Radicals methane chlorination

Termination steps are m general less likely to occur than the propagation steps Each of the termination steps requires two free radicals to encounter each other m a medium that contains far greater quantities of other materials (methane and chlorine mol ecules) with which they can react Although some chloromethane undoubtedly arises via direct combination of methyl radicals with chlorine atoms most of it is formed by the propagation sequence shown m Figure 4 21... 

Chemical initiation generates organic radicals, usually by decomposition of a2o (11) or peroxide compounds (12), to form radicals which then react with chlorine to initiate the radical-chain chlorination reaction (see Initiators). Chlorination of methane yields all four possible chlorinated derivatives methyl chloride, methylene chloride, chloroform, and carbon tetrachloride (13). The reaction proceeds by a radical-chain mechanism, as shown in equations 1 through. Chain initiation... 

Like many radical reactions in the laboratory, methane chlorination requires three kinds of steps initiation, propagation, and termination. 

Reactions 1 and 2 involve dissociation of chlorine into chlorine atoms and the breaking of a C-H bond of methane to give a methyl radical and a hydrogen atom. The methyl radical, like chlorine and hydrogen atoms, has one electron not involved in bonding. Atoms and radicals usually are highly reactive, so... 

Chain reactions may be considered to involve three phases. First, chain initiation must occur, which for methane chlorination is activation and conversion of chlorine molecules to chlorine atoms by light. Second, chain-propagation steps convert reactants to products with no net consumption of atoms or radicals. The propagation reactions occur in competition with chainterminating steps, which result in destruction of atoms or radicals. Putting everything together, we can write ... 

In addition to forming chloromethane, the second propagation step produces another chlorine radical. The chlorine radical can react with another molecule of methane, giving HC1 and a methyl radical, which reacts with Cl2 to give chloromethane and regenerate yet another chlorine radical. In this way, the chain reaction continues until the supply of the reactants is exhausted or some other reaction consumes the radical intermediates. The chain reaction explains why many molecules of methyl chloride and HC1 are formed by each photon of light that is absorbed. We can summarize the reaction mechanism as follows. 

Let s look at a speciBc example of a radical reaction—the chtonnatlon of metl>ane—to see its characteristics. A more detailed discussion of this radical substitution reaction is given in Chapter 10. For the present, it s only nec issar> to know that methane chlorination is a multistep process. 

There are one or more chain-propagating steps, each of which consumes a reactive particle and generates another here they are the reaction of chlorine atoms with methane (step 2), and of methyl radicals with chlorine (step 3). 

Reaction pathways represent the routes by which the chemical species are formed and consumed. For example, in the methane chlorination above, the four reactions indicate the pathways by whieh the various species are generated or consumed. In practiee, each of these reactions may involve the formation and destruction of intermediates and unstable speeies (e.g., free radicals). The rates of the reaction pathways should be known in order to determine the rate of formation (or depletion) of the chemieal species in the reactor. 

All four chlorinated methanes are produced using methane chlorination or catalytic oxychlorination of methane. The methane chlorination is initiated thermally or photochemical ly. The strongly exothermic free-radical reaction is conducted without external heat at 400-450°C at slightly raised pressure [158],... 

Note that the reverse reactions to those shown in Scheme 2.1.1 play a very important role for the formation of new covalent bonds. In addition, radicals or charged species can attack neutral compounds to form different radicals and charged species involving new chemical bonds. Scheme 2.1.2 gives examples of some practical relevance in chemical technology. In transformation (a), a methyl radical attacks a chlorine molecule to form chloromethane and a chlorine radical. This reaction is one of the key steps in technical methane chlorination. In transformation (b), an isopropyl carbocation attacks water to form isopropanol with the release of a proton, the key mechanism in the technical production of isopropanol and all higher secondary and tertiary alcohols. In transformation (c), an anionic methanolate ion acts as starter for an anionic polymerization reaction - one possible starting step in technical anionic polymerization. 

Scheme 2.1.2 Reactivity of radicals, carbocations, and carbanions exemplified for reactions of relevance in chemical technology reaction of (a) the methyl radical with chlorine key-step in methane chlorination) (b) the isopropylium ion with water (key-step in isopropanol synthesis from propene) ...

This allylic bromination with NBS is analogous to the methane chlorination reaction discussed in Section 6.3 and occurs hy a similar radical chain reaction mechanism. As in methane halogenation, Br- radical abstracts an allylic hydrogen atom of the alkene, thereby forming an allylic radical plus HBr. This allylic radical then reacts with Br2 to yield the product and a Br-radical, which cycles hack into the first step and carries on the chain. The Br2 results from reaction of NBS with the HBr formed in the first step. 

Figure 3-6 Potential-energy diagram of the reaction of methane with a chlorine atom. Partial bonds in the transition state are depicted by dotted lines. This process, propagation step 1 in the radical chain chlorination of methane, is slightly endothermic.

Interpretation. Radicals and free atoms are capable of undergoing direct covalent bonding with one another. In the methane chlorination process, three such combination processes are possible, one of which—the reaction of two methyl groups—furnishes ethane. The concentrations of radicals and free atoms in the reaction mixture are very low, however, and hence the chance of one radical or free atom finding another is small. Snch combinations are therefore relatively infrequent. When such an event does take place, the propagation of the chains giving rise to the radicals or atoms is terminated. We thns describe these combination processes as termination steps. 

Like carbocations most free radicals are exceedingly reactive species—too reac tive to be isolated but capable of being formed as transient intermediates m chemical reactions Methyl radical as we shall see m the following section is an intermediate m the chlorination of methane... 

FIGURE 4 21 The initiation and propagation steps in the free radical mechanism for the chlorination of methane Together the two propaga tion steps give the overall equation for the reaction... 

Chlorine atom Methane Hydrogen chloride Methyl radical... 

Chlorination of methane and halogenation of alkanes generally proceed by way of free radical intermediates Alkyl radicals are neutral and have an unpaired electron on carbon... 

Chlorine atoms obtained from the dissociation of chlorine molecules by thermal, photochemical, or chemically initiated processes react with a methane molecule to form hydrogen chloride and a methyl-free radical. The methyl radical reacts with an undissociated chlorine molecule to give methyl chloride and a new chlorine radical necessary to continue the reaction. Other more highly chlorinated products are formed in a similar manner. Chain terrnination may proceed by way of several of the examples cited in equations 6, 7, and 8. The initial radical-producing catalytic process is inhibited by oxygen to an extent that only a few ppm of oxygen can drastically decrease the reaction rate. In some commercial processes, small amounts of air are dehberately added to inhibit chlorination beyond the monochloro stage. 

Chlorination of Methane. Methane can be chlorinated thermally, photochemicaHy, or catalyticaHy. Thermal chlorination, the most difficult method, may be carried out in the absence of light or catalysts. It is a free-radical chain reaction limited by the presence of oxygen and other free-radical inhibitors. The first step in the reaction is the thermal dissociation of the chlorine molecules for which the activation energy is about 84 kj/mol (20 kcal/mol), which is 33 kJ (8 kcal) higher than for catalytic chlorination. This dissociation occurs sufficiendy rapidly in the 400 to 500°C temperature range. The chlorine atoms react with methane to form hydrogen chloride and a methyl radical. The methyl radical in turn reacts with a chlorine molecule to form methyl chloride and another chlorine atom that can continue the reaction. The methane raw material may be natural gas, coke oven gas, or gas from petroleum refining. 

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

Bonds may also be broken symmetrically such that each atom retains one electron of the pair that formed the covalent bond. This odd electron is not paired like all the other electrons of the atom, i.e. it does not have a partner of opposite spin. Atoms possessing odd unpaired electrons are termed free radicals and are indicated by a dot alongside the atomic or molecular structure. The chlorination of methane (see later) to produce methyl chloride (CH3CI) is a typical free-radical reaction ... 

---