Organic reactions bond dissociation energy

Thomson --TV Click Organic Interactive to use bond dissociation energies to predict organic reactions and radical stability. 

Three- and pentacoordinate organic phosphorus compounds can be oxidized through a free radical Arbuzov reaction, i.e., formation and p-scission of a phosphoranyl radical (Scheme 24). The P-scission is regioselective homolysis occurs on a ligand located in an equatorial site. Both a- and P-scissions are strongly dependent on the strength (bond dissociation energy) of the cleaved... 

Although we will deal with organic radicals in solution, it is worth mentioning that the reactivity of atoms and small organic radicals with silanes in the gas phase has been studied extensively. For example, the bond dissociation energies of a variety of Si-H bonds are based on the reaction of iodine or bromine with the corresponding silanes.1... 

This type of organic reaction is important and deserves some comments. The addition of one electron to a molecnle generates an anion-radical. This results in an increase in its reactivity. Particularly, the bond dissociation energies in anion-radicals are much smaller than those in the corresponding nentral molecnles. Thns, snbstitntion reactions proceed more easily in the case of anion-radicals. The reactions of the type are good examples of this feature. Two possible schemes (a andb) for the reaction conrse are listed as follows ... 

A relative scale of PA(B) has been established by examining the reaction shown in equation 23 for a large number of organic and inorganic bases. These data can be calibrated by reference to a variety of species for which absolute values of PA(B) may be derived from appearance potential measurements. The molecular ionization potentials IP(B) and the homo lytic bond dissociation energies D(B+—H) are linear functions of the proton affinity PA(B) for a homologous series of amines117-121. 

Table V shows the efficient organization of this reaction chemistry into five reaction families. Bond fission, for example, is the elementary step that creates two free radicals from a parent molecule. In chain processes this will often be the initiation step. Thermochemical estimates often show that the logarithm of the Arrhenius A factor (logioA) is of the order 14-17, whereas the activation energy is essentially equivalent to the bond dissociation energy (19,42). This equality is the result of the essentially unactivated reverse reaction step, radical recombination.

Carbon-centered radicals play an important role in organic synthesis, biological chemistry, and polymer chemistry. The radical chemistry observed in these areas can, to a good part, be rationalized by the thermodynamic stability of the open shell species involved. Challenges associated with the experimental determination of homolytic bond dissociation energies (BDEs) have lead to the widespread use of theoretically calculated values. These can be presented either directly as the enthalpy for the C-H bond dissociation reaction described in Equation 5.1, the gas-phase thermodynamic values at the standard state of 298.15K and 1 bar pressure being the most commonly reported values. 

The breaking of chemical bonds under the influence of heat is the result of overcoming bond dissociation energies. Organic substances such as polymers are highly heat sensitive due to the limited strength of the covalent bonds that make np their structures. Scission can occur either randomly or by a chain-end process, often referred to as an unzipping reaction. 

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