Interhalogen Compounds

Interhalogen compounds are molecules composed solely of halogen atoms (fluorine, chlorine, bromine, iodine, or astatine) without any atoms from other element groups. These compounds form when different halogens react, typically resulting in binary compounds.

1.0Introduction

Interhalogen compounds are formed when different halogens interact with each other. The general formula for interhalogen compounds is XY_n​, where X is the less electronegative halogen, Y is the more electronegative halogen, and n can be 1,3,5 or 7. The value of n in interhalogen compounds is odd due to the odd valencies of the halogens. These compounds undergo hydrolysis and ionize to form polyhalogen ions. When astatine is involved, the polyhalogen formed has a very short half-life because astatine is highly radioactive.

2.0Types of Interhalogen Compounds

Interhalogen compounds are classified into four types based on the number of atoms in the molecule:

1. XY: Diatomic interhalogens
2. XY₃: Tetratomic interhalogens
3. XY₅: Hexatomic interhalogens
4. XY₇: Octatomic interhalogens

• "X" represents the larger and less electronegative halogen in these compounds, while "Y" represents the smaller and more electronegative halogen.
• The number of atoms in an interhalogen molecule can be determined using the radius ratio:
• Radius Ratio=Radius of Larger Halogen Atom / Radius of Smaller Halogen 

As the radius ratio increases, the number of atoms in the molecule also increases. Among all interhalogen compounds, iodine heptafluoride (IF₇) has the highest number of atoms per molecule because it has the largest radius ratio.

3.0Structures of Interhalogen Compounds

1. Diatomic Interhalogens (XY): Diatomic interhalogens, like chlorine monofluoride (ClF), have properties between their parent halogens. The bond is ionic, and the larger halogen (X) becomes partially favourable. ClF is the lightest and colourless gas.
2. Tetra-Atomic Interhalogens (XY₃): Chlorine trifluoride (ClF₃) is a colourless gas that turns into a green liquid and a white solid. It is highly reactive and has a T-shaped, planar structure with two lone pairs in equatorial positions.
3. Hexa-Atomic Interhalogens (AX₅): Chlorine pentafluoride (ClF₅) is a colourless gas with a trigonal bipyramidal structure. It forms by reacting ClF₃ with fluorine and is highly reactive with water and metals.
4. Octa-Atomic Interhalogens (AX₇): Iodine heptafluoride (IF₇) is a colourless gas with a pentagonal bipyramidal structure. It is chemically inert and mimics sulfur hexafluoride (SF₆) due to its lack of lone pairs in the valence shell. It is the only interhalogen with iodine bonding to seven fluorine atoms.

4.0Preparation of Interhalogen Compounds

Interhalogen compounds are typically prepared using two main methods:

1. Direct Mixing of Halogens: Halogen atoms can react directly to create an interhalogen compound. For instance, equal volumes of chlorine and fluorine combine at 473 K to produce chlorine monofluoride (ClF).

                                       Cl_2​+F_2​→2ClF(at 473 K)

                                        I₂+Cl₂→2ICl

2. Reaction of Halogens with Lower Interhalogen Compounds: A halogen atom can also react with a lower interhalogen compound to form a higher interhalogen compound. For example, fluorine reacts with iodine pentafluoride (IF₅) at 543 K to form iodine heptafluoride (IF₇).

                                         IF_5​+F₂​→IF₇​  (at 543 K)

5.0Physical Properties

Interhalogen compounds can exist in all three states of matter:

• Gases: ClF, BrF, ClF₃, IF₇
• Liquids: BrF₃, BrF₅, IF₅
• Solids: ICl, IBr, IF₃, ICl₃

The colour of interhalogen compounds generally depends on their molecular mass, with heavier halogens typically forming coloured compounds.

Interhalogen compounds are diamagnetic because all of their electrons are paired.

The thermal stability of interhalogen compounds increases with the size of the central atom. The typical order of thermal stability is:

IF_7​ > BrF₅ ​> ClF₃ ​> IF_3​ > IBr > BrCl

Additionally, two halogens with similar electronegativities can interact to form interhalogens of the types XY and XY₃.

6.0Chemical Properties

• Interhalogen compounds are generally more reactive than diatomic halogen molecules due to the weaker bonds in interhalogens (except for F₂).
• When interhalogens react with water, they typically form halide and oxyhalide ions. This reaction can be particularly vigorous, as seen with bromine pentafluoride (BrF₅):

                        ICl + H₂O → HCl + HOI

                        BrF_{5 ​}+ 3H_2​O → 5HF + HBrO_3​

• When exposed to silicon dioxide or metal oxides, interhalogens react to form free diatomic halogens and diatomic oxygen.
• Most interhalogens are halogen fluorides. All except three (IBr, AtBr, and AtI) are halogen chlorides among the remaining interhalogens.
• Iodine can form bonds with up to seven fluorine atoms, while chlorine and bromine can bond with five other halogen atoms.

7.0Uses of Interhalogen Compounds

• Non-Aqueous Solvents: Interhalogen compounds are used as solvents in non-aqueous environments.
• Catalysts: They serve as catalysts in certain chemical reactions.
• Production of Uranium Hexafluoride: Chlorine trifluoride (ClF₃) and bromine trifluoride (BrF₃) are used in the production of uranium hexafluoride (UF₆), which is essential for the enrichment of uranium-235:

                       U(s) + 3ClF₃​(l) → UF₆​(g) + 3ClF(g)

• Fluorinating Agents: Interhalogen compounds are employed as fluorinating agents in various chemical processes.