Halogens and Hydrogen Halides

Halogens and Hydrogen Halides.— The earliest work with photochemical hot atoms involved halogens and hydrogen halides and interest in these reactions has continued with molecular beam studies and trajectory calculations as well as further photochemical work. 

In the presence of a sufficient excess of inert gas most atoms are thermalized before reaction and 

Reactions of hydrogen atoms with HI and with I2 have been examined in a similar manner, yielding 29/ 27 = 4 1 and 33/ 32 = 13 2 at 300 K, with only a slight temperature dependence. The thie old for both reactions (32) and (33) is probably very small. 

Some insight into the reactions involved in these systems has been gained through several quasiclassical trajectory studies, which have been performed for comparison with the results of thermal, photochemical, and molecular beam experiments. - This is illustrated by reference to the reactions of hot D atoms with DCl and with CI2, (27 ) and (29 ). Cross-sections for each of these reactions were computed as a function of relative translational energy using quasiclassical 

Wood examined the production of HD and D2 in the photolysis of mixtures of DCl and H2 and concluded that for X = Cl abstraction was the more important process at room temperature, k ilk o = 0.05 0.01. The relative extent of exchange deceased with increasing temperature with 4o/ 4i = (850 200) exp[-(9300 800)/8.314T]. Use of dato for the related abstraction reaction (32) indicates values of A and a for the exchange reaction (41) of the order of 2 x 10 dm mol s and 5 kJ mol respectively. 

PURPOSE OF EXPERIMENT Study some preparations, properties, and reactions of some halogens and hydrogen halides. 

In this experiment you will prepare chlorine, bromine, and iodine and will investigate their physical and chemical properties. The preparative reaction involves the oxidation of the -1 halide anion by potassium permanganate in an acidic solution. In addition, you will prepare the hydrogen halides and study their physical and chemical properties. 

Grasp a piece of copper foil with a pair of crucible tongs, and heat one end of the foil to redness. Immediately remove the watch glass, and dip the hot Cu foil into the test tube of CI2 gas, being careful not to drop the foil into the tube. Then remove the foil, and cover the test tube with the watch glass. Describe what happened to the Cu foil in TABLE 39.1A1. 

Place your test tube of CI2 gas in the hood in a beaker labeled with your name, remove the watch glass, add 2 mL of water, and swirl the test tube. Describe in TABLE 39.1A1 what you observe regarding the solubility of CI2 gas in water. The resulting solution is called chlorine water. Since such a solution affords a convenient supply of chlorine for certain reactions, it is a common laboratory reagent. When chlorine dissolves in water, some of the chlorine reacts chemically with the water and disproportionates to form species in which the chlorine atoms have higher and lower oxidation states than they have in CI2. Thus CI2 has been both oxidized and reduced. 

Obtain 1 mL of chlorine water from the reagent bench. Test its effect on blue litmus paper, starch-potassium iodide paper, and dyed cotton cloth (Laboratory Methods P), and describe your results in TABLE 39.1A1. Add 10. drops of cyclohexane to the chlorine water contained in a test tube, stopper the test tube with a cork, shake the contents, and then let the contents stand a few moments. Describe what you observe in TABLE 39.1A1. 

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Their physieal properties are essentially those of the alkanes. It is the unsaturated linkages that dominate the ehemistry and the main reaetion is one of addition (e.g. hydrogen, halogen, and hydrogen halides) aeross the double bond to produee saturated eompounds. This reaetivity is utilized in the manufaeture of long-ehain polymers, e.g. polyethylene and polypropylene. 

The reactions of halogens and hydrogen halides with alkenes are electrophilic addition reactions. This means that the initial attack on the organic molecule is by an electron-deficient species that accepts a lone pair of electrons to form a covalent bond. This species is called an electrophile. In the case of the reaction with hydrogen bromide, the mechanism for the reaction is as shown. 

It is important to be able to look at a molecular structure and deduce the possible reactions it can undergo. Take an alkene, for example. It has a 7t bond that makes it electron-rich and able to attack electrophiles such as water, halogens and hydrogen halides in electrophilic addition reactions. Haloalkanes, on the other hand, contain polar carbon-halogen bonds because the halogen is more electronegative than carbon. This makes them susceptible to attack by nucleophiles, such as hydroxide, cyanide and alkoxide ions, in nucleophilic substitution reactions. 

Addition of halogens and hydrogen halides May require mercury catalyst... 

It is fair to say the chemistry of 1-3 and their derivatives is dominated by reactions with nucleophiles. The molybdenum species Cp2Mo2(CO)4 does, however, also react with halogens and hydrogen halides (182, 183). 

By Addition of Halogen and Hydrogen Halide to Unsaturated Compounds... 

Reactions of the organometallic compounds InCp and TlCp with halogens and hydrogen halides involve attack on the cyclopentadienyl group . However, I2 reacts with InCp by oxidative addition to yield Cplnl2 , which forms adducts with suitable donor ligands (e.g., 2,2 -dipyridyl). Loss of the cyclopentadienyl ligand accompanies... 

Reactions at a Group Viii Metal Center The Ni, Pd, Pt Triad 10.3.8.6. Reactions of Halogens and Hydrogen Halides... 

The kinetics of the gas-phase reactions of HBr with Cl2 and of HI with Cl2, Br2, and IC1 have been investigated by stopped-flow methods.27 The rate constants for the family of reactions can be rationalized if halogen and hydrogen halide molecules react to form a transition state in which the proton is near the centre of a triangle of three halogen atoms. In a study of the reaction ... 

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