Iodination, of methane

A positive value for AH° signifies an endothermic reaction. The reactants are more stable than the products, and so iodination of alkanes is not a feasible reaction. You would not want to attempt the preparation of iodomethane by iodination of methane. 

Investigate the energies (AH0) of possible chain mechanisms for the light-induced monobromination of methane and compare with those for chlorination. What are the prospects for iodination of methane ... 

If the mechanisms for fluorination, bromination, and iodination of methane are the same as for its chlorination, we can explain the wide variation in reactivity of the halogens by a careful examination of A//° and act for each step. 

The thermodynamic quantities for iodination of methane make it clear that the chain-initiating step is not responsible for the observed order of reactivities F2 > CI2 > Bt2 > I2. The iodine-iodine bond is even weaker than the fluorine-fluorine bond. On this basis alone, one would predict iodine to be the most reactive of the halogens. This clearly is not the case. Once again, it is the hydrogen atom-abstraction step that correlates with the experimentally determined order of reactivities. The energy of activation of this step in the iodine reaction (140 kJ moF is so large that only two collisions out of every have sufficient energy to produce reactions at 300°C. As a result, iodination is not a feasible reaction experimentally. 

Free radical fluorination of methane is uncontrollably violent and free radical iodination of methane is a very poor reaction. Explain these observations in light of bond energies. 

Don t forget The Br that forms is a neutral atom, not a negatively charged bromide ion 3. Write any termination step for iodination of methane. 

A number of dihydroquinolines have been prepared by treating aniline derivatives with acetone or mesityl oxide in the presence of iodine. In these cases aromatization to the fully unsaturated quinoline would require the loss of methane, a process known as the Riehm quinoline synthesis. Such Skraup/Doebner-von Miller-type reactions are often low yielding due to large amounts of competing polymerization. For example, aniline 37 reacts with mesityl oxide to give dihydroquinolines 39, albeit in low yield. ... 

In another set of experiments the effect of prior dehydrogenation on the yields of gases (especially that of methane) was studied. As shown earlier, dehydrogenation with either sulfur (17, 18) or iodine (22) leads to the complete inhibition of tar formation and fixation of the corresponding carbon (alicyclic) in char. It is thus possible to study the contribution, if any, of the hydroaromatic structure towards gas formation by partial or complete fixation of the hydroaromatic carbon in char. 

The thermal reaction, catalysed by Cu(acac)2, of thiobenzophenones with ylides coming from bis arylsulphonyl methane is also likely to proceed by an initial transylidation the main products are here benzo[c]thiophenes [35,36], The car-banionic carbon of iodonium ylides is devoid of nucleophilic character, yet PhI=C(S02Ph)2 gave, with iodomethane, the methylated iododisulphone MeC(I)(S02Ph)2 (68%). This reaction, performed at room temperature without any catalyst, is probably the result of a nucleophilic attack from iodine of iodomethane to iodine of the ylide [37]. 

When a small amount of iodine is added to a mixture of chlorine and methane, it prevents chlorination from occurring. Therefore, iodine is a free-radical inhibitor for this reaction. Calculate A H° values for the possible reactions of iodine with species present in the chlorination of methane, and use these values to explain why iodine inhibits the reaction. (The I—Cl bond-dissociation enthalpy is 211 kJ/mol or 50 kcal/mol.)... 

Chlorination of methane to methyl chloride, methylene chloride, chloroform, and carbon tetrachloride is practiced industrially worldwide on a large scale. Bromination and iodi-nation are similarly practiced on a smaller scale to prepare the corresponding bromine and iodine derivatives which are important pharmaceutical, synthetic, and fireproofing raw materials. 

In a similar experiment, a mixture of 6.0 cc. (CH3)3A1 and excess HNF2 were allowed to stand for 1 hour at —80° C., then overnight at 25° C. 12.7 cc. HNF2 were consumed, yielding 12.9 cc. methane. The hydrolysis of the nonvolatile products of these reactions in neutral or acidic iodide solution liberated only traces of iodine but was accompanied by evolution of methane. 

The condensation of two molecules of primary selenoamides proceeds with bromine under extrusion of selenium to give selenadiazoles [93]. The reaction of selenocarbonates, selenothiocarbonates [94], and selenourea [95] with bromine and iodine has been widely tested. The products depend on the amount of halogen used. For example, in the reaction of N-methylthiazoline-2(3H)-selone, the use of one equivalent of bromine gives hypervalent lO-Se-3 complexes 44, whereas two equivalents of bromine cleave the carbon selenium double bond to give product 45. A similar hypervalent compound is formed from 4-imidazolin-2-selone,but the iodination of bis(imidazolin-2-selone)methane gives iodinated product 46. The availability of some of halogen adducts has been tested as a conducting material. 

The relatively large quantity of methane formed in the gas phase photolysis of methyl iodide is hard to reconcile with step (3) for which the activation energy would be about 9 kcal.mole . Schultz and Taylor showed that methane formation is independent of temperature over the range 40-100 °C and is also reasonably independent of iodine concentration. They therefore proposed that methane formation proceeds via a hot radical mechanism as follows... 

Primary process IV. (/) n = (uninhibited) = ch3i (inhibited). The ways in which the primary quantum yields can be evaluated are given below. 

There is ample experimental evidence also against the occurrence of primary process IV. In the presence of iodine, only traces of methane could be detected at 3130 and 2654 A the small amounts observed at 2537 A, in the temperature range 60-140 °C, support the participation of hot radicals rather than the direct formation of methane in the primary step. There is, however, definite evidence for the occurrence of reaction IV at 1470 A , and one may expect some contribution of this primary step at wavelengths shorter than about 2000 A. Methane formation was observed at high intensities Slagg and Marcus demonstrated that under such circumstances the CH4 formation is, in all likelihood, the result of hot methyl radical reactions. 

It has already been mentioned that free radicals are produced almost exclusively in the primary process. This is supported by the observation that the amounts of methane and ethane, formed in the experiments made in the presence of iodine or nitric oxide are the same within the limits of experimental error. However, the fact that a small amount of acetone was also formed in the presence of iodine led Bell and Blacet to assume the occurrence of primary process... 

This is the only tetra-halogen substitution product of methane which will be mentioned. It is produced when methane is chlorinated to its limit. It may also be made by further chlorination of chloroform. The reaction by which it is made industrially is, however, entirely different. It consists in chlorinating carbon di-sulphide in the presence of a carrier such as iodine. In this reaction, which probably takes place in several steps, the two sulphur atoms, in the carbon di-sulphide, are replaced by four chlorine atoms. 

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

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. 

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