Iodides hydrogen

Hydrogen iodide is prepared in a similar way to hydrogen bromide, by the action of water on a mixture of iodine and violet phosphorus. TTie hydrogen iodide evolved may be collected by downward delivery or may be condensed (b.p. 238 K) it reacts with mercury and so cannot be collected over the latter. 

Hydrogen iodide. This gas may be conveniently prepared by allowing a solution of two parts of iodine in one part of hydriodic acid, sp. gr. 1 7 (for preparation, see Section 11,49,2), to drop on to excess of red phosphorus. The evolution of hydrogen iodide takes place in the cold when the evolution of gas slackens considerably, the mixture should be gently warmed. 

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  

Rupture by hydrogen iodide. Ethers are usually broken up by hot hydrogen iodide thus  

When potassium iodide or hydrogen iodide is added to an aqueous solution of a diazonium salt, nitrogen is readily evolved (no catalyst being necessary) and the corresponding iodo-compound is formed. lodo-compounds can thus 

An aqueous solution of hydrogen iodide, up to 50% strength, may be prepared by passing hydrogen sulphide (or sulphur dioxide) into a suspension of iodine in water  

Iodine and hydrogen react reversibly to give hydrogen iodide  

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

Scheme 98) (523). Hydrogen chloride is unsatisfactory as a cyclizing agent, whereas hydrogen iodide causes further reduction and leads directly to derivatives of the previously inaccessible 4-aminothiazoles (see Section VII. 2). 

The hydrolysis of phosphorus tribromide or triiodide is used in the preparation of hydrogen bromide and hydrogen iodide respectively  

The order of reactivity of the hydrogen halides parallels their acidity HI > HBr > HCl >> HF Hydrogen iodide is used infrequently however and the reaction of alco hols with hydrogen fluoride is not a useful method for the preparation of alkyl fluorides Among the various classes of alcohols tertiary alcohols are observed to be the most reactive and primary alcohols the least reactive 

Heat under reflux i g. of anisole and 10 ml. of constant-boiling hydrogen iodide for 30 minutes. Now distil off the volatile methyl iodide and identify it in the distillate (see pp. 390-391). 

Comparable results are not obtained with the less reactive iodine, because the hydrogen iodide formed tends to reduce the iodo compound and a condition of equilibrium is produced  

Lead dioxide in acetic acid solution gives lead tetra acetate which oxidises hydrogen bromide (and also hydrogen iodide), but has practically no cflFect under the above experimental conditions upon hydrogen chloride. 

These values indicate a rapid fall in thermal stability of the halide from fluorine to iodine, and hydrogen iodide is an endothermic compound. If we now examine the various enthalpy changes involved. we find the following values (in kJ)  

However, if an oxidising agent (fuming nitric acid or sodium persulphate) is present to destroy the hydrogen iodide as it is formed, the equilibrium is displaced and the iodo compound may be conveniently prepared, for example  

This preparation illustrates the direct iodination of a primary aromatic amine by iodine the sodium bicarbonate removes the hydrogen iodide as formed  

The parent acid is unstable and always reverts to the lactone form shown. It is reconverted to acetoacetic ester by boiling alcoholic potassium hydroxide. Reduced by hydrogen iodide to dimethylpyrone. 

For 2-amino-4- m-nitrophenyl) seienazole, the yield is particularly high. This has been explained by the oxidizing effect of the nitro group, which liberates iodine from the hydrogen iodide eliminated in the condensation reaction. 

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