Iodine, reaction with alkanes

Chlorination of alkanes is less exothermic than fluonnation and bromination less exothermic than chlorination Iodine is unique among the halogens m that its reaction with alkanes is endothermic and alkyl iodides are never prepared by lodmation of alkanes... 

Halogenation of alkanes is common with Br2 and Clj. Fluorine, Fj, is seldom used because its reactions with alkanes are so exothermic that they are difficult to control and can actually cause C—C bond cleavage and even explosions. Iodine, Ij, is seldom used because the reaction is endothermic and the position of equilibrium favors alkane and I2 rather than iodoalkane and HI. 

Heat or light is usually needed to initiate this halogenation. Reactions of alkanes with chlorine and bromine proceed at moderate rates and are easily controlled. Reactions with fluorine are often too fast to control, however. Iodine reacts very slowly or not at all. We will discuss the halogenation of alkanes in Chapter 4. 

In this reaction, X can be fluorine, chlorine, or bromine, but not iodine. Iodine does not react well with alkanes. A common use of a halogented hydrocarbon is shown in Figure 23-3. 

Replacement of iodine by bromine in this reaction leads to a mixture of cis-and fran -1-alkenyl bromides and the corresponding n-alkanal. After some experimentation, Hamaoka and Brown found that alkenylboronic acids can be converted into a-bromo dimethyl acetals by reaction with bromine and sodium methoxide in methanol at -78° ... 

Iodine in combination with [bis(acyloxy)iodo]arenes is a classical reagent combination for the oxidative iodination of aromatic and heteroaromatic compounds [99], A typical iodination procedure involves the treatment of electron-rich arenes with the PhI(OAc)2-iodine system in a mixture of acetic acid and acetic anhydride in the presence of catalytic amounts of concentrated sulfuric acid at room temperature for 15 min [100,101]. A solvent-free, solid state oxidative halogenation of arenes using PhI(OAc)2 as the oxidant has been reported [102]. Alkanes can be directly iodinated by the reaction with the PhI(OAc)2-iodine system in the presence of f-butanol under photochemical or thermal conditions [103]. Several other iodine(in) oxidants, including recyclable hypervalent iodine reagents (Chapter 5), have been used as reagents for oxidative iodination of arenes [104-107]. For example, a mixture of iodine and [bis(trifluoroacetoxy)iodo]benzene in acetonitrile or methanol iodinates the aromatic ring of methoxy substituted alkyl aryl ketones to afford the products of electrophilic mono-iodination in 68-86% yield [107]. 

Only fluorine, chlorine, and bromine react this way with alkanes. Iodine is essentially unreactive due to unfavorable reaction energetics. 

In these reactions a halogen atom replaces one or more of the hydrogen atoms of the alkane and the corresponding hydrogen halide is formed as a by-product. Only fluorine, chlorine, and bromine react this way with alkanes. Iodine is essentially unreactive, for a reason that we shall explain later. 

---