series 1990, 428, 133. Copyright 1990 American Chemical Society.

Calculated (B3LYP) enthalpies of addition and the effects of phosphine methylation (substitution of PMca for PH3) in reactions of some iridium complexes with C-H compounds (as well as with molecular hydrogen) are summarized in Table VI. 3 [46], It can be seen that the addition of aryl and especially acetylene C-H bonds is thermodynamically more favorable than the addition of simple alkyl C-H bonds. Addition of an aryl C-H bond has been found to be at least 16 kcal mol less exothermic than H2 addition. However, on the basis of the Bryndza-Bercaw relationship [47]... 

The low enthalpy of addition of CO as shown in reaction (13) can be compared to the more exothermic value for reaction (14) determined in the gas phase by pulsed laser techniques (11) ... 

The mechanistic basis of free-radical reactions was considered in Chapter 12 of Part A. Several mechanistic points are crucial in development of free-radical reactions for synthetic applications. Successful free-radical reactions are usually chain processes. The lifetimes of the intermediate radicals are very short. To meet the requirement of synthesis for high selectivity and efficiency, all steps in a desired process must be fast in comparison with competing reactions. Because of the requirement that all steps be quite fast, only steps that are exothermic or very slightly endothermic can participate in chain processes. Comparison of two sets of radical processes can illustrate this point. Let us compare the enthalpy of addition of a radical to a carbon-carbon double bond with addition to a carbonyl group ... 

Maria, P.C., Elegant, L., Azzaro, M. et al. (1972) Phosphorus-containing heterocycles. XV. Behavior of a series of 2-oxo-l, 3,2-dioxaphospholanes toward boron trifluoride determined from enthalpy of addition. Thermochim. Acta, 4, 505-511. 

The eo values calculated from experimental bre parameters made it possible to evaluate correctly the contribution of the enthalpy of addition to its activation energy. As an example. Table 7.6 presents the results of this comparison for two groups of addition reactions. 

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