Iodination of alkanes

Chlorination of alkanes is less exothennic than fiuorination, and bromination less exothennic than chlorination. Iodine is unique fflnong the halogens in that its reaction with alkanes is endothennic and alkyl iodides are never prepared by iodination of alkanes. 

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. 

The figures for fluorination reflect the weakness of the F—F [150kJ (36 kcal) mol ],and the strength of the H—F [560 kJ(134 kcal) mol" ], bonds. Fluorination normally requires no specific initiation (cf. p.324), and is explosive unless carried out at high dilution. That fluorination does proceed by a radical pathway, despite not requiring specific initiation, is demonstrated by the fact that chlorination may be initiated in the dark, and at room temperature, by the addition of small traces of F2. Bromination is a good deal slower than chlorination, under comparable conditions, as step (1)—H-abstraction by Br—is commonly endothermic. This step is usually so endothermic for I that direct iodination of alkanes does not normally take place. 

Halogenation of an alkane is required here. Iodination of alkanes, however, is not a feasible reaction. We can make alkyl iodides from alcohols or from alkenes by treatment with HI. A reasonable synthesis using reactions that have been presented to this point proceeds as shown ... 

The effect of temperature is clearly seen in the iodination of alkanes and cycloalkanes with CCI4-AII3-I2, where only monoiodination is observed at — 20°C, but further products are also obtained at 0°C (eq 37).61... 

A new method for homolytic iodination of alkanes involves the homolysis of perfluoroalkyl iodides (for example n-C4F9I in the presence of catalytic amounts of... 

Iodination of alkanes using iodine (I2) is usually an unfavorable reaction. (See Problem 4-17, for example.) Tetraiodomethane (CI4) can be used as the iodine source for iodination, in the presence of a free-radical initiator such as hydrogen peroxide. Propose a mechanism (involving mildly exothermic propagation steps) for the following proposed reaction. Calculate the value of AH for each of the steps in your proposed mechanism. 

Elemental chlorine can be used in free-radical halogenation reactions, too, but these reactions are less easily controlled, because the Cl- radical is more reactive than the Br- radical and hence less selective. The reagents t-BuOCl and SO2CI2 are used as alternative chlorinating agents. The F- radical is so reactive, and the reaction F-F + C-H —> H-F + C-F is so exothermic, that free-radical fluorinations result in violent and uncontrollable exotherms (explosions). At the other extreme, free-radical iodinations of alkanes do not work well at all, as the H- abstraction step is too endothermic. 

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