Chlorination of propane

Chlorination of propane with chlorine at 480-640°C yields a mixture of perchloroethylene (Perchlor) and carhon tetrachloride  

Carhon tetrachloride is usually recycled to produce more perchloroethylene 2CCI4 CCl2=CCl2 + 2CI2 

Perchlor may also he produced from ethylene dichloride (1,2-dichloroethane) through an oxychlorination-oxyhydrochlorination process. Trichloroethylene (trichlor) is coproduced (Chapter 7). 

Perchlor and trichlor are used as metal degreasing agents and as solvents in dry cleaning. Perchlor is also used as a cleaning and drying agent for electronic equipment and as a fumigant. 

---

Perchloro- and trichloroethylene may also be produced from chlorination of propane (Chapter 6). 

However, it is instructive to consider radical chlorination of alkanes just a little further, to appreciate the mechanistic concepts. If we carry out light-induced chlorination of propane, then we obtain... 

Assuming the relative rate of secondary to primary hydrogen atom abstraction to be the same in the chlorination of propane as it is in that of butane, calculate the relative amounts of propyl chloride and isopropyl chloride obtained in the free-radical chlorination of propane. 

Explain why the product distribution in the chlorination of propane by sulfuryl chloride is expected to differ according to whether the hydrogen-abstraction step is accomplished by Cl- or -S02CI. 

Propane has six primary hydrogens and two secondary. In the chlorination of propane, the relative proportions of hydrogen atom removal are given by the product of the statistical distribution and the relative rate per hydrogen. Given that a secondary hydrogen is abstracted 3.9 times faster than a primary one, we write the expression for the amount of chlorination at the primary relative to that at the secondary position as ... 

A series of reactions then occurs that involves a chain-reaction. Consider the chlorination of propane, for example. A Cl- atom can attack, the CH3 group at one end of the molecule. 

In methane, all four hydrogen atoms are identical, and it does not matter which hydrogen is replaced. In the higher alkanes, replacement of different hydrogen atoms may lead to different products. In the chlorination of propane, for example, two monochlorinated (just one chlorine atom) products are possible. One has the chlorine atom on a primary carbon atom, and the other has the chlorine atom on the secondary carbon atom. 

The mechanism for free-radical chlorination of propane. The first propagation step forms either a primary radical or a secondary radical. This radical determines whether the final product will be the primary alkyl chloride or the secondary alkyl chloride. 

What would be the product ratio in the chlorination of propane if all the hydrogens were abstracted at equal rates ... 

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... 

Reaction-energy diagram for the first propagation step in the chlorination of propane. Formation of the secondary radical has a lower activation energy than does formation of the primary radical. 

Figure 4-11 summarizes the energy diagrams for the first propagation steps in the bromination and chlorination of propane. Together, these energy diagrams explain the enhanced selectivity observed in bromination. 

The energy diagrams for bromination and chlorination of propane, (a) In the endothermic bromination, the transition states are closer to the products (the radicals) in energy and in structure. The difference in the 1° and 2° activation energies is about 9 kJ (2.2 kcal), nearly the entire energy difference of the radicals. 

Although we discussed its mechanism at length in Section 4-3, free-radical halogenation is rarely an effective method for the synthesis of alkyl halides. It usually produces mixtures of products because there are different kinds of hydrogen atoms that can be abstracted. Also, more than one halogen atom may react, giving multiple substitutions. For example, the chlorination of propane can give a messy mixture of products. 

How can we explain the ratios of products that are formed The key is to look at the relative stabilities of the radicals involved in the reaction and the strengths of the bonds that are formed and broken. First, the chlorination of propane. A chlorine radical, produced by photolysis, can abstract either a primary hydrogen atom, from the end of the molecule, or a secondary hydrogen atom, from the middle. For the first process, we have these energy gains and losses. 

As an example let us take chlorination of propane. The relative amounts of /7-propyl chloride and isopropyl chloride obtained depend upon the relative rates at which /i-propyl radicals and isopropyl radicals are formed. If, say, isopropyl radicals are formed faster, then isopropyl chloride will be formed faster, and will make up a larger fraction of the product. As we can sec, /7-propyl radicals are formed by abstraction of primary hydrogens, and isopropyl radicals by abstraction of secondary hydrogens. 

By what two quantitative methods could you show that a product isolated from the chlorination of propane was a monochloro or a dichloro derivative of propane Tell exactly what results you would expect from each of the methods. 

Freund synthesis. Formation of alicyclic hydrocarbons by the action of sodium (Freund) or zinc (Gustavson) on open-chain dihalo compounds 1,3-dichloropropane derived from the chlorination of propane, obtained from natural gas, is cyclized in the Hass process by treating with zinc dust in aqueous alcohol in the presence of sodium iodide as catalyst. 

Chain reactions consist of three kinds of elementary steps initiation (or activation), propagation, and termination. As an illustration, consider the chlorination of propane (PrH). The experimental evidence suggests the following sequence ... 

Example, 2-9 Experimental measurements for the photochlorination of propane at 25°C and 1 atm pressure showed that the rate of consumption of Cb was independent of propane, second order in chlorine, and first order in light intensity. If the controlling termination step is the heterogeneous termination of propvl radicals. sh.ow-that-the description of elementary steps given earlier for the chlorination of propane satisfactorily explains the experimental data. 

The addition of chlorine to the chloroethylenes has been the most carefully studied of the various chlorine addition reactions. The rate coefficients are also calculated here by using an internal standard, the rate of chlorination of propane, and the k s therefore, depend on the validity of that value (log k = 11.01 — 0.98/2.31 T) as measured by Fettis and Knox... 

An example of a vapor-phase complex series reaction is the chlorination of propane, which is discussed in Chap. 6. 

Fig. 6-3. Chlorination of propane, at 300 C, Effect of pressure on relative chlorination rates of primary and secondary hydrogen atoms.

---