Other Radical Halogenations of Methane

Write out the overall equation and the propagation steps of the mechanism for the chlorination of chloromethane to furnish dichloromethane, CH2CI2. (Caution Write out each step of the mechanism separately and coirpletely. Be sure to include full Lewis stmctures of all species and all appropriate arrows to show electron movement.) 

Fluorine and bromine, but not iodine, also react with methane by similar radical mechanisms to furnish the corresponding halomethanes. The dissociation energies of X2 (X = F, Br, I) are lower than that of CI2, thus ensuring ready initiation of the radical chain (Table 3-4). 

Fluorine is most reactive, iodine least reactive 

On the other hand, reaction of I- with CH4 has a very high E, [at least as large as its endothermicity, +34 kcal mol (+142 kJ mol ) Table 3-5]. Thus, the transition state is not reached until the H-C bond is nearly completely broken and the H-I bond is almost fully formed. The transition state is said to occur late in the reaction pathway. It is substantially further along the reaction coordinate and is much closer in structure to the products of this process, CH3- and HI. Late transition states are frequently typical of relatively slow, endothermic reactions. Together these rules concerning early and late transition states are known as the Hammond postulate. 

Let us now consider the second propagation step for each halogenation in Table 3-5. This process is exothermic for all the halogens. Again, the reaction is fastest and most exothermic for fluorine. The combined enthalpies of the two steps for the fluorination of methane result in a AH° of -103 kcal mol (-431 kJ mol ). The formation of chloromethane is less exothermic and that of bromomethane even less so. In the latter case, the appreciably endothermic nature of the first step [A//° = +18 kcal mol (+75 kJ mol )] is barely overcome by the enthalpy of the second [A7/° = 24 kcal mol (-100 kJ mol )], resulting in an 

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Other Radical Halogenations of Methane Similarities and differences. 

What happens in the radical halogenation of other alkanes Will the different types of R-H bonds—namely, primary, secondary, and tertiary— react in the same way as those in methane As we saw in Exercise 3-4, the monochlorination of ethane gives chloroethane as the product. 

Halogenation of alkanes had long been known, and in 1930 the kinetics of the chlorination of chloroform to carbon tetrachloride were reported by Schwab and Heyde (equation 40), while the kinetics of the chlorination of methane were described by Pease and Walz in 1931. Both of these studies showed the currently accepted mechanism, which was extended to reactions in solution by Hass et al. in 1936. The free radical halogenation mechanism of other alkanes was described by Kharasch and co-workers, ° including side chain halogenation of toluene. 

In the light of the more complete study of ring-halogenated triphenylchloro-methanes in this paper, the free radical hypothesis was back - if it ever was excluded in the previous paper - in the final discussion of the constitution of triphenylmethyl , now with two tautomeric triphenylmethyl radical structures in equilibrium with each other and the Jacobson dimer 1 (Scheme 2). Note that the radical was symbolized by an open valence (a thick line is used here for clarity). The strong results obtained with 3 (Scheme 1) were explained by removal of the quinoid bromine atom from 4 giving a radical 6 which tautomerized to the triphenylmethyl analogue 7. By analogy with the... 

Because a high yield of one particular compound is usually needed, and the separation of product mixtures is often difficult, radical halogenations are rarely used with substrates other than simple hydrocarbons. This is not to say that this reaction type is unimportant the chlorination of methane is a major industrial process. Chloromethane is not the only product obtainable if the ratio of the reactants and the reaction conditions are varied, dichloromethane (CH2CI2), trichloromethane (chloroform, CHCI3) and tetrachloromethane (carbon tetrachloride, CCI4) can all be produced. 

In the previous section, we explored the proposed mechanism for the chlorination of methane. We will now explore whether this reaction can be accomplished with other halogens as well. Is it possible to achieve a radical fluorination, bromination, or iodination To answer this question, we must explore some thermodynamic aspects of haiogenation. 

According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere, bromotrichloromethane, which has a measured vapor pressure of 39mmHg at 25°C, is expected to exist solely as a vapor in the ambient atmosphere. Based on bromotrichloromethane s structural similarity to bromotrifluoromethane, it is expected to slowly degrade in the atmosphere by reaction with photochemically produced hydroxyl radicals the half-life for bromotrichloromethane s reaction in air is estimated to be greater than 44 years. Photolysis may occur based on bromotrichloromethane s structural similarity to other halogenated methane compounds but not at an environmentally relevant rate. 

We have seen that alkanes undergo chemical transformations when subjected to pyrolysis, and that these processes include the formation of radical intermediates. Do alkanes participate in other reactions In this section, we consider the effect of exposing an alkane (methane) to a halogen (chlorine). A chlorination reaction, in which radicals again play a key role, takes place, producing chloromethane and hydrogen chloride. We shall analyze each step in this transformation to establish the mechanism of the reaction. 

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