Propagation steps chlorination

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... 

Write equations for the initiation and propagation steps for the formation of dichloromethane by free radical chlorination of chloromethane... 

In practice side reactions intervene to reduce the efficiency of the propagation steps The chain sequence is interrupted whenever two odd electron species combine to give an even electron product Reactions of this type are called chain terminating steps Some commonly observed chain terminating steps m the chlorination of methane are shown m the following equations... 

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... 

These two products arise because m one of the propagation steps a chlorine atom may abstract a hydrogen atom from either a methyl or a methylene group of butane... 

Figure 10.1 Mechanism of the radical chlorination of methane. Initiation step Three kinds of steps are required initiation, propagation, and termination. The propagation steps are a repeating cycle, with Cl- a reactant in step 1 and a product in...

Chain reaction (Section 5.3) A reaction that., once initiated, sustains itself in an endlessly repeating cycle of propagation steps. The radical chlorination of alkanes is an example of a chain reaction that is initiated by irradiation with light and then continues in a series of propagation steps. 

Recall from Section 2.9 that most radicals are very reactive. Because one of the products is another radical, this reaction is a propagation step (a step in which one reactive radical intermediate produces another). In a second propagation step, the methyl radical may react with a chlorine molecule ... 

The chlorosulfonation of organic molecules with chlorine and sulfur dioxide is called the Reed reaction In scope and range of products obtained, the reaction is similar to 14-1. The mechanism is also similar, except that there are two additional main propagation steps ... 

Finally, when we are running out of cyclohexane, the process terminates by the interaction of two radical species, e.g. two chlorine atoms, two cyclohexyl radicals, or one of each species. The combination of two chlorine atoms is probably the least likely of the termination steps, since the Cl-Cl bond would be the weakest of those possible, and it was light-induced fission of this bond that started off the radical reaction. Of course, once we have formed cyclohexyl chloride, there is no reason why this should not itself get drawn into the radical propagation steps, resulting in various dichlorocyclohexane products, or indeed polychlorinated compounds. Chlorination of an alkane will give many different products, even when the amount of chlorine used is limited to molar ratios, and in the laboratory it is not going to be a particularly useful process. 

Propagation. In this step, the intermediate reacts with a stable molecule to produce another reactive intermediate and a product molecule. The propagation step yields a new electrophilic species, the methyl radical, which has an unpaired electron. In a second propagation step, the methyl radical abstracts a chlorine atom from a chloromethane molecule, and generates a chlorine radical. 

There is evidence (unusually high amounls of multiply chlorinated products) that under certain conditions in the reaction of RH with Cl2, the products of the second propagation step (RX + X-) are enclosed within a solvent cage. See Skell Baxter J. Am. Chem. Soc. 1985, 107, 2823 Raner Lusztyk Ingold J. Am. Chem. Soc. 1988, 110, 3519 Tanko Anderson J. Am. Chem. Soc. 1988, 110, 3525. 

A free radical chain reaction proceeds through a succession of free radicals. In the photochemical chlorination of an alkane, the initiating step is the homolytic lission of chlorine molecules to produce chloroalkanc molecules and chlorine free radicals. These two reactions constitute the propagating step. However, the chlorine free radicals may also combine to form chlorine molecules or react with the alkane free radicals to form chloroalkane molecules. Both of these reactions constitute terminating steps of the chain reaction. Il should be noted, however, that the foregoing sequence cannot take place in the dark. Exposure to light allows the series of reactions then to proceed rather violently. 

Cl2 can induce the chlorination of methane by the chain-propagating steps ... 

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 ... 

Exercise 4-13 Calculate AH0 for each of the propagation steps of methane chlorination by a mechanism of the type... 

The overall rates of chain reactions usually are slowed very much by substances that can combine with atoms or radicals and convert them into species incapable of participating in the chain-propagation steps. Such substances are called radical traps, or inhibitors. Oxygen acts as an inhibitor in the chlorination of methane by rapidly combining with a methyl radical to form the comparatively stable (less reactive) peroxymethyl radical, CH300-. This effectively terminates the chain ... 

Chlorination with sulfuryl chloride of alkanes with more than one kind of hydrogen gives a mixture of alkyl chlorides resembling that obtained with chlorine itself. However, in some circumstances the mixture of chlorides is not the same mixture obtained with chlorine itself and when this is true, the hydrogen-abstraction step probably involves -SOzCl rather than C1-. The alternative propagation steps then are... 

The foregoing discussion adds further to our understanding of the selectivity observed in the halogenation reactions discussed in Chapter 4. When propene is chlorinated in sunlight, the product is 3-chloropropene, and we may explain this on the basis that the radical-chain reaction involves propagation steps in which a chlorine atom attacks the hydrogen corresponding to the weakest C-H bond ... 

RR SiCr, Na+ (equation (7)). The propagation step (equation (8)), which is rapid but nevertheless rate determining, is the reaction of anion-terminated chains with dichlorosi-lane to add one silicon unit and produce a chlorine-ended chain. The latter is thought to be reduced rapidly to the anionic form by reaction with sodium. The polysilane chains will alternately be terminated with silyl anions or Si-Cl. This is consistent with evidence from chemical derivatization that both may be present in the products of the reaction. ... 

A chlorine atom abstracts a hydrogen atom from chloromethane in the first propagation step. 

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