Interhalogen Compounds: Types, Preparation, Properties and Uses

Interhalogen compounds are the important portion of compounds in chemistry that are formed when halogen group elements react with each other. In other easier words, it is a molecule that consists of two or more different elements of group 17. We can also say that when different halogens react with each other, they form interhalogen compounds. 

Key Terms: Interhalogen Compounds, Electron, Halogen, Group 17 Elements, VSEPR Theory, Chlorine

Read More: Phosphorus Pentachloride

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Types of Inter – Halogen Compounds

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The inter – halogen compounds are of four different types varying in the increasing and decreasing number of atoms in a molecule. A halogen element of group 17 reacts with another element having the opposite rate of electropositivity and size. Therefore, the number of atoms in a molecule increases as per the rate of increase in the ratio of the radius of the larger and the smaller halogen.

Four different types of inter – halogen compounds could be classified as

• Diatomic Inter – Halogen (XX’)
• Tetra Atomic Inter – Halogens (XX’₃)
• Hexa Atomic Inter – Halogens (XX’₅)
• Octa Atomic Inter – Halogens (XX’₇)

For instance, Inter – Halogens are assigned the general compositions, such as XX′, XX₃ ′ , XX₅ ′ and XX₇ ′ .

Where, X is halogen of larger size and X′ is of smaller size and X is more electropositive than X′ .

As the ratio between radii of X and X ′ increases, the number of atoms per molecule will also increase. Thus, iodine (VII) fluoride shall have a maximum number of atoms as the ratio of radii between I and F should be maximum. That is why it has its formula as IF7 (having maximum number of atoms). 

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Preparation of Inter – Halogen Compounds

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The formation of inter – halogen compounds depends on various specific conditions under which compounds of different elements are formed. These compounds could either be produced directly combining with each other or they may be formed by the action of halogen in the lower inter – halogen compound. Although, as mentioned earlier, any kind of formation of inter – halogen compounds shall depend on the specific conditions. These conditions could vary from the properties of elements, contemporary temperature and atmosphere, etc.

For example, at the temperature of 437 K, chlorine reacts with fluorine of equal volume to form ClF. For the production of group 17 fluorides, this method is widely used.

Cl₂ +F₂ → 2ClF (437K), showing equal volume

Cl₂ + 3F₂ → 2ClF₃ (573), showing excess

I₂ + Cl₂ → 2ICl Equimolar

Another example –

Br₂ + 3F₂ → 2BrF₃ (diluted with water)

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Properties of Inter – Halogen Compounds

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• These molecules are covalent in nature due to less electronegativity difference between bonded atoms and are also diamagnetic in nature because of its nature of having only the lone pairs and bond pairs.
• The bonds formed between these compounds are more reactive than diatomic halogen bonds. It also has an exception as Fluorine. This is because A-X bond in inter - halogens is weaker than X-X bond in halogens except F-F bond.
• Structures of these different types of interhalogens are different from one another which can be explained using V.S.E.P.R Theory. (The theory is explained below)
• The physical properties of these molecules are transitional between its constituents. Inter - halogen compounds exist in all the three states that are the gaseous state, liquid state (or) solid state.
• Most of these compounds are volatile solids (or) liquids at 298 K while the other compounds are present in gaseous form.
• For instance, chlorine mono - fluoride is gas while bromine tri - fluoride and iodine tri - fluoride are found in liquid state and sometimes also in the solid state respectively.
• These inter – halogen compounds also differ in their shapes depending upon the atoms and types of elements used such as the molecular structure of AX₃ molecules is bent T shaped while the structure of an AX₅ molecule is square or pyramidal and the structure of AX₇ is bi - pyramidal or pentagonal.
• The length of the bonds depends upon the size of the constituent halogens.
• If we talk about the colour of the halogens and their compounds then we can say that the molecule which consists of lighter group 17 elements is colourless to some extent but one which is made up of higher halogens is deeper in colour which is due to the rise in the molecular weight.

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Explanation of V.S.E.P.R Theory

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• In chlorine tri - fluoride the central atom chlorine has seven electrons in the valence shell. Three of these electrons will form three bond pairs with three fluorine atoms leaving the four electrons separately.
• According to V.S.E.P.R theory, these electrons will occupy the corners of the trigonal bi - pyramid. The lone pairs will occupy the equatorial positions while bond pairs will occupy the other three positions.
• And then in order to minimize the repulsions of the ion pairs, the axial bond pairs will bend towards the equatorial positions. Hence its shape is bent T-shape.

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Applications of Inter - Halogen Compounds

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• These interhalogen compounds are capable of being used as non-aqueous solvents.
• They are also used as an essential catalyst in few reactions that help in maintaining specific conditions in the performing reactions.
• UF₆ which is used in the enrichment of ²³⁵ U is produced by using ClF₃ and BrF₃.

For instance, U (s) + 3ClF₃ (l) → UF₆ (g) + 3ClF (g)

• They also have the potential to be used as fluorinating compounds.