Exothermic, propagation step

Iodination of alkanes using iodine (I2) is usually an unfavorable reaction. (See Problem 4-17, for example.) Tetraiodomethane (CI4) can be used as the iodine source for iodination, in the presence of a free-radical initiator such as hydrogen peroxide. Propose a mechanism (involving mildly exothermic propagation steps) for the following proposed reaction. Calculate the value of AH for each of the steps in your proposed mechanism. 

Polymerization thermodynamics has been reviewed by Allen and Patrick,323 lvin,JM [vin and Busfield,325 Sawada326 and Busfield/27 In most radical polymerizations, the propagation steps are facile (kp typically > 102 M 1 s l -Section 4.5.2) and highly exothermic. Heats of polymerization (A//,) for addition polymerizations may be measured by analyzing the equilibrium between monomer and polymer or from calorimetric data using standard thermochemical techniques. Data for polymerization of some common monomers are collected in Table 4.10. Entropy of polymerization ( SP) data are more scarce. The scatter in experimental numbers for AHp obtained by different methods appears quite large and direct comparisons are often complicated by effects of the physical state of the monomei-and polymers (i.e whether for solid, liquid or solution, degree of crystallinity of the polymer). 

The energetics of the propagation step are dominated by the fact that it involves a decrease in entropy. Therefore, propagation can only take place if it is exothermic and if no other... 

Reactions (Rl) and (R12) are the two most important elementary reactions in combustion. H + O2 is the essential chain-branching reaction, while CO + OH is a chain-propagating step that regenerates the H atom from OH. Furthermore the CO + OH reaction is highly exothermic and responsible for a large fraction of the heat release that occurs in combustion of hydrocarbon fuels. Under moist conditions, reactions of CO with O and O2 are not competitive, but (RIO) may serve as an initiation step. 

The two chain-propagating steps, taken together, are exothermic by 16 kcal and have a fairly reasonable energy balance between the separate steps. The reaction chains apparently are rather long, because the addition is strongly inhibited by radical traps and only traces of peroxide catalyst are needed. 

If we consider as an example the addition of HC1 to ethylene, we find that whereas the propagation step for polymerization will be exothermic by about 30 kcal mole-1,146 abstraction of H from HC1 by the R—CH2- radical will be endothermic by 5 kcal mole-1. Activation energies for typical polymerization propagation steps are in the range of 6-10 kcal mole-1,147 and that for abstraction from HC1 will have to be greater than the 5 kcal mole-1 endothermicity. These data are at least indicative that radical addition of HC1 will not be favorable experimentally, it is indeed rare, but can be made to occur with excess HC1.148 With HBr the situation is different. Now the hydrogen abstraction is exothermic by about 10 kcal mole-1 and occurs to the exclusion of telomeriza-tion.149 Hydrogen iodide does not add successfully to olefins because now the initial addition of the iodine atom to the double bond is endothermic. 

A very important characteristic of polymerization reactors is their thermal stability as discussed by Sebastian (6). Chain addition polymerizations are thermally simple reactions, in that the polymerization exotherm is attributable almost in its entirety to the chain propagation step. For chain addition polymerization reactors the rate of reaction r is proportional to the product of the square root of initiator concentration, cu and to monomer concentration, cm... 

The first propagation step of chlorination is exothermic for all alkanes except methane. For methane it is slightly endothermic, about +4 kj/mol (+1 kcal/mol). 

The first propagation step is endothermic for bromination but exothermic for chlorination. 

The regenerated bromine radical reacts with another molecule of the alkene, continuing the chain reaction. Both of the propagation steps are moderately exothermic, allowing them to proceed faster than the termination steps. Note that each propagation step starts with one free radical and ends with another free radical. The number of free radicals is constant, until the reactants are consumed, and free radicals come together and terminate the chain reaction. 

As explained earlier in this chapter (Section IIA), the propagation step for the homopolymerisation of tetrafluoroethene is approximately 71kJmol more exothermic than for ethene [2], in spite of adverse polar effects of fluorine substitution on the addition step. It is therefore easy to obtain telomers with tetrafluoroethene and this is also the case with, for example, trifluoroethene, chlorotrifluoroethene and 1,1-difluoroethene. 

If one propagation step is endothermic, its standard-enthalpy variation must be smaller than that of the exothermic step. 

If both propagation steps are highly exothermic, both are irreversible. 

If one propagation step is endothermic and the other is exothermic, the endothermic step may be reversible, the exothermic step is not. 

Lastly, the termination mechanism can be conjectured on the basis of thermochemical data. The propagation step with the larger drop in free energy, or the exothermic step if one is endothermic, is apt to be "faster. " The chain carrier consumed by this step is depleted while the other accumulates. Since termination normally is controlled by coupling of the more plentiful chain carrier ... 

Both propagation steps for the addition of HBr are exothermic, so propagation is exothermic (energetically favorable) overall. For the addition of HCl or HI, however, one of the chain-propagating steps is quite endothermic, and thus too difficult to be part of a repeating chain mechanism. Thus, HBr adds to alkenes under radical conditions, but HCl and HI do not. 

Following an initiating process consisting of reactions (xlii) and (iii), the chain propagating step (xliii) seems the natural one leading to the major observed products. It is exothermic 61.6... 

An electron transfer process is also involved in the second propagation step of a cation radical chain reaction, i.e., the step in which the product cation radical is neutralized by electron transfer from the neutral substrate molecule. For an efficient chain reaction, this step should, of course, be exothermic. If the assumption is made... 

Initiation has been discussed in the previous section. In order for propagation to occur, many product molecules need to be formed from just one radical produced in the initiation step, and this occurs only if the propagation steps are exothermic. Termination steps include coupling, disproportionation, and abstraction. Abstraction by a chain-transfer agent removes a radical from a propagation step, and a new radical is generated in its place. If this newly generated radical is sufficiently stable, the chain transfer results in termination. 

Is there another possible reaction pathway Could the chlorine radical be the chain-carrying radical If so, equation 3, an exothermic reaction, could represent one of the propagation steps. 

Consider the reactions shown in Example 5.2. If both propagation steps are exothermic, the chain length will be long enough that the thermochemistry of the initiation step(s) is unimportant relative to that of the propagation steps. 

We will calculate the enthalpy change for the reactions in the propagation steps. In equation 1, the C—H bond broken has a bond dissociation energy (BDE) of 91.0 kcal/mol, and the O—H bond formed has a BDE of 103 kcal/mol. Therefore, this first reaction is exothermic by 91 - 103 or -12 kcal/mol. 

Most chain-propagating steps are exothermic and one can use the strength of bonds that are broken and formed as a rough guide to the rate of the reaction (thermodynamic parameter). 

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