Hydrogen iodide, bonds

This equilibrium is established when hydrogen iodide is heated, hydrogen-iodine bonds being broken. 

Cleavage of alkyl aryl ethers by hydrogen halides always proceeds so that the alkyl-oxygen bond is broken and yields an alkyl halide and a phenol as the final prod nets Either hydrogen bromide or hydrogen iodide is normally used... 

Hydrogen iodide is the strongest reducing agent. This trend is expected when the bond enthalpies of the hydrogen halides are considered. 

The opposite of the stabilisation of an ester is its activation. If we include in the concept ester the alkyl halides, their Friedel-Crafts reactions provide familiar examples of this phenomenon. An unusual example especially relevant to our present considerations is provided by some results made available to me in advance of publication by Giusti and Andruzzi. Their results [38] on the polymerisation of styrene by iodine and hydrogen iodide can be interpreted in terms of an organic iodide derived from styrene, probably 1-phenylethyl iodide, being activated by the co-ordination of one or two molecules of iodine. This process appears to polarise the C—I bond to such an extent that the normally stable ester becomes activated to a chain-propagating species and induces a pseudocationic polymerisation ... 

The mechanism is well understood, involving complexation of the rhodium with iodine and carbon monoxide, reaction with methyl iodide (formed from the methanol with hydrogen iodide), insertion of CO in the rhodium-carbon bond, and hydrolysis to give product with regeneration of the complex and more hydrogen iodide. 

Hydrogen iodide adds to double bond of alkenes undergoing Markovnikov addition. The reaction is faster than that with other hydrogen halides ... 

Since the bond dissociation energy of hydrogen iodide is less than that of hydrogen bromide [DH-i, 70.5 kcal. DH-Br, 86.5 kcal. (3)], Reactions 6, 7, and 8 should occur more readily with hydrogen iodide. This would account for the fact that the promoting effect of iodine compounds on ignition is less than that of bromine compounds. 

In all of the molecules discussed above the bond is intermediate between theeovalent extreme M X and the ionic extreme M+X , varying from an essentially covalent bond with only a small amount of ionic character (hydrogen iodide), through a bond with about equal amounts of covalent and ionic character (hydrogen fluoride), to an essentially ionic bond with only a small amount of covalent character (cesium chloride). 

Exercise 10-25 Calculate the AH° values for initiation and chain-propagation steps of radical addition of hydrogen fluoride, hydrogen chloride, and hydrogen iodide to an alkene. Would you expect these reagents to add easily to double bonds by such a mechanism ... 

In general, it is very difficult to break the aromatic C-O bond of arenols. Thus concentrated halogen acids do not convert simple arenols to aryl halides, and alkoxyarenes are cleaved with hydrogen bromide or hydrogen iodide in the manner ArO—R rather than Ar—OR ... 

If we consider as an example the addition of HC1 to ethylene, we find that whereas the propagation step for polymerization will be exothermic by about 30 kcal mole-1,146 abstraction of H from HC1 by the R—CH2- radical will be endothermic by 5 kcal mole-1. Activation energies for typical polymerization propagation steps are in the range of 6-10 kcal mole-1,147 and that for abstraction from HC1 will have to be greater than the 5 kcal mole-1 endothermicity. These data are at least indicative that radical addition of HC1 will not be favorable experimentally, it is indeed rare, but can be made to occur with excess HC1.148 With HBr the situation is different. Now the hydrogen abstraction is exothermic by about 10 kcal mole-1 and occurs to the exclusion of telomeriza-tion.149 Hydrogen iodide does not add successfully to olefins because now the initial addition of the iodine atom to the double bond is endothermic. 

The term iodane refers to hydrogen iodide (HI), a colorless non-flammable gas. According to IUPAC rules, compounds with nonstandard bonding number are shown by the lambda notation thus, H3I is called A3-iodane and H5I A5-iodane. The most common ArIL2 (L heteroatom ligands) with decet structure is named aryl-A3-iodane and ArIL4 with dodecet structure aryl-A5-iodane. 

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