Termination steps chlorination

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... 

Chain termination. The chlorination of alkanes by rm-butyl hypochlorite is believed to follow a chain mechanism, but there is a dispute about the termination step.10 Derive the steady-state rate equation for each, making the long-chain approximation. 

The rate law (5.6) is in agreement with experimental results. At high chlorine pressure, the termination step (iv) may be replaced by following equilibria... 

Irradiation of mixtures of hydrocarbons and chlorine at suitable wavelengths leads to chlorination of the organic molecule (Scheme 1.3). Reactions have overall quantum yields in excess of 106 (>106 propagation cycles for each termination step). 

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. 

Two possible termination steps are shown, one of which produces the same product as the chain reaction. Recombination of two chlorine radicals is feasible, but less likely. Either of the termination products could be suggested as an alternative product but, in practice, what we are going to get is derivatives with more than one chlorine substituent. It is difficult to control a radical process once the chain reaction is under way, so we usually get a mixture of products. Note particularly that further substitution is on the original or alternative... 

The number-average molecular weight of the initially formed polymer is 20,000. A 1.00-g sample of the polymer contains 3.0 x 10-5 moles of OH groups it does not contain chlorine. Show the reaction sequence of initiation, propagation, and termination steps for this polymerization and derive the appropriate expressions for the rate and degree of polymerization. Indicate clearly any assumptions made in the derivations. 

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. 

The reaction of methane, CH4, with chlorine gas occurs by a mechanism similar to that described in Exercise 13.80. Write the mechanism, (a) Identify the initiation, propagation, and termination steps. 

By-products resulting from the chain-termination steps would be butane and any chlorination products derived from it. 

Typical radical reactions are substitution and addition reactions as shown below (Scheme b). A typical substitution reaction is the halogenation of methane with chlorine gas under photolytic conditions, and generally available chlorohydrocarbons are prepared by this method. The chlorination reaction proceeds through a chain pathway via the initiation step, propagation step, and termination step as shown below (Scheme 1.1). 

In this case, the termination steps are much less important than in the last case we looked at, and typically the chain reaction can continue for 106 steps for each initiation event (photolysis of chlorine). Be warned reactions like this can be explosive in sunlight. 

Finally, there are chain-terminating steps, in which reactive particles are consumed but not generated in the chlorination of methane these would involve the union of two of the reactive particles, or the capture of one of them by the walls of the reaction vessel. 

The chlorine atom formed may take part in the other propagation step or attack a CH3CI molecule to eventually form CH2CI2. CHCI3 and CCI4 can also be formed by a continuation of this process. A termination step occurs when two radicals combine to form a nonradical product, as in the reaction Cl + CH3 CH3CI... 

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. 

This is the desired result, giving the rate as second order in chlorine and first order in light intensity. By choosing different termination steps to be controlling, we can obtain different expressions for —d[C 2 ldt, some involving [Pr], [CI2], and [7] to other powers. Comparison of the various results with experimentally determined rates allows us to choose the best form of the rate equation and to evaluate the ratio of rate constants for example, k tp-Jk-, in Eq. (E). From measurements only on stable species, individual values of k cannot, in general, be established. 

Consider that the photochlorination of propane occurs according to the reactions in Sec. 2-11. If the controlling termination step is the heterogeneous termination of chlorine radicals. 

Any two radicals in the reaction mixture can combine to form a molecule in which all the electrons are paired. The combination of two radicals is called a termination step because it helps bring the reaction to an end by decreasing the number of radicals available to propagate the reaction. The radical chlorination of alkanes other than methane follows the same mechanism. A radical chain reaction, with its characteristic initiation, propagation, and termination steps, was first described in Section 4.10. 

Alkanes are called saturated hydrocarbons because they do not contain any double or triple bonds. Since they also have only strong cr bonds and atoms with no partial charges, alkanes are very umeactive. Alkanes do undergo radical substitution reactions with chlorine (Cl 2) or bromine (Br2) at high temperatures or in the presence of light, to form alkyl chlorides or alkyl bromides. The substitution reaction is a radical chain reaction with initiation, propagation, and termination steps. Unwanted radical reactions are prevented by radical inhibitors—compounds that destroy reactive radicals by creating umeactive radicals or compounds with only paired electrons. 

Figure 3.2 (a) Epoxy resin synthesis generalized reaction (b) reaction of hydroxyl group of NaOH with proton (c) displacement of the chlorine atom by the hydroxyl group (d) chain propagation (e) termination step. 

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