Iodine, reaction with methane

For the endothermic reaction of an iodine atom with methane. Fact can be no less than 33 kcal, and is probably somewhat larger. Even for this minimum value of 33 kcal, an iodine atom must collide with an enormous number of methane molecules (lO or a million million at 275°) before reaction is likely to occur. Virtually no iodine atoms last this long, but instead recombine to form iodine molecules the reaction therefore proceeds at a negligible rate. Iodine atoms are easy to form it is their inability to abstract hydrogen from methane that prevents iodination from occurring. 

Bis(iodozincio)methane (3), which possesses two nucleophilic sites on a single carbon, has the possibility to react sequentially with two different electrophiles. It will act as a molecular hinge which connects two electrophiles. It was found that the reactivity of the first C—Zn bond of 3 is much higher than that of the resulting methylzinc in the reaction with water or iodine (equation 27). These results indicate that it is possible to use these two C—Zn bonds individually44. 

The preparation of acetic acid represents a special case. Olah and coworkers as well as Hogeveen and coworkers have demonstrated that CO can react with methane under superacidic conditions, giving the acetyl cation and by subsequent quenching acetic acid or its derivatives (see Section 7.2.3). Monosubstituted methanes, such as methyl alcohol (or dimethyl ether), can be carbonylated to acetic acid.115 Similarly, methyl halides undergo acid-catalyzed carbonylation.115,116 Whereas the acid-catalyzed reactions can be considered as analogs of the Koch reaction, an efficient Rh-catalyzed carbonylation of methyl alcohol in the presence of iodine (thus in situ forming methyl iodide) was developed by Monsanto and became the dominant industrial process (see Section 7.2.4). 

Preparation of Alkyl Halides.—We have spoken of the formation of the alkyl halides by the direct action of the halogen upon the saturated hydrocarbon. In the case of chlorine this action takes place at ordinary temperatures as in the reaction between methane and chlorine in the sunlight. Bromine, however, does not act directly at ordinary temperatures but by heating in a sealed tube. Iodine does not act directly with the hydrocarbons. In any case the result is a mixture of several substitution products, and the method is not, therefore, of practical value. Where direct action does not occur the presence of iodine chloride or antimony chloride, which act as carriers, is necessary. The two reactions of most importance in the preparation of these compounds are those involving either alcohols or unsaturated hydrocarbons. These will be taken up when these compounds are studied. 

Problem 2.2 Calculate for the corresponding steps in the reaction of methane with (a) bromine, (b) iodine, (c) fluorine. 

With this background, let us return to the reaction between methane and the various halogens, and see if we can account for the order of reactivity given before, F2 > CI2 > Br2 > I2, and in particular for the fact that iodine does not react at all. 

Compare the second reaction in each pair methyl radical reacting with chlorine is more exothermic than methyl radical reacting with iodine this does not explain how iodine prevents the chlorination reaction. Compare the first reaction in each pair chlorine atom reacting with iodine is very exothermic whereas chlorine atom reacting with methane is slightly endothermic. Here is the key chlorine atoms will be scavenged by iodine before they have a chance to react with methane. Without chlorine atoms, the reaction comes to a dead stop. 

SULFICYLBIS (METHANE) (67-68-5) CjHjOS (CH3)2S0 Combustible liquid [explosion limits in air (vol %) 2.6 to 63.0 flashpoint 203°F/95°C oc autoignition temp 419°F/215°C Fire Rating 2]. Violent or explosive reaction with strong oxidizers, acryl halides, aryl halides and related compounds, alkali metals p-bromobenzoyl acetanilide, boron compounds, especially hydrides iodine pentafluoride, magnesium perchlorate, methyl bromide, perchloric acid, periodic acid, silver fluoride, sodium hydride, potassium permanganate. Forms powerfully explosive mixtures with metal salts of oxoacids [iron(III)nitrate, phosphonic acid, sodium perchlorate]. On small fires, use dry chemicals or COj extinguishers. 

In oxychlorination, methane is oxidized to methyl chlorides using chlorine. Bromine or iodine can also be used, forming corresponding halides. The typical reaction with chlorine is... 

Although chlorine and bromine react with methane and other saturated hydrocarbons, iodine enters into combination only under exceptional circumstances. Chlorine and bromine react with hydrogen with the evolution of heat, whereas under the same conditions (400 C) hydrogen iodide is formed with the absorption of heat. Since the heat of formation is a measure of the strength of the bond, then, compared with the halogens of lower molecular weight, iodine exhibits a stronger tendency to combine only loosely and to enter into reversible reactions. 

---