Dissociation energy of bond

The average dissociation energy of bonds forming the structure of a macromolecule appears thus to be a first criterion for estimating the thermal stability of a given polymer. The fraction of bonds that reaches the energy equal to dissociation energy D is determined by the Boltzman s factor... 

Table 1 Dissociation energies of bonds A-B in kj/mol that may form the polymer structure... 

Possible pathways of the degradation reaction may be visualized for a linear hydrocarbon chain in which the reaction centre ( ) is formed by the effect of initiation (heat, light, oxygen, shear stress, etc.), see Scheme la. A complementary reaction site is denoted as (-). For example, when ( ) is a free radical site, (-) is also a free radical site, if ( ) is a cation, then (-) is an anion, etc. The three stages of the reaction depicted in Scheme la, are initiation, propagation and termination, respectively. The dissociation energies of bonds situated in a /(-position to the reaction site ( ) are considerably lower than those... 

Fig. 6.4. Thermochemical description of Stevenson s rule Dab homol5dic bond dissociation energy of bond A-B, IE ionization energy.

The dissociation energy of bonds present in a polymer provides a guide to its thermal stability (Table 6.3). The fraction of bonds with sufficient energy to dissociate can be calculated from the Boltzmann factor ... 

The effect on dissociation energies of bonding chlorine atoms to carbon, as compared wdth hydrogen atoms can be seen in the following set of dissociation energies ... 

Tkble I. Dissociation Energies of Bonds Typically Present in Polymers... 

TABLE 3.2. Dissociation Energies of Bonds in Gas Phase Molecules... 

The heats of formation of the gaseous atoms, 4, are not very different clearly, it is the change in the bond dissociation energy of HX, which falls steadily from HF to HI, which is mainly res ponsible for the changes in the heats of formation. 6. We shall see later that it is the very high H—F bond energy and thus the less easy dissoeiation of H—F into ions in water which makes HF in water a weak aeid in comparison to other hydrogen halides. 

The bond dissociation energy of the hydrogen-fluorine bond in HF is so great that the above equilibrium lies to the left and hydrogen fluoride is a weak acid in dilute aqueous solution. In more concentrated solution, however, a second equilibrium reaction becomes important with the fluoride ion forming the complex ion HFJ. The relevant equilibria are ... 

A more useful quantity for comparison with experiment is the heat of formation, which is defined as the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states. The heat of formation can thus be calculated by subtracting the heats of atomisation of the elements and the atomic ionisation energies from the total energy. Unfortunately, ab initio calculations that do not include electron correlation (which we will discuss in Chapter 3) provide uniformly poor estimates of heats of formation w ith errors in bond dissociation energies of 25-40 kcal/mol, even at the Hartree-Fock limit for diatomic molecules. 

As the table indicates C—H bond dissociation energies m alkanes are approxi mately 375 to 435 kJ/mol (90-105 kcal/mol) Homolysis of the H—CH3 bond m methane gives methyl radical and requires 435 kJ/mol (104 kcal/mol) The dissociation energy of the H—CH2CH3 bond m ethane which gives a primary radical is somewhat less (410 kJ/mol or 98 kcal/mol) and is consistent with the notion that ethyl radical (primary) is more stable than methyl... 

The dissociation energy of the terminal C—H bond m propane is exactly the same as that of ethane The resulting free radical is primary (RCH2) m both cases... 

Bond Dissociation Energies of Some Representative Compounds ... 

Similarly by comparing the bond dissociation energies of the two different types of C—H bonds m 2 methylpropane we see that a tertiary radical is 30 kJ/mol (7 kcal/ mol) more stable than a primary radical... 

The bond dissociation energy of the other reactant methane is much higher It IS 435 kj/mol (104 kcal/mol)... 

The same conclusion is reached using bond dis sociation energies The following equation shows the bond dissociation energies of the reactants and prod ucts taken from Table 4 3... 

The degree to which allylic radicals are stabilized by delocalization of the unpaired electron causes reactions that generate them to proceed more readily than those that give simple alkyl radicals Compare for example the bond dissociation energies of the pri mary C—H bonds of propane and propene... 

The benzyhc position m alkylbenzenes is analogous to the allyhc position m alkenes Thus a benzyhc C—H bond like an allyhc one is weaker than a C—H bond of an alkane as the bond dissociation energies of toluene propene and 2 methylpropane attest... 

Secondary bonds are considerably weaker than the primary covalent bonds. When a linear or branched polymer is heated, the dissociation energies of the secondary bonds are exceeded long before the primary covalent bonds are broken, freeing up the individual chains to flow under stress. When the material is cooled, the secondary bonds reform. Thus, linear and branched polymers are generally thermoplastic. On the other hand, cross-links contain primary covalent bonds like those that bond the atoms in the main chains. When a cross-linked polymer is heated sufficiently, these primary covalent bonds fail randomly, and the material degrades. Therefore, cross-linked polymers are thermosets. There are a few exceptions such as cellulose and polyacrylonitrile. Though linear, these polymers are not thermoplastic because the extensive secondary bonds make up for in quantity what they lack in quahty. 

Resonance theory can also account for the stability of the allyl radical. For example, to form an ethylene radical from ethylene requites a bond dissociation energy of 410 kj/mol (98 kcal/mol), whereas the bond dissociation energy to form an allyl radical from propylene requites 368 kj/mol (88 kcal/mol). This difference results entirely from resonance stabilization. The electron spin resonance spectmm of the allyl radical shows three, not four, types of hydrogen signals. The infrared spectmm shows one type, not two, of carbon—carbon bonds. These data imply the existence, at least on the time scale probed, of a symmetric molecule. The two equivalent resonance stmctures for the allyl radical are as follows ... 

Although the mean dissociation energy of tin—carbon bonds is less than that normally associated with carbon—carbon bonds (Dgn-c = 188 230 kJ/mol (45—55 kcal/mol), = 335 — 380 kJ/mol (80—90 kcal/mol) (75), the difference is not great enough to render the... 

Table 1. Dissociation Energies of Carbon—Hydrogen Bonds ...

Figure 7.8. Relationship of energy absorbed by molecule to wavelength of incident light. Dissociation energies of various bonds are indicated to show wavelength below which breakdown may occur.

Somewhat surprisingly perhaps, it has been found that [l.l.l]propellane is considerably less reactive than [2.2.1]propellane. Use the theoretically calculated enthalpy data below to estimate the bond dissociation energy of the central bond in each of the three propellanes shown. How might this explain the relative reactivity of the [1-1.1]- and [2.2. Ijpropellanes ... 

Assume that the bond dissociation energy of the bridgehead hydrogens in each bicycloalkane is 104kcal/mol. Indicate and discuss any other assumptions you have made. 

The radical stabilization provided by various functional groups results in reduced bond dissociation energies for bonds to the stabilized radical center. Some bond dissociation energy values are given in Table 12.6. As an example of the effect of substituents on bond dissociation energies, it can be seen that the primary C—H bonds in acetonitrile (86 kcal/mol) and acetone (92kcal/mol) are significantly weaker than a primaiy C—H... 

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