Hydrogen with iodine

Consider again the gaseous reaction of hydrogen with iodine ... 

The composition of an equilibrium mixture is unaffected by a change in pressure if the reaction involves no change in the number of moles of gas. For example, the reaction of hydrogen with iodine has 2 mol of gas on both sides of the balanced equation ... 

Derivation (1) By passing hydrogen with iodine vapor over warm platinum sponge and absorption in water. (2) By the action of iodine on a solution of hydrogen sulfide. 

The gas phase reaction of hydrogen with iodine, investigated extensively by Bodenstein in the 1890s, provides a classic example of opposing second-order reactions. Between 300 and 500 °C the reaction proceeds at conveniently measurable rates. If we assume that the mechanism is simple, consisting of one elementary reaction and its reverse, ... 

Excellent examples are found in the hydrolytic reactions of esters with alkalis, the interaction of alkyl halides with hydroxyl ions, the benzoylation of amines, and the union of tertiary amines with alkyl halides to give quaternary ammonium salts. Among gas-phase reactions the decomposition of hydrogen iodide, on the one hand, and the union of hydrogen with iodine on the other provide the classical examples. The absolute rates in this last case are rather closely given by collision number... 

FIGURE 19.33 Treatment of a ketone containing an a hydrogen with iodine, bromine, or chlorine in acid leads to a halogenation. 

The information below gives the data for the reaction of hydrogen with iodine at 500 "C. 

Consider the gas-phase reaction of hydrogen with iodine monochloride to form hydrogen chloride and iodine. 

Hypohahtes replace the acetylenic hydrogen with chlorine, bromine, or iodine (203). 

Hydrogen iodide iodinates triaLkylsilanes in good yield in boiling carbon tetrachloride with no aluminum haUde present (116). This can perhaps be explained on the basis that some free iodine is always present in equiUbrium with hydrogen iodide. 

Preparation. Thiophosgene forms from the reaction of carbon tetrachloride with hydrogen sulfide, sulfur, or various sulfides at elevated temperatures. Of more preparative value is the reduction of trichi oromethanesulfenyl chloride [594-42-3] by various reducing agents, eg, tin and hydrochloric acid, staimous chloride, iron and acetic acid, phosphoms, copper, sulfur dioxide with iodine catalyst, or hydrogen sulfide over charcoal or sihca gel catalyst (42,43). 

Pyridazinethiones are readily oxidized to the corresponding disulfides with iodine, aqueous iron(III) chloride, hydrogen peroxide in acetic acid, potassium permanganate in acetic acid, and upon long exposure to air. 

These workers also prepared the thio analog of 17 (R = H) by treating 16 (R = H) with aqueous ammonia to provide the P-oxoamide, which was converted into the corresponding enolized P-thioxoamide 18 by treatment with hydrogen sulfide and hydrogen chloride in ethanol. Compound 19 was synthesized by oxidation of 18 with iodine in ethanol under basic conditions. 

The filtrate was adjusted to a pH of 9 by adding concentrated ammonia, and than a 1 N aqueous iodine-potassium iodide solution was added dropwise, whereby the tetrahydro-pyrimido-[5,4-d] pyrimidine obtained by hydrogenation with zinc in formic acid was converted by oxidation into 2,6-bis-(diethanolamino)-8-piperidino-pyrimido-[5/4-d]-pyrimidine. The completion of the oxidation was checked by means of a starch solution. The major amount of the oxidation product already separated out as a deep yellow crystalline precipitate during the addition of the iodine solution. After the oxidation reaction was complete, the reaction mixture was allowed to stand for a short period of time, and than the precipitate was separated by vacuum filtration, washed with water and dried. It had a malting point of 157°C to 158°C. The yield was 8.0 g, which corresponds to 95% theory. 

It is important to point out one of the limitations of mechanism studies. Usually more than one mechanism is compatible with the same experimentally obtained rate expression. To make a choice between alternative mechanisms, other evidence must be considered. A classic example of this situation is the reaction between hydrogen and iodine... 

The preparation of Pans-1,2-cyclohexanediol by oxidation of cyclohexene with peroxyformic acid and subsequent hydrolysis of the diol monoformate has been described, and other methods for the preparation of both cis- and trans-l,2-cyclohexanediols were cited. Subsequently the trans diol has been prepared by oxidation of cyclohexene with various peroxy acids, with hydrogen peroxide and selenium dioxide, and with iodine and silver acetate by the Prevost reaction. Alternative methods for preparing the trans isomer are hydroboration of various enol derivatives of cyclohexanone and reduction of Pans-2-cyclohexen-l-ol epoxide with lithium aluminum hydride. cis-1,2-Cyclohexanediol has been prepared by cis hydroxylation of cyclohexene with various reagents or catalysts derived from osmium tetroxide, by solvolysis of Pans-2-halocyclohexanol esters in a manner similar to the Woodward-Prevost reaction, by reduction of cis-2-cyclohexen-l-ol epoxide with lithium aluminum hydride, and by oxymercuration of 2-cyclohexen-l-ol with mercury(II) trifluoro-acetate in the presence of ehloral and subsequent reduction. ... 

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