When halogen group elements react with each other, interhalogen compounds are formed. In other terms, it’s a molecule made up of two or more distinct group 17 elements. Interhalogen chemicals are divided into four categories: The group 17 elements, Fluorine (F)(F), Chlorine (Cl)(Cl), Bromine (Br)(Br), Iodine (l)(l), and Astatine (As)(As) of the modern periodic table, are called halogens. 

The word halogen is derived from Greek words; halo means sea salt, and gen means producer. The halogen forms several compounds like hydrochloric acid, carbon tetrachloride, CFCs, etc. In this article, you will explore a special type of compound, its properties, types, etc., formed by the bonding between two or more halogens. These are called interhalogen compounds.

What are Interhalogen Compounds?

Compounds containing two or more halogens are called interhalogen compounds. Their general formula is \({\rm{X}}{{\rm{X}}_{\rm{n}}}’\) where \({\rm{X}}\) is a less electronegative halogen (halogen of larger size) while \({\rm{X’}}\) is a more electronegative halogen (halogen of smaller size) and \({\rm{n}}\) is its number.

Types of Interhalogen Compounds

The halogen exhibit four oxidation states, i.e.,\({\rm{ + 1, + 3,  + 5}}\,{\rm{or}}\,{\rm{ + 7}}{\rm{.}}\) Based on the oxidation state, halogen is divided into four types,
\({\rm{XX}}’,{\rm{X}}{{\rm{X}}_3}’,{\rm{X}}{{\rm{X}}_5}’\) and \({\rm{X}}{{\rm{X}}_7}’\)

Types	\({\rm{XX}}’\)	\({\rm{X}}{{\rm{X}}_3}’\)	\({\rm{X}}{{\rm{X}}_5}’\)	\({\rm{X}}{{\rm{X}}_7}’\)
Examples	\({\rm{ClF,BrF}}\)	\({\rm{Cl}}{{\rm{F}}_3},\,{\rm{Br}}{{\rm{F}}_3}\)	\({\rm{Cl}}{{\rm{F}}_{\rm{5}}}\)	–
	\({\rm{BrCl}}\)	\({\rm{I}}{{\rm{F}}_3}\)	\({\rm{Br}}{{\rm{F}}_{\rm{5}}}\)	–
	\({\rm{ICI,}}\,{\rm{IBr,}}\,{\rm{IF}}\)	\({\rm{IC}}{{\rm{I}}_3}\)	\({\rm{I}}{{\rm{F}}_5}\)	\({\rm{I}}{{\rm{F}}_7}\)

Nomenclature of Interhalogen Compounds

The interhalogen compounds are named halogen halides. The halogen with a positive oxidation state (i.e.,\({\rm{X}}\)) is named as such, and the halogen with the negative oxidation state \({\rm{X}}’\)

Example \(1\): The \({\rm{BrCl}}\) is named Bromine chloride.

Example \(2\): The \({\rm{IC}}{{\rm{l}}_3}\) is named Iodine trichloride.

Example \(3\): The \({\rm{I}}{{\rm{F}}_7}\) is named Iodine heptafluoride.

Preparation of Interhalogen Compounds

The interhalogen compound can be prepared by the direct reaction with halogen or by the halogenation of lower interhalogen compounds.

1. By the direct reaction of halogen: Except \({\rm{I}}{{\rm{F}}_7},\) all other interhalogen compounds can be prepared by this method.
   \({\rm{C}}{{\rm{l}}_{\rm{2}}} + {{\rm{F}}_2} \to 2{\rm{ClF,}}\,{\rm{at}}\,473\,{\rm{K}}\)
   \({\rm{C}}{{\rm{l}}_{\rm{2}}} + 3{{\rm{F}}_2} \to 2{\rm{Cl}}{{\rm{F}}_3},\,{\rm{at}}\,{\mkern 1mu} 473\,{\rm{K}}\)
   \({{\rm{I}}_2} + 3{\rm{C}}{{\rm{l}}_2} \to 2{\rm{IC}}{{\rm{l}}_3}\)
   \({\rm{B}}{{\rm{r}}_2} + 5{{\rm{F}}_2} \to 2{\rm{Br}}{{\rm{F}}_5}\)
   
2. By halogenation of lower interhalogen compounds: The lower interhalogen reacts with halogen to give higher interhalogens.
   \({\rm{ClF}}\, + \,{{\rm{F}}_2} \to {\rm{Cl}}{{\rm{F}}_3}\)
   \({\rm{Cl}}{{\rm{F}}_3} + {{\rm{F}}_2} \to {\rm{Cl}}{{\rm{F}}_5}{\mkern 1mu} \,{\rm{at}}{\mkern 1mu} \,350\,^\circ {\rm{C}}\)
   \({\rm{Br}}{{\rm{F}}_3} + {{\rm{F}}_2} \to {\rm{Br}}{{\rm{F}}_5}{\mkern 1mu} \,{\rm{at}}\,{\mkern 1mu} 473\,{\rm{K}}\)
   \({\rm{I}}{{\rm{F}}_5} + {{\rm{F}}_2} \to {\rm{Br}}{{\rm{F}}_5}{\mkern 1mu} \,{\rm{at}}\,{\mkern 1mu} 473\,{\rm{K}}\)

Properties of Interhalogen Compounds

1. Physical state: The interhalogen compound may exist as gases \({\rm{(ClF, BrF, Cl}}{{\rm{F}}_3},{\rm{I}}{{\rm{F}}_7})\), liquids \(({\rm{Br}}{{\rm{F}}_3},\,{\rm{Br}}{{\rm{F}}_5},\,{\rm{I}}{{\rm{F}}_5})\) and solids \(({\rm{ICl}},\,{\rm{IBr}},\,{\rm{I}}{{\rm{F}}_3},\,{\rm{IC}}{{\rm{l}}_3})\)
2. Colour: The compound containing heavier halogen are coloured. The colour depends on the increase in molar mass.
3. Nature of compounds: These are covalent compounds due to the small difference in electronegativity.
4. Magnetic properties: These are diamagnetic in nature since all the electrons are paired in interhalogen compounds.
5. Thermal stability: The stability of the interhalogen compound increases as the size of the central atom increases.
   Thermal stability: \({\rm{IF}} > {\rm{BrF}} > {\rm{CIF}} > {\rm{ICI}} > {\rm{IBr}} > {\rm{BrCI}}\)
   Electronegativity difference: \({\rm{1}}{\rm{.5}}\,{\rm{ > }}\,{\rm{1}}{\rm{.2}}\,\,{\rm{ >  1}}{\rm{.0}}\,\,{\rm{ >  0}}{\rm{.5}}\,\,{\rm{ > }}\,{\rm{0}}{\rm{.3}}\,\,{\rm{ > }}\,{\rm{0}}{\rm{.2}}\)
6. Oxidising nature: The interhalogen compounds are strong oxidising agents. In these compounds, the smaller and the more electronegative atom is assigned a negative oxidation number.
7. Hydrolysis: These compounds on hydrolysis yield halogen acid and oxohalogen acid. The larger central atom forms the oxoacids.
   Example: \({\rm{ICl}}\,{\rm{ + }}\,{{\rm{H}}_2}{\rm{O}}\, \to \,{\rm{HCl}}\, + {\rm{HOI}}\) (Hypoiodous acid)
8. Reactivity: The interhalogen compounds, i.e.,\({\rm{X – X}}’\) has a relatively weaker bond thanbond in halogen. The overlapping of orbitals of two dissimilar atoms in an interhalogen compound is less effective than the overlapping of orbitals of similar atoms in halogens. Therefore, interhalogens are more reactive than halogen (except \({{\rm{F}}_2}\)) The sequence of reactivity of various interhalogen compounds decreases in the following order: \({\rm{Cl}}{{\rm{F}}_3} > {\rm{Br}}{{\rm{F}}_5} > {\rm{I}}{{\rm{F}}_7} > {\rm{ClF}} > {\rm{Br}}{{\rm{F}}_3} > {\rm{I}}{{\rm{F}}_{\rm{5}}} > {\rm{BrF}} > {\rm{I}}{{\rm{F}}_3} > {\rm{IF}}.\)
9. Ionisation: Interhalogen compounds often ionise in solution or in the liquid state.
   \(2{\rm{ICl}} \mathbin{\lower.3ex\hbox{$\buildrel\textstyle\leftharpoonup\over{\smash{\rightharpoondown}}$}} \,{{\rm{I}}^ + } + {\rm{IC}}{{\rm{l}}_2}^ – \)
   \(2{\rm{IC}}{{\rm{l}}_3} \mathbin{\lower.3ex\hbox{$\buildrel\textstyle\leftharpoonup\over{\smash{\rightharpoondown}}$}} \,{\rm{IC}}{{\rm{l}}_2}^ +  + {\rm{IC}}{{\rm{l}}_4}^ – \)
10. Hydrolysis: Interhalogen compounds on hydrolysis give halide and oxohalide ion. The oxohalide ion is always formed from the larger halogen.

\({\rm{ICl}}\,{\rm{ + }}\,{{\rm{H}}_2}{\rm{O}}\, \to \,{\rm{HCl}}\,{\rm{ + }}\,{\rm{HOl}}\,{\rm{(Hypoiodous}}\,\,{\rm{acid)}}\)
\({\rm{Br}}{{\rm{F}}_5} + 3{{\rm{H}}_2}{\rm{O}}\, \to 5{\rm{HF}}\,{\rm{ + }}\,{\rm{HBr}}{{\rm{O}}_3}({\rm{Bromic}}\,{\rm{acid}})\)

Structures

The interhalogen compounds are covalent in nature. The molecular structure of interhalogen compounds can be explained on the basis of \({\rm{VSEPR}}\) theory as follows.

1. The \({\rm{X}}{{\rm{X}}_3}’\) compound shows \({\rm{s}}{{\rm{p}}^{\rm{3}}}{\rm{d}}\) hybridisation and has a trigonal bipyramidal arrangement.
2. The \({\rm{X}}{{\rm{X}}_5}’\) compound shows \({\rm{s}}{{\rm{p}}^{\rm{3}}} {{\rm{d}}^2}\) hybridisation and has an octahedral arrangement.
3. The \({\rm{I}}{{\rm{F}}_7}\) compound shows \({\rm{s}}{{\rm{p}}^3}{{\rm{d}}^3}\) hybridisation and has a pentagonal bipyramidal arrangement.

Uses

1. Interhalogen compounds are used as non-aqueous solvents.
2. These are strong oxidising agents.
3. These are used as a reagent for producing mixed halohydrocarbons and derivatives.
4. It used in medicines.
5. It is used as an oxidiser for rocket propellants.
6. The \({\rm{Cl}}{{\rm{F}}_3}\) and \({\rm{Br}}{{\rm{F}}_{\rm{3}}}\) are used for the production of \({\rm{U}}{{\rm{F}}_6}\) during the enrichment of uranium – \(235,\) \(\left( {{}_{92}^{235}{\rm{U}}} \right).\)
7. These are used in separating the fission products from spent fuel rods. Plutonium and most of the fission products form non-volatile tetrafluorides like \({\rm{Pu}}{{\rm{F}}_4}.\)

\(4{\rm{Cl}}{{\rm{F}}_3}\, + \,3{\rm{Pu}}\, \to \,3{\rm{Pu}}{{\rm{F}}_4} + \,2{\rm{C}}{{\rm{l}}_2}\)

Summary

In this article, you have acquired knowledge about a special type of halogen compound, i.e., interhalogen compound. With this knowledge, you can recall the different methods of preparation of interhalogen compounds, their types, properties, uses, etc.

FAQs

Q.1. What are the different types of interhalogen compounds?
Ans: Based on the oxidation state \(( + 1,{\mkern 1mu} + 3,{\mkern 1mu} + 5{\mkern 1mu} \,{\rm{or}}\,{\mkern 1mu} + 7)\) of halogen is divided into four types, i.e., \({\rm{XX’, X}}{{\rm{X}}_{\rm{3}}}’,\,{\rm{X}}{{\rm{X}}_{\rm{5}}}’\,{\rm{and}}\,{\rm{X}}{{\rm{X}}_{\rm{7}}}’.\)

Q.2. Why are interhalogen compounds diamagnetic?
Ans: The interhalogen compounds are diamagnetic in nature because all the electrons are paired in interhalogen compounds.

Q.3. What are the uses of interhalogen compounds?
Ans: The interhalogen compounds are used as non-aqueous solvents, strong oxidising agents, reagents for producing mixed halohydrocarbons and derivatives, oxidisers for rocket propellants, etc.

Q.4. Why are interhalogens formed?
Ans: The interhalogen compounds are formed due to the difference in the electronegativity of bonding halogen atoms.

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