Pseudo Halide Ions and Pseudo Halogens: As the name suggests, Pseudo halogens are polyatomic molecules that resemble halogens but are not true halogens. They can be used as a substitute for halogen in different chemical compounds. Pseudo halides are the ions of these halogens that have properties like halide ions. Example of some Pseudo halides includes cyanides \(\left( {{\rm{C}}{{\rm{N}}^{\rm{ – }}}} \right)\), isocyanides \(\left( {{\rm{N}}{{\rm{C}}^{\rm{ – }}}} \right)\), Cyanate \(\left( {{\rm{OC}}{{\rm{N}}^ – }} \right)\), isocyanate \(\left( {{\rm{NC}}{{\rm{O}}^ – }} \right)\), etc.

They have resemblance with true halides such as \({{\rm{F}}^ – },{\rm{C}}{{\rm{l}}^ – },{\rm{B}}{{\rm{r}}^ – },{{\rm{I}}^ – }\). Mostly, these pseudo halogens contain nitrogen. They are mainly used as an oxidizing agent and are also used to determine unsaturation in organic compounds. Mostly pseudo halides are weak Lewis base and bear \(-1\) charge. Heavy metal pseudo halides have greater reactivity and are covalent in nature.

What are Pseudo Halide Ions and Pseudo Halogens?

Pseudo halogens are commonly known as ‘Fake halogens’ as they are polyatomic analogues of halogens that resemble the true halogens. They act as a substitute for halogens in various classes of chemical compounds.

Pseudo halogens also exist as pseudo halogen molecules. These are inorganic molecules of the general form \({\rm{Ps – Ps}}\) or \({\rm{Ps – X}}\) (where \({\rm{Ps}}\) represents a pseudo halogen group). They form dipseudohalogens as well as inter-pseudohalogens. Examples of pseudo halogens include cyanide, cyanate, thiocyanate, azide, etc.

Pseudo Halides
When pseudo halogens form, univalent anions which form hydracids with hydrogen and form insoluble salts with \({\rm{Ag}}\) (Silver) are known as pseudo halides. For example, cyanides, cyanates, isocyanates, thiocyanates and isothiocyanates, azides, etc.

What are Halogens?

Halogens are chemical elements located in group \(17\) of the periodic table. The five members of the Halogen family are: Fluorine \({\rm{(F)}}\), Chlorine \({\rm{(Cl)}}\), Bromine \({\rm{(Br)}}\), Iodine \({\rm{(I)}}\), and Astatine \({\rm{(At)}}\). These halogens are represented by the general symbol \({\rm{X}}\). The term halogen is derived from its nature of “salt-producing”.

This is because these chemical elements can react with metals to form anions and produce a wide variety of salts. These elements present in group \(17\) are the only group in the periodic table that contains elements that exist in all three states of matter at standard temperature and pressure. For example, fluorine and chlorine exist as gases, bromine exist as a liquid, and iodine exists as a solid at room temperature. These elements are highly reactive.

By obtaining one electron to their outermost orbital, they easily form anion having \(-1\) charge because these elements have one unpaired electron in their valence shell of p orbital. Halogens form a wide range of compounds that include hydrogen halides, metal halides, interhalogen compounds (contains two halogens), organohalogen compounds (halogens bonded to organic molecules), etc.

Difference Between Halogen and Pseudo Halogen

The differences between halogen and pseudo halogen are as follows:

Halogen	Pseudo Halogen
1. Halogens are comprised of group \(17\) elements of the periodic table.	1. Pseudo halogens are false halogens as they are formed by the combination of different elements.
2. Halogens form salts by forming anions.	2. Pseudo halogens have the chemical characteristics of halogens as the formation of covalent compounds and complexes like halogens.
3. Halogens are elements.	3. Pseudo halogens are chemical compounds.
4. Halogens alone are monoatomic species.	4. Pseudo halogens are polyatomic species.

Similarities Between Halogen and Pseudo Halogen

The similarities between halogen and pseudo halogens are as follows:

• Both halogens and pseudo halogens are chemical species that have similar chemical behaviour.
• Both Halogens and Pseudohalogens are capable of forming strong acids.
• Halogens and Pseudohalogens undergo reactions with metals to form salts.
• Ionic species of both types, i.e., halides and pseudo halides, carry a unit negative electrical charge \((-1)\).

Types of Pseudo Halogens

The different types of pseudo halogens are as follows:

• Pseudohalogen molecules are inorganic molecules formed by the combination of two or more chemical elements. Based on the structure, they are of two types:
  a. Symmetrical Pseudohalogen molecules such as cyanogen \(\left( {{{({\rm{CN}})}_2}} \right)\).
  b. Asymmetrical Pseudohalogen molecules such as \({\rm{BrCN}}\).
• Pseudohalide anions have a negative charge. For example, cyanide ion.
• Pseudohalides are present in inorganic acids such as \({\rm{HCN}}\).
• Pseudohalides are also present as ligands in coordination compounds such as ferricyanides.
• Pseudohalides are also present as functional groups in organic molecules such as the nitrile group.
• These compounds can act as chemical components that resemble halogens, and the presence of single or double bonds do not affect their chemical behaviour.
• They are able to form strong acids which resemble halogenic acids of \({\rm{HX}}\) type (for example, \({\rm{HCo}}{({\rm{CO}})_4}\) resembles \({\rm{HCl}}\)).
• They can also react with metals to form compounds that resemble MX types of halogenic compounds (for example \({\rm{Na}}{{\rm{N}}_3}\) resembles \({\rm{NaCl}}\)).

Examples of Pseudo Halide Ions

Some of the example of pseudo halide ions are as follows:

Pseudo Halide	Structural Formula
Cyanide	\({\rm{C}}{{\rm{N}}^{\rm{ – }}}\)
Isocyanide	\({\rm{N}}{{\rm{C}}^{\rm{ – }}}\)
Cyaphide	\({\rm{C}}{{\rm{P}}^{\rm{ – }}}\)
Hydroxide	\({\rm{O}}{{\rm{H}}^{\rm{ – }}}\)
Hydrosulfide	\({\rm{-S}}{{\rm{H}}^{\rm{ – }}}\)
Cyanate	\({\rm{ [-O – C}} \equiv {\rm{N]}}\)
Isocyanate	\({\rm{R – N = C = O}}\)
Selenocyanate	\({\rm{R – Se – C}} \equiv {\rm{N}}\)
Thiocyanate	\({\rm{R – S – C}} \equiv {\rm{N}}\)
Isothiocyanate	\({\rm{R – N = C = S}}\)
Azide	\({\rm{N}}_{\rm{3}}^{\rm{ – }}\)
Ttellurocyanate	\({\rm{R – Te – C}} \equiv {\rm{N}}\)

Summary

Pseudohalogens are chemical species having a resemblance with halogens. The primary difference between halogens and pseudo halogens is that the halogens are present in the group \(17\) elements in the modern periodic table, whereas the pseudohalogens are combinations of different chemical elements that have similar chemical characteristics to that of halogens. Pseudo halogens are polyatomic molecules that resemble halogens but are not true halogens. Hence, they are also called fake halogens. They can be used as a substitute for halogen in different chemical compounds.

Pseudo halides are the anions of these halogens that have properties like halide ions. Example of some Pseudo halides includes cyanides \(\left( {{\rm{C}}{{\rm{N}}^{\rm{ – }}}} \right)\), isocyanides \(\left( {{\rm{N}}{{\rm{C}}^{\rm{ – }}}} \right)\), Cyanate \(\left( {{\rm{OC}}{{\rm{N}}^{\rm{ – }}}} \right)\), isocyanate \({\rm{(R – NCO)}}\) etc. They have resemblance with true halides such as \({{\rm{F}}^{\rm{ – }}}{\rm{,C}}{{\rm{l}}^{\rm{ – }}}{\rm{,B}}{{\rm{r}}^{\rm{ – }}}{\rm{,}}{{\rm{I}}^{\rm{ – }}}.\) Mostly, these pseudo halogens contain nitrogen. Pseudo halides are mainly used as an oxidizing agent and are also used to determine unsaturation in organic compounds. Pseudo halides are weak Lewis base and bear \(-1\) charge. Though there are few differences between halogens and pseudo halogens, still they are interrelated to each other.

FAQs on Pseudo Halide Ions and Pseudo Halogens

Q.1. Define Pseudo Halogens.
Ans: Pseudo halogens are commonly known as ‘Fake halogens’ as they are polyatomic analogues of halogens that resemble the true halogens. They act as a substitute for halogens in various classes of chemical compounds. Pseudo halogens also exist as pseudo halogen molecules.

These are inorganic molecules of the general form \({\rm{Ps – Ps}}\) or \({\rm{Ps – X}}\) (where \({\rm{Ps}}\) represents a pseudo halogen group). They form dipseudohalogens as well as inter-pseudohalogens. Examples of pseudo halogens include cyanide, cyanate, thiocyanate, azide, etc.

Q.2. Define Pseudo halides.
Ans: When pseudo halogens form, univalent anions which form hydracids with hydrogen and form insoluble salts with \({\rm{Ag}}\) (Silver) are known as pseudo halides. For example, cyanides, cyanates, isocyanates, thiocyanates and isothiocyanates, azides, etc. 

Q.3. What is the difference between halogen and pseudo halogen?
Ans: The basic difference between halogen and pseudo halogen are: Halogens are comprised of group \(17\) elements of the periodic table. While Pseudo halogens are false halogens as they are formed by the combination of different elements. Halogens are elements, and Pseudo halogens are chemical compounds.

Halogens form salts by forming anions, and Pseudo halogens have the chemical characteristics of halogens as the formation of covalent compounds and complexes like halogens. Halogens alone are monoatomic species, and Pseudo halogens are polyatomic species.

Q.4. What are some examples of pseudohalides?
Ans: Some examples of pseudohalides are as follows: cyanides \(\left( {{\rm{C}}{{\rm{N}}^{\rm{ – }}}} \right)\), isocyanides \(\left( {{\rm{N}}{{\rm{C}}^{\rm{ – }}}} \right)\), Cyanate \(\left( {{\rm{OC}}{{\rm{N}}^{\rm{ – }}}} \right)\), isocyanate \({\rm{(R – NCO)}}\), cyaphide \(\left( {{\rm{C}}{{\rm{P}}^{\rm{ – }}}} \right)\), hydrosulfide \({\rm{( – SH)}}\), selenocyanate \({\rm{( – Se – C}} \equiv {\rm{N)}}\), etc.

Q.5. Discuss different types and uses of Pseudo halogens and pseudo halides.
Ans: Based on the structure, Pseudohalogens are of two types: (i) Symmetrical Pseudohalogen molecules such as cyanogen \(\left( {{{({\rm{CN}})}_2}} \right)\). (ii) Asymmetrical Pseudohalogen molecules such as \({\rm{BrCN}}\). Pseudohalide anions have a negative charge. For example, cyanide ion.

Pseudohalides are present in inorganic acids such as \({\rm{HCN}}\), as ligands in coordination compounds such as ferricyanides, as functional groups in organic molecules such as the nitrile group. These compounds can act as chemical components that resemble halogens. Pseudo halides are mainly used as an oxidizing agent and are also used to determine unsaturation in organic compounds.

Q.6. What are halogens?
Ans: Halogens are chemical elements located in group \(17\) of the periodic table. The five members of the Halogen family are: Fluorine \({\rm{(F)}}\), Chlorine \({\rm{(Cl)}}\), Bromine \({\rm{(Br)}}\), Iodine \({\rm{(I)}}\), and Astatine \({\rm{(At)}}\). These halogens are represented by the general symbol \({\rm{X}}\).

The term halogen is derived from its nature of “salt-producing”. This is because these chemical elements can react with metals to form anions and produce a wide variety of salts. These elements present in group \(17\) are the only group in the periodic table that contains elements that exist in all three states of matter at standard temperature and pressure.

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