Interhalogen Compounds

An interhalogen compound is a molecule that contains two or more separate halogen atoms (fluorine, chlorine, bromine, iodine, or astatine) and no atoms of any other group of elements.  Interhalogen compounds are compounds formed when halogen-group elements react with each other. Most of the interhalogen compounds are binary (composed of only two distinct components). The general formula of the interhalogen compounds is expressed as XYn where n= 1,3,5 and 7 where X is a halogen which is less electronegative than the other halogen which is Y (the more electronegative one). 

The value of n is odd due to the odd valencies of the halogens. They all undergo hydrolysis and are ionized to give rise to polyhalogen ions. The poly hydrogen that is formed from astatine has a very short half-life as astatine is highly radioactive in nature.

In other words, it is a molecule composed of two or more separate elements of group 17. There are four forms of interhalogen compounds:

• Diatomical interhalogens (AX)

• Tetratomic interhalogens (AX₃)

• Hexatomical interhalogens (AX₅)

• Octatomical interhalogens (AX₇)

A halogen with a large size and high electropositivity interacts with a group of 17 products with a small size and lower electropositivity. As the ratio of the radius of larger and smaller halogens increases, so does the number of atoms in the molecule.

Structure of Interhalogen Compounds

The various structures of the interhalogen compounds are suggested according to the VSEPR (Valence Shell Electron Pair Repulsion) theory. For XY₃ the shape determined is T-shaped. It has a lone pair of electrons that is located in the equatorial region of the trigonal bipyramid. For XY₅ the shape is a square pyramid that has unpaired electrons located in the axial position of the octahedral. Whereas XY₇ is a pentagonal bipyramid. 

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Types of Inter-halogen Compounds

According to the number of atoms in the molecule, the interhalogens are classified into four groups. They are:

• XY: The properties of the diatomic interhalogen lies somewhat between the properties of the two individual halogens from which it is made up. There are ionic characteristics to the covalent bonding that happens between the two halogens forming the respective interhalogen. Here, X which is the larger element becomes oxidised and gains a positive charge. F, Cl, Br and I are the halogens whose most combinations are known but not all are stable. 

1. Chlorine monofluoride (ClF) is the lightest colourless interhalogen gas that is known with a boiling point of  173 °K.

2. Bromine monofluoride (BrF) cannot be obtained in the pure state and thus, its dissociation results in the formation of trifluoride and free bromine.

3. Iodine fluoride (IF) disproportionate rapidly at room temperature as it is an unstable compound. 5IF → 2I2 + IF5. Thus, its molecular properties have been identified spectroscopically. The iodine-fluorine distance is 190.9 pm and the I-F bond dissociation energy is around 277 kJ mol-1. ΔHf° = -95.4 kJ mol-1 and ΔGf° = -117.6 kJ mol-1, both at 298 K. It can be obtained by following reactions:

I2 + F2 → 2IF at -45 °C in CCl3F;

I2 + IF3 → 3IF at -78 °C in CCl3F;

I2 + AgF → IF + AgI at 0 °C.

4. Bromine monochloride (BrCl) is a red-brown coloured gas that is unstable and has a boiling point of  5 °C.

5. Iodine monochloride is a red transparent crystal that melts at 27.2 degrees centigrade to form a choking brownish liquid. It forms a strong hydrogen chloride when it reacts with hydrochloric acid. The crystal structure of iodine monochloride consists of a puckered zigzag chain that has a very strong interaction between the chains.

6. By a direct combination of elements, iodine monobromide is formed as a dark red crystalline solid. It boils at 116 degrees centigrade to form a partially dissociated vapour whereas it melts at 42 degrees centigrade.

• XY₃: 

1. Chlorine trichloride is a colourless gas that freezes to white solid and condenses to green liquid. It is made by reacting chlorine with excess fluorine at 250 degrees centigrade in a nickel tube. Chlorine reacts more violently than fluorine often resulting in an explosive reaction. The molecules generally obtained T-shaped or planar.

2. Bromine trifluoride generally conducts electricity and is a yellow-green liquid. It conducts electricity because it ionizes to form [BrF2+] + [BrF4-]. In order to form similar identities, it reacts with many metals and metal oxides but with some others, it generally forms the methyl fluoride with free bromine and oxygen. Therefore, it is used as a fluorinating agent in organic chemistry. As that chlorine trifluoride, it has the same molecular structure.

3. Iodine trifluoride decomposes at above -28 degrees centigrade and is a yellow solid. It can easily be synthesized from elements but extra precautions need to be taken to avoid the formation of iodine pentafluoride. The fluorination reaction I2 + 3XeF2 → 2IF3 + 3Xe can be used alternatively at low temperature. Iodine tetrafluoride is unstable. 

• XY₅: 

1. Chlorine pentachloride is made by the reaction of chlorine trifluoride with chlorine at high temperature and high pressure and it forms to be a colourless gas. It has a very violent reaction with water, most metals and non-metals. Bromine pentafluoride is a fuming liquid that is colourless in nature and is formed by the reaction of bromine trifluoride with chlorine at 200 degrees centigrade. Though it is stable in nature, it reacts violently with water and most metals and non-metals.

2. Iodine pentafluoride is made by reaction of iodine with silver chloride or by reaction of iodine pentoxide with fluorine, and it is a colourless liquid. It is very non-reactive in nature and therefore also reacts very slowly with glass. Elements, oxides and carbon halides also react with iodine pentafluoride. The molecule takes the structure of a tetragonal pyramid. Primary amines after hydrolysis with water react with iodine pentafluoride to form nitriles.

R-CH₂-NH₂ —> R-CN

• XY₇: Iodine fluoride is a colourless gas. It is generated by reacting pentafluoride with fluorine. It is chemically inert and has no lone pair of electrons in the valence shell. It resembles sulphur hexafluoride in structure. Its molecular structure is pentagonal bipyramidal. This is the only compound that can be formed when n large atom carries 7 small atoms.

"X" is a larger (or less electronegative halogen and "Y" is a smaller (or more) electronegative halogen.

Using the radius ratio, we can calculate the number of particles in the atom.

Radius Ratio = \[\frac{\text{Radius of Bigger Halogen Particle}}{\text{Radius of Smaller Halogen Molecule}}\]

As the radius proportion increases the number of atoms per molecule, so does the rise. So, Iodine heptafluoride has the largest number of particles per atom out of all interhalogen compounds because it has the most impressive radius proportion.

Preparation of Interhalogen Compounds

These interhalogen compounds can be produced using two main methods. One of them involves the direct mixing of halogens and the other involves the reaction of halogens to the lower interhalogen compounds under specific conditions. 

• Halogen atoms combine to form an interhalogen compound. One example is the reaction when the volume of chlorine reacts with an equal volume of fluorine at 473K. The resultant product is chlorine monofluoride. This process is commonly used in the manufacture of group 17 fluorides.

Cl₂ +F₂ → 2ClF (473K)

I₂ + Cl₂ → 2ICl

• In other cases, the halogen atom acts with a lower interhalogen to form an interhalogen compound. For example, fluorine reacts to 543 K with iodine pentafluoride. This results in a compound of Iodine Heptafluoride.

Properties of Interhalogen Compounds

We may find interhalogen compounds in gas, solid, or liquid state. A number of these substances unstable at 298 K are solids or fluids. There are also a few other substances that are gases. For example, chlorine monofluoride is a gas. On the other hand, trifluoride bromine and trifluoride iodine are both solid and liquid.

• Most of these compounds are liquid solids (or fluids) at 298 K, while the rest are gaseous.

• For example, chlorine monofluoride exists as a gas, while bromine trifluoride and iodine trifluoride exist separately as a solid and liquid.

• All of these compounds are covalent in nature due to a less electronegativity distinction between bonded molecules. For example, chlorine monofluoride, bromine trifluoride, iodine heptafluoride are covalent in nature.

• Both of these interhalogen compounds are diamagnetic in nature because they have only bond pairs and lone pairs. 

• Interhalogen compounds are highly reactive. Fluorine is an exception to this. This is because the bond of A-X in interhalogens is much weaker than the bond of X-X in halogens, except for the bond of F-F.

Uses of Interhalogen Compounds

• They are used as non-aqueous solvents / non-watery solvents.

• We use these compounds as a catalyst in a number of reactions.

• UF6 used to enrich 235 U is provided using ClF₃ and BrF₃.

• U(s) + 3ClF₃(l) and UF₆(g) + 3ClF(g)

Interhalogen compounds are halogen subordinates. Compounds containing two distinct forms of halogens are referred to as interhalogen compounds. Example: monofluoride chlorine, trifluoride bromine, pentafluoride iodide, heptafluoride iodide, etc. The article discusses all the important concepts of interhalogen compounds.