Disproportionation endothermicity

In order to document the radical disproportionation reaction, we have used FT-IR spectroscopy to characterize the irradiation products. Upon irradiation of 1 in pentane, the formation of the characteristic peak near 2100 cm-1 due to Si-H stretching vibrations was readily apparent. The IR spectrum obtained in perdeuterated pentane was identical, suggesting that radical processes other than abstraction from the solvent are involved. Furthermore the ESR spectrum obtained in this solvent is identical to that already described. This raises the question whether the initially formed silyl radicals really abstract hydrogen from carbon with the formation of carbon-based radicals as suggested (13), particularly in light of the endothermicity of such a process. 

When one of the aromatic groups of the triarylmethyl free radical is replaced by an alkyl group, a decrease in stability due to a loss of resonance stabilization is to be expected. The paramagnetism and reactions associated with these less stable radicals will therefore appear only when the ethane is heated well above room temperature, the dissociation being endothermic. The rate of formation, but not the equilibrium constant, is experimentally accessible for these radicals since the radical once formed is subject to rearrangement, cleavage, and disproportionation reactions ... 

The disproportionation of borirene to acetylene and diboretene (58) is slightly endothermic (by 5 kcal/mol), whereas the activation barrier of this BH transfer reaction equals 14.6 kcal/mol (86JA3960) The estimation of the aromatic stabilization energy for isodesmic reaction (59) yields values of 47.5 (6-31G //6-31G ) (86JA3960), 47.1 kcal/mol (6-31G //STO-3G), [81JA( 103)2589] which makes about 70% of the stabilization energy... 

No disproportionation lias been observed for 2CH, — CH2 + CH4. Although it is doubtful if it would be easily detectable if it did occur, it is more likely not to happen because of its endothermicity, estimated at about 5-7 kcal./mole. 

As mentioned above, xenon trioxide is an endothermic compound which explodes violently at the slightest provocation. Aqueous solutions are stable but powerfully oxidizing. These solutions are weakly acidic ( xenic acid ) and contain molecular XeOj When these solutions are made basic. HXeO ions are formed and alkali hydrogen xenates, MHXe04, may be isolated from them. Hydrogen xenale ions disproportionate in alkaline solution to yield perxenates ... 

This reaction is endothermic. There is a wide range of temperatures where the partial pressure of TiC14 allows the disproportionation [9]. 

The reactions shown in Eqs. (14.1) through (14.3) are known as the Alkylation reactions. They are exothermic and highly irreversible, except for Eq. (14.3). The reactions in Eqs. (14.4) through (14.6) are known as Disproportionation reactions. They are reversible and are endothermic. The alkylation reactions dictate the rate of consumption of methanol and are somewhat faster than the disproportionation rates that govern the selectivity of the three amines. 

The following observation emphasizes the influence of the temperature on ion-solvation equilibrium. The reduction product of 1 with lithium metal in methyltetrahydrofuran is temperature-dependent11. At —120 °C only the radical anion (1 ) could be observed by ESR, while at higher temperatures the paramagnetism disappears and the dianion (12 ) is detected. This reaction must be endothermic it therefore seems that disproportionation is driven by entropy and not by energy, due to ion-pair-solvation equilibrium. It is noteworthy that 12 cannot be observed by NMR spectroscopy due to its special electronic structure12. 

We see that the rate of production of products is determined by two quantities, the first a quasi-thermodynamic quantity, the equilibrium concentration of free radicals, and the second a kinetic quantity, namely, the rate at which each radical can go through a chain cycle. When the cycle is made up of two steps of disproportionate speed as in the present case, it is the slower step (in this case Br + H2) which is of importance in determining the over-all rate. It is this feature which explains in this case the specific inhibition by HBr even though the over-all reaction is essentially not reversible. The slow step in a chain will in general (though not always) be endothermic. This implies that its reverse is exothermic and hence of lower activation energy, and so faster. We can thus always expect inhibition by products in chain reactions except in those cases in which the fast steps are of unusual speed. 

All six possible interhalogens of this type exist. Out of these, the gaseous CIF is very stable, ICl and IBr are moderately stable and are pure crystalline materials at room temperature, whereas BrCl dissociates easily and reversibly into its elements (equation 36) BrF and IF disproportionate into a higher fluoride and Br2 or I2 (equation 37). IBr, a solid resulting from direct combination of the elements, is endothermic and extensively dissociated in the vapor. The stracture of CIF, determined at - 188 °C, represents the only... 

Under higher-pressure conditions, particularly at low temperatures, collisional deactivation of excited association products producing ClOOCl and CIOCIO may be important kinetically. In fact for the most stable intermediate, ClOOCl, its formation is the dominant process under the stratospheric condition [107, 108, 1 12,1 14, 116]. On the other hand, the disproportionation processes producing ClOO and OCIO by (12b) and (12c), respectively, are endothermic by 3 - 4 kcal/mol [119] accordingly, they cannot compete effectively with the recombination reaction in the stratosphere (with low temperature and medium pressure). These endothermic reactions may, however, become dominant processes in the combustion of the AP propellant. 

Figure 5 shows the temperature dependence of disproportionation of the sodium salts of radical anions of tetraphenylethene. Several conclusions may be drawn from these data. Both disproportionations are endothermic, as shown by their enthalpy changes (AH) AH18 = 19 2 kcal/mol, and AH 19 = 13 2 kcal/mol. These observations may not be surprising because the electron-electron repulsion in the dianion may account for the endo-thermicity of the disproportionation. However, the large entropy gain (AS)... 

In spite of the CIDNP polarization pattern, we believe the sulfinyl mechanism can be dismissed. First, the SO bond in a sulfinyl radical is very strong. Using Benson s estimate for the heat of formation of the phenylsulfinyl radical (13 kcal/mol) [63] and standard values for the other relevant compounds [98], the S-0 bond energy is ca. 102 kcal/mol, whereas the C-S bond is some 35 kcal/mol weaker. Transfer of an O atom from phenylsulfinyl to a methyl radical is endothermic by 11 kcal/mol, and to epoxidize ethylene endothermic by 40 kcal/mol. (The relevance of the latter example will become clear below.) Furthermore, from the a-cleavage work discussed previously, it is clear that the expected product from reaction to an arylsulfmyl radical and a carbon radical is a sulfenic ester or disproportionation product. 

The reaction with dimethyl acetylenedicarboxylate proceeds less cleanly for 1,3-dimethylindole, possibly because this indole is a better electron donor [40c]. Seven products are obtained these are the 2-1-2 cycloadduct and the geometrical isomers of 17-19 shown in Scheme 8. The substitution products 17 dominate in polar, protic solvent and are possibly formed via photochemical electron transfer from 1,3-dimethylindole to the alkyne this substitution mechanism is discussed further in Section V. In nonpolar or aprotic media, 17 is still formed, although only as a minor product under these conditions, where electron transfer could be endothermic, it is possible that 17 is formed by a route involving intramolecular disproportionation of the triplet 1,4-biradical 20 that is also the likely precursor of the 2-1-2 photocycloadduct. The geometrical isomers 18 and 19 are the major products formed when the indole concentration is high Davis and Neckers speculate that these arise from addition of biradical 20 to 1,3-dimethylindole [40c]. However, the lifetimes of... 

However, if Cu (aq) is formed in a clean aqueous medimn, the disproportionation reaction has often a relatively long induction period as reaction (4a) is endothermic due to the Cu°(s) lattice energy. 

The latter two reactions proceed via the inner-sphere mechanism (see below), that is, they require access of the substrate to the central Cu(I) ion. The disproportionation reaction requires the contact of the central copper ion with a smface, preferably a Cu°(s) surface, as the formation of a Cu° atom is extremely endothermic due to the lattice energy of copper, - 301.4 kJmol (5). Thus ligands that block sterically the approach of a substrate or of a surface to the central copper ion stabilize it (19). An extreme example is 1,4,5,7.7,8,11,12,14,14-decamethyl-l,4,8,ll-tetraazacyclotetradecane, (27). Thus [Cu(I)L ] is stable even in aerated aqueous solutions (27). In analogy, some enzymes with Cud) as the active site, for example, CuSOD, inhibit disproportionation or the reaction with O2 by inhibiting the approach of two Cu(I) central ions to each other which is required for these reactions which are thermodynamically exothermic. 

As stated above, Cu" (aq) is unstable and disproportionates via reaction (4). However, due to the fact that reaction (4a) is very endothermic as AG°(Cu°—Cu(s)) = —301.4 kJmol (5) one can prepare over saturated Cu" (aq) acidic aqueous solutions. (The acidity is required due to the very low solubility of Cu0H/Cu20 (6).). The lifetime of deaerated Cu (aq) solutions depends on the purity of the system, that is, on the niunber of crystallization centers available in the solution or the walls of the container. 

---