Hydrogen dissociation enthalpy

The decomposition of tri- and tetrasulfane in CCI4 solution (0.2 mol 1 ) at 70 °C and in the absence of oxygen has been studied by H NMR spectroscopy [64]. Initially, tetrasulfane decomposes to a mixture of tri- and pentasul-fane but slowly and after an induction period hydrogen sulfide and disulfane are formed in addition. These results have been interpreted in terms of a radical-chain reaction. The initial step is assumed to be the homolytic cleavage of the central SS bond which has by far the lowest dissociation enthalpy of the molecule ... 

Table I includes the relative bond dissociation enthalpies obtained for some group 14 hydrides by photoacoustic calorimetry,7 10 The data demonstrate that, for the trialkyl-substituted series, the bond strengths decrease by 6.5 and 16.5 kcal/mol on going from silane to germane and to stannane, respectively. The silicon-hydrogen bonds can be dramatically weakened by successive substitution of the Me3Si group at the Si-H functionality. A substantial decrease in the bond strength is also observed by replacing alkyl with methylthio groups.

In Table 2.4, we have collected background information for discussion in the following chapters. Recommended C—H bond dissociation enthalpies of selected organic compounds are reported in the first two columns, followed by a variety of heteroatom-hydrogen bond strengths including N—H, O—H, S—H, Ge—H, and Sn—H bonds. 

Many AB5 hydrides have AH 8 kcal so that at 0-100°C (of interest for chemical heat pump action), the hydrogen dissociation pressures change roughly by one order of magnitude. This implies that a metal hydride pair could function in the heat pump mode even if their enthalpies were exactly the same, provided... 

To be effective as autoxidation inhibitors radical scavengers must react quickly with peroxyl or alkyl radicals and lead thereby to the formation of unreactive products. Phenols substituted with electron-donating substituents have relatively low O-H bond dissociation enthalpies (Table 3.1 even lower than arene-bound isopropyl groups [68]), and yield, on hydrogen abstraction, stable phenoxyl radicals which no longer sustain the radical chain reaction. The phenols should not be too electron-rich, however, because this could lead to excessive air-sensitivity of the phenol, i.e. to rapid oxidation of the phenol via SET to oxygen (see next section). Scheme 3.17 shows a selection of radical scavengers which have proved suitable for inhibition of autoxidation processes (and radical-mediated polymerization). 

In the chlorination of propane, the secondary hydrogen atom is abstracted more often because the secondary radical and the transition state leading to it are lower in energy than the primary radical and its transition state. Using the bond-dissociation enthalpies in Table 4-2 (page 143), we can calculate AH° for each of the possible... 

The energy differences between chlorination and bromination result from the difference in the bond-dissociation enthalpies of H—Cl (431 kJ) and H—Br (368 kJ). The HBr bond is weaker, and abstraction of a hydrogen atom by Br- is endothermic. This endothermic step explains why bromination is much slower than chlorination, but it still does not explain the enhanced selectivity observed with bromination. 

Peroxides are often added to free-radical reactions as initiators because the oxygen-oxygen bond cleaves homolytically rather easily. For example, the bond-dissociation enthalpy of the O —O bond in hydrogen peroxide (H —O —O —H) is only 213 kJ/mol (51 kcal/mol). Give a mechanism for the hydrogen peroxide-initiated reaction of cyclopentane with chlorine. The BDE for HO — Cl is 210 kJ/mol (50 kcal/mol). 

Stability of Allylic Radicals Why is it that (in the first propagation step) a bromine radical abstracts only an allylic hydrogen atom, and not one from another secondary site Abstraction of allylic hydrogens is preferred because the allylic free radical is resonance-stabilized. The bond-dissociation enthalpies required to generate several free radicals are compared below. Notice that the allyl radical (a primary free radical) is actually 13 kJ/mol (3 kcal/mol) more stable than the tertiary butyl radical. 

The bond dissociation enthalpy for normal hydrogen bonds is ca. 13... 42 kJ/mol (3. .. 10 kcal/mol) For comparison, covalent single bonds have dissociation enthalpies of 210... 420 kJ/mol (50... 100 kcal/mol). Thus, hydrogen bonds are approx, ten times weaker than covalent single bonds, but also approx, ten times stronger than the non-... 

The decomposition products are LiH and MgH2 which could be further decomposed to the elements at higher temperatures. The first step between 100 and 130 °C is exothermic which makes the materials not applicable for hydrogen storage. The second step between 150 and 180 °C is endothermic. The intermediate LiMgAlHg contains 9.4 wt.% H2 of which 4.8 wt.% are released in the second step. When heating to about 250 °C, another 3.6 wt.% can be released. From the peak areas of DSC measurements dissociation enthalpies of about —15kJ mol for the first decomposition step (exothermic) and 13 kj mol for the second step (endothermic) were calculated [166]. 

The bond dissociation enthalpies A// of the carbon-hydrogen bond in a series of environments is as follows ... 

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