CHEMISTRY OF INTERHALOGEN COMPOUNDS PDF

An interhalogen compound is a molecule which contains two or more different halogen atoms fluorine , chlorine , bromine , iodine , or astatine and no atoms of elements from any other group. Most interhalogen compounds known are binary composed of only two distinct elements. The value of "n" in interhalogens is always odd, because of the odd valence of halogens. They are all prone to hydrolysis , and ionize to give rise to polyhalogen ions. Those formed with astatine have a very short half-life due to astatine being intensely radioactive. No interhalogen compounds containing three or more different halogens are definitely known, [1] although a few books claim that IFCl 2 and IF 2 Cl have been obtained, [2] [3] [4] [5] and theoretical studies seem to indicate that some compounds in the series BrClF n are barely stable.

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At room temperature, all of the halogen halides are gases and have sharp, acid smells. They can be prepared by direct combination of the halogens with H 2 or by the action of a concentrated acid nonoxidising for HBr and HI on metal halides. Sulfuric acid is too strong an oxidising agent to be used in the generation of HBr and HI resulting in partial oxidation, for example:.

Aqueous solutions of HX are generally referred to as hydohalic acids and we will look at some chemistry of both the anhydrous hydrogen halides and the hydrohalic acids. Commercial production of anhydrous Hydrogen Fluoride began in the 's and by the 's at least 16 countries were involved in generating over 1 million tonnes worldwide. Initially the HF was used in making refrigerants and for synthetic cryalite Na 2 AlF 6 for aluminium production as well as in uranium processing.

A small amount is used in glass etching. Hydrofluoric acid attacks glass by reaction with silicon dioxide to form gaseous or water soluble silicon fluorides. The dissolution process proceeds as follows:. This property has been known since the 17th century, even before a general procedure for the preparation of large quantities of hydrofluoric acid had been devised by Scheele in Image used with permission from Wikipedia. HF is miscible with water in all proportions and phase diagrams show several distinct species including.

The construction of a deuterium fluoride laser resembles a rocket engine. In the combustion chamber, ethylene is burned in nitrogen trifluoride. This reaction produces free excited fluorine radicals. Just after the nozzle, the mixture of helium and hydrogen or deuterium gas is injected to the exhaust stream; the hydrogen or deuterium reacts with the fluorine radicals, producing excited molecules of deuterium or hydrogen fluoride.

The excited molecules then undergo stimulated emission in the optical resonator region of the laser. The HF laser is somewhat cheaper and operates at 2. The DF laser operates at 3. Hydrogen chloride is one of the the largest volume chemicals to be manufactured as either the gas or aqueous acid. When a cheap source of NaCl was available the two-stage Leblanc process, developed during the Industrial Revolution of the 's, was used:.

Here salt is converted to sodium sulfate, using sulfuric acid, giving hydrogen chloride as by-product. Initially, this gas was released to the air, but the Alkali Act of in the UK prohibited such release, the manufacturers then absorbed the HCl waste gas in water, producing hydrochloric acid on an industrial scale. Modern production of HCl is still as a byproduct that is recovered from large scale processes but now more likely to be from an Industrial Organic Scheme.

Pickling or removal of rust iron oxide scale from steel involves the following reaction:. The HCl is then regenerated via a closed loop reaction scheme that has been developed so that Fe 2 O 3 is recovered as a by-product as well. When high-purity HCl is required, then burning of dihydrogen in dichlorine is used.

This is a highly exothermic reaction. High quality HCl is used in the food and pharmaceutical industry as well as for water treatment. The acid in our stomachs gastric acid consists mainly of HCl around 0. The pH is usually around The infrared spectrum of gaseous hydrogen chloride consists of a number of sharp absorption lines grouped around cm -1 and for deuterium chloride vapor this has shifted to cm These classic molecular spectra can be analyzed to provide information about both rotational and vibrational energies of the molecules.

The rotational angular momentum changes by 1 during such transitions. The splitting of the lines shows the difference in rotational inertia of the two chlorine isotopes 35 Cl Calculation of the bond length for H- 35 Cl for example gives a value of pm, close to the accepted value of pm.

Both HBr and HI can be produced by direct combination, using platinum catalysts and elevated temperatures. An alternate method for the production of HI is the quantitative reaction of iodine with hydrazine. A constant boiling point mixture exists at about Hydrogen iodide is extremely soluble in water dm -3 in 1 dm -3 , which is roughly equivalent to only 4 water molecules per HI molecule. As with the dihalogens, it is expected that with increasing size and polarizability the MP's and BP's should increase.

The position of HF though needs further explanation. Solid HF exists as an H-bonded polymer. In water, the hydrohalic acids are formed by reaction with the gaseous hydrogen halides and apart from HF these are completely dissociating strong acids. The reaction representing the pK is given by:. The factors that influence the degree of dissociation can not all be readily measured but can be simplified as the following 6 steps in the Hess cycle.

Step 1 must be predicted since it is based on having undissociated HX aq , and apart from HF the hydrogen halides are completely dissociated in aqueous solution.

Step 2 is the cleavage of the HX bond. Step 4 is related to the Electron Affinity and step 6 is the hydration of the gaseous X- ion. The predicted value for HF is 1. In contrast to the weak acidicity of dilute aqueous HF solutions, in concentrated hydrogen fluoride solution, F- ions form a [HF 2 ] - aq complex by addition to HF molecules.

HF molecules remain ionised to compensate the loss of F- ions. Anhydrous hydrogen fluoride is an extremely strong acid comparable in strength to anhydrous sulfuric acid. The fact that it is a weak acid in dilute solutions does not mean it is any less of a hazard and the poorly dissociated HF actually penetrates tissue more quickly than typical acids. Symptoms of exposure to hydrofluoric acid may not be immediately evident and unfortunately any delay in treatment can lead to serious consequences.

The severe pain that comes on is thought to be caused by dissolution of Ca in the bone to form insoluble CaF 2 and it is has been found necessary in some cases to amputate affected limbs. Some properties of interhalogen compounds are listed below. They are all prepared by direct combination of the elements although since in some cases more than one product is possible the conditions may vary by altering the temperature and relative proportions.

For example under the same conditions difluorine reacts with dichlorine to give ClF with dibromine to give BrF 3 but with diiodine to give IF 5. The premise of the VSEPR is that the valence electron pairs surrounding an atom mutually repel each other, and will therefore adopt an arrangement that minimizes this repulsion, thus determining the molecular geometry.

The number of electron pairs surrounding an atom, both bonding and nonbonding, is called its steric number. The VSEPR theory thus provides a simple model for predicting the shapes of such species, in particular for main group compounds. The model combines original ideas of Sidgwick and Powell 's with extensions developed by Nyholm and Gillespie 's. The VSEPR theory works best for simple halides of the p-block elements, but may also be applied to species with other substituents.

However, the model does not take steric factors i. The structures found for the various interhalogens conform to what would be expected based on the VSEPR model. For XY 3 the shape can be described as T-shaped with 2 lone pairs sitting in equatorial positions of a trigonal bipyramid.

For XY 5 the shape is a square pyramid with the unpaired electrons sitting in an axial position of an octahedral and XY 7 is a pentagonal bipyramid. The interhalogens with formula XY have physical properties intermediate between those of the two parent halogens. The covalent bond between the two atoms has some ionic character, the larger element, X, becoming oxidised and having a partial positive charge.

Most combinations of F, Cl, Br and I are known, but not all are stable. Chlorine trifluoride, ClF 3 was first reported in and it is primarily used for the manufacture of uranium hexafluoride, UF 6 as part of nuclear fuel processing and reprocessing, by the reaction:. U isotope separation is difficult because the two isotopes have very nearly identical chemical properties, and can only be separated gradually using small mass differences. A cascade of identical stages produces successively higher concentrations of U.

Each stage passes a slightly more concentrated product to the next stage and returns a slightly less concentrated residue to the previous stage.

These both make use of the volatility of UF 6. ClF 3 has been investigated as a high-performance storable oxidizer in rocket propellant systems. Handling concerns, however, prevented its use. Hypergolic means explode on contact with no need for any activator. It is, of course, extremely toxic, but that's the least of the problem. It can be kept in some of the ordinary structural metals-steel, copper, aluminium, etc.

If, however, this coat is melted or scrubbed off, and has no chance to reform, the operator is confronted with the problem of coping with a metal-fluorine fire. For dealing with this situation, I have always recommended a good pair of running shoes. It is believed that prior to and during World War II, ClF 3 code named N-stoff "substance N" was being stockpiled in Germany for use as a potential incendiary weapon and poison gas.

The plant was captured by the Russians in , but there is no evidence that the gas was actually ever used during the war. Robert J. Hydrogen chloride Hydrogen chloride is one of the the largest volume chemicals to be manufactured as either the gas or aqueous acid.

Melting and Boiling Points As with the dihalogens, it is expected that with increasing size and polarizability the MP's and BP's should increase. Acidic properties In water, the hydrohalic acids are formed by reaction with the gaseous hydrogen halides and apart from HF these are completely dissociating strong acids.

Safety In contrast to the weak acidicity of dilute aqueous HF solutions, in concentrated hydrogen fluoride solution, F- ions form a [HF 2 ] - aq complex by addition to HF molecules.

Interhalogens Some properties of interhalogen compounds are listed below. Valence-shell electron-pair repulsion theory, VSEPR The premise of the VSEPR is that the valence electron pairs surrounding an atom mutually repel each other, and will therefore adopt an arrangement that minimizes this repulsion, thus determining the molecular geometry.

Each valence shell electron pair of the central atom E in a molecule EX n containing E-X single bonds is stereochemically significant, and repulsions between them determine the molecular shape. Repulsions between the bonding pairs in EX n depend on the difference between the electronegativities of E and X; electron-electron repulsions are less the more the E.

X bonding electron density is drawn away from the central atom E. Basic Geometry 0 lone pair 1 lone pair 2 lone pairs 3 lone pairs 2 linear 3 trigonal planar bent 4 tetrahedral trigonal pyramid bent 5 trigonal bipyramid seesaw T-shaped linear 6 octahedral square pyramid square planar 7 pentagonal bipyramid pentagonl pyramid. Structure of the Interhalogens The structures found for the various interhalogens conform to what would be expected based on the VSEPR model. XY diatomic interhalogens The interhalogens with formula XY have physical properties intermediate between those of the two parent halogens.

Bromine monofluoride BrF has not been obtained pure - it dissociates into the trifluoride and free bromine.

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Interhalogen Compounds

The halogens react with each other to form interhalogen compounds. The compounds which are formed by the union of two different halogens are called inter halogen compounds. There are never more than two types of halogen atoms in an interhalogen molecule. There are of four general types:. The interhalogen compounds of type AX and AX 3 are formed between the halogen having very low electronegative difference e. The interhalogen compounds of type AX 5 and AX 7 are formed by larger atoms having low electronegativity with the smaller atoms having high electronegativity.

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17.7A: Interhalogen Compounds

Preparations of Interhalogen Compounds. Interhalogen Compounds are the subordinates of halogens. Compounds containing two unique sorts of halogens are termed as interhalogen compounds. Example : Chlorine monofluoride, bromine trifluoride, iodine pentafluoride, iodine heptafluoride, etc. Contingent upon the number of atoms in the particle, interhalogens are characterized into four sorts. As the radius proportion expands the number of atoms per molecule likewise increments. So out of all interhalogen compounds Iodine heptafluoride has the highest number of particles per atom as it has most astounding radius proportion.

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