Reactions with hydrogen iodide

The monobenzo compound (107 R1=R2 = H) forms a stable salt with methyl iodide. Reaction with TCNE results in elimination of hydrogen cyanide and formation of the tricyanovinyl derivative (107 R1 = C(CN)=C(CN)2, R2 = H) (67JA2626). 

Example 4.2. The hydrogen-iodide reaction. The reversible formation of hydrogen iodide from hydrogen and iodine in the gas phase, with stoichiometry... 

On the other hand, a power-law rate equation with integer reaction orders cannot be taken as evidence against fast pre-dissociation. An example is the hydrogen-iodide reaction, which involves fast pre-dissociation although the reaction orders are integers (see Example 4.2 in Section 4.2). Here, conversion 21 + H2 — 2HI of the dissociation products is second order in these, so that the square-root factor is squared, making the reaction first order in I2. 

Reduction of aryl iodides has been accomplished by catalytic hydrogenation using hydrogen or hydrogen donors over palladium or Raney nickel catalysts (equation 66), by reduction with complex hydrides (equations 62-64),with metals and their compounds,with hydrogen iodide, and with organic compounds (equations 65 and 67). Table 5 shows reaction conditions for reduction of iodobenzene to benzene. 

This explanation indicates that the action of sodium upon alcohol is really the action of sodium upon water, partially olefinated. The action of ethyl iodide upon sodium ethoxide is not the replacement of the sodium by ethyl so much as the action of olefinated hydrogen iodide upon olefinated sodium hydroxide. It is the same type of reaction as the neutralization of sodium hydroxide by hydriodic acid in aqueous solution. The hydrated hydrogen iodide reacts with hydrated sodium hydroxide. The water bears the same relation to the latter reaction that the olefin does to the former. 

How can we find out whether the proposed mechanism for a particular reaction is correct In the case of hydrogen peroxide decomposition we might try to detect the presence of the IO ions by spectroscopic means. Evidence of their presence would support the reaction scheme. Similarly, for the hydrogen iodide reaction, detection of iodine atoms would lend support to the two-step mechanism. For example, I2 dissociates into atoms when it is irradiated with visible light Thus, we might predict that the formation of HI from H2 and I2 would speed up as the intensity of light is increased because that should increase the concentration of I atoms. Indeed, this is just what is observed. 

Species releasing hydrogen atoms upon irradiation are also used as sources of atomic hydrogen. For instance, hydrogen iodide irradiated with UV light (X<3 100 A) decomposes as HI + hv -> H + I. The iodine atoms that are considerably less active than hydrogen atoms only slight distort the reaction mechanism. 

Hence hydrogen iodide cannot be produced by the reaction of sulphuric acid with an iodide. Hydriodic acid is slowly oxidised by air (more rapidly in light) liberating iodine ... 

Oxidative addition of alkyl halides to Pd(0) is slow. Furthermore, alkyl-Pd complexes, formed by the oxidative addition of alkyl halides, undergo facile elimination of /3-hydrogen and the reaction stops at this stage without undergoing insertion or transmetallation. Although not many examples are available, alkynyl iodides react with Pd(0) to form alkynylpalladium complexes. 

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