Haloalkanes and Haloarenes Classification: DDT stands for dichloro-diphenyl-trichloroethane, which is the first chlorinated organic insecticide. It is a chlorinated hydrocarbon that poses a serious threat to the environment. Compounds like DDT can be grouped under a general category of hydrocarbons in which hydrogen atom(s) of a hydrocarbon, aliphatic or aromatic, are replaced by a halogen atom(s). This replacement results in the formation of alkyl halide (haloalkane) and aryl halide (haloarene), respectively. In this article, let’s discuss the Classification of Haloalkanes and Haloarenes in detail.

Haloalkanes and Haloarenes

Halogens are the Modern Periodic Table’s group \(17\left( {{\rm{VIIA}}} \right)\) elements, which consist of five chemically related elements: fluorine \(\left( {\rm{F}} \right)\), chlorine \(\left( {\rm{Cl}} \right)\), bromine \(\left( {\rm{Br}} \right)\), iodine \(\left( {\rm{I}} \right)\), and astatine \((\text {At})\). The halogen atom(s) is bonded to the carbon atom of an alkyl group in haloalkanes. Haloarenes, on the other hand, have halogen atoms bonded to the carbon atoms of an aryl group.

Haloalkane

A haloalkane is an alkane with one or

more halogens bonded to the \(\text {sp}^{3}\) hybridised carbon atom of the alkyl group. These are generally represented as \(\mathrm{R}-\mathrm{X}\), where \(\mathrm{R}\) represents the alkyl group, and \(\text {X}\) denotes halogen atom, which can be fluorine \(\left( {\rm{F}} \right)\), chlorine \(\left( {{\rm{Cl}}} \right)\), bromine \(\left( {\rm{Br}} \right)\), or iodine \(\left( {\rm{I}} \right)\).

Classification of Haloalkanes

Haloalkanes can be grouped into different classes depending on the number of halide groups attached to the chain of carbon atoms and to which carbon atom the halide is attached. 

1. Classification based on which carbon atom is bonded to the halide group.

Primary (1°) Halides

If the halide (X) group is attached to a primary carbon atom or \(1^{\circ}\) carbon atom, which is bonded to only one other carbon atom, then the compound is a primary halide. The halogen group is attached to the carbon atom that has only one neighbouring alkyl group. Some examples of primary halides are:

Methyl chloride, \(\mathrm{CH}_{3} \mathrm{Cl}\), also known as Chloromethane, is classified as primary halide even though no alkyl groups are attached to the carbon atom carrying the halogen group.

Secondary (2°) halide

If the halide(X) group is attached to a secondary carbon atom or \(2^{\circ}\) carbon atom, which is bonded to two other carbon atoms, then the compound is a secondary halide. In this type of alkyl halide, the halogen group is attached to the carbon atom with two neighbouring alkyl groups, which may be the same or different. Examples include the following:

Tertiary (3°) Halide

If the halide(X) group is attached to a tertiary carbon atom or \(3^{\circ}\) carbon atom, which is bonded to three other carbon atoms, then the compound is a tertiary halide. In this type of alkyl halide, the halogen group is attached to the carbon atom with three neighbouring alkyl groups, which may be any combination of the same or different groups. Examples of tertiary halides are given below:

2. Classification based on the number of halogen groups attached to the carbon chain.

Monohaloalkane

Haloalkanes that have only one halogen group attached to the carbon chain are known as monohaloalkanes.

Monohaloalkanes are further classified according to the type of hybridisation of the carbon atom to which the halogen group is attached.

(1) Compounds containing \(\text {C}\left(\text {sp}^{3}\right)-\text {X}\) bond

1. Alkyl Halides

In these halides, the halogen \((\mathrm{X})\) group is attached to a \(\text {sp}^{3}\) hybridised carbon atom of the alkyl group and are represented as \(\text {R}-\text {X}\). They form a homologous series represented by \({{\text{C}}_{\text{n}}}{{\text{H}}_{2{\text{n}} + 1}}{\text{X}}\). They are further classified as primary, secondary, and tertiary halides in which the halogen group is attached to primary, secondary, and tertiary carbon atoms, respectively.

2. Allylic Halides

In these halides, the halogen group is attached to a \(\text {sp}^{3}\)- hybridised carbon atom next to the carbon-carbon double bond, i.e., allylic carbon. Allylic halides may be primary, secondary, and tertiary, depending on which carbon atom is attached with the halogen atom.

(2) Compounds containing \({\text{C}}\left( {{\text{s}}{{\text{p}}^2}}\right) – {\text{X}}\) bond (Vinylic halides)

In these monohaloalkanes, the halogen group is attached to a \(\text {sp}^{2}\) hybridised carbon atom,
e.g., vinylic carbon. These halides are also known as vinylic halides.

Dihaloalkane

Haloalkanes that have two halogen groups in their molecules are called dihaloalkanes. Dihaloalkanes can be categorised into Geminal dihalides and Vicinal dihalides.   

1. Geminal dihalides: These dihaloalkanes have the halogen atom on the same carbon atom. For example – \(2,2\)-Dichloro propane is a Gem-dihalide, as shown below.

2. Vicinal dihalides: These dihaloalkanes have the halogen atom on the neighboring carbon atoms. For example – \(2,3\)-Dibromo ethane is a Vic-dihalide, as shown below.

Trihaloalkane

Haloalkanes that have three same or different halogen atoms in their molecules are known as trihaloalkanes. For example:

Haloarenes

A haloarene is an aromatic compound with one or more halogens bonded to the \(\text {sp}^{2}\) hybridised carbon atom of the aryl group. These are generally represented as \(\text {Ar}-\text {X}\), where \(\text {Ar}\) represents the aryl group, and \(\text {X}\) denotes halogen atom, which can be fluorine \(\left( {\rm{F}} \right)\), chlorine \(\left( {\rm{Cl}} \right)\), bromine \(\left( {\rm{Br}} \right)\) or iodine \(\left( {\rm{I}} \right)\).

Classification of Haloarene

Haloarene can be grouped into different classes depending on how many and how the halogen atoms are attached to the benzene ring. 

Based on the number of halogen groups attached, haloarenes can be classified as:

Mono Haloarene

Haloarenes having only one halogen group in their molecules are called monohaloarenes.

Monohaloarenes are further classified according to the type of hybridisation of the carbon atom to which the halogen group is attached.

(1) Compounds containing \({\text{C}}\left( {{\text{s}}{{\text{p}}^3}}\right) – {\text{X}}\) bond

Benzylic halides: In these halides, the halogen group is attached to a \(\text {sp}^{3}\) -hybridised carbon atom next to an aromatic ring. The halogen atom is present in the side chain attached to an aromatic ring. Benzylic halides may be primary, secondary, and tertiary halides.

(2) Compounds containing \({\text{C}}\left( {{\text{s}}{{\text{p}}^2}}\right) – {\text{X}}\) bond 

Aryl Halides: These monohaloarenes have a halogen group attached to a \(\text {sp}^{2}\) hybridised carbon atom of an aromatic ring.

Dihaloarene

Haloarenes that have two same or different halogen groups in their molecules are called dihaloarenes. For example:

Trihaloarene

Haloarenes that have three same or different halogen groups in their molecules are called trihaloarenes. For example:

Summary

Haloalkanes and haloarenes are organic molecules with a wide range of uses in industry and everyday life. They are employed as starting materials for the synthesis of a variety of organic compounds as well as solvents for non-polar molecules. These compounds can be classified into numerous classes based on the halogen atom’s bonding to the aryl or alkyl chain.

FAQs on Classification of Haloalkanes and Haloarenes

Q.1. What is the difference between haloalkanes and Haloarenes?
Ans: A haloalkane is an alkane with one or more halogens bonded to the \(\text {sp}^{3}\) hybridised carbon atom of the alkyl group. These are generally represented as \(\mathrm{R}-\mathrm{X}\), where \(\mathrm{R}\) represents the alkyl group, and \(\text {X}\) denotes halogen atom, which can be fluorine \(\left( {\rm{F}} \right)\), chlorine \(\left( {\rm{Cl}} \right)\), bromine \(\left( {\rm{Br}} \right)\), or iodine \(\left( {\rm{I}} \right)\).
A haloarene is an aromatic compound with one or more halogens bonded to the \(\text {sp}^{2}\) hybridised carbon atom of the aryl group. These are generally represented as \(\text {Ar}-\text {X}\), where \(\text {Ar}\) represents the aryl group, and \(\text {X}\) denotes halogen atom, which can be fluorine \(\left( {\rm{F}} \right)\), chlorine \(\left( {\rm{Cl}} \right)\), bromine \(\left( {\rm{Br}} \right)\) or iodine \(\left( {\rm{I}} \right)\).

Q.2. What are the uses of haloalkanes and haloarenes?
Ans: Haloalkanes and haloarenes are used for many industrial and day-to-day purposes. Some of its uses are:
1. As flame retardants, solvents for non-polar compounds.
2. As propellants, refrigerants, fire extinguishants.
3. They are used for medicinal purposes, such as chloramphenicol for the treatment of typhoid fever.
4. Chloroquine is used for the treatment of people suffering from malaria.
5. DDT is utilised as an insecticide.

Q.3. How do you classify alkyl halides?
Ans: Alkyl halides can be classified into three main groups, namely primary \(\left(1^{\circ}\right)\), secondary \(\left(2^{\circ}\right)\),
Primary or \(\left(1^{\circ}\right)\) alkyl halides consist of the halogen atom attached to the carbon that is further attached to only one adjacent carbon atom. For example- Ethyl chloride \(\left( {{\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{Cl}}} \right)\).
Secondary or \(\left(2^{\circ}\right.)\) alkyl halides consist of the halogen atom attached to the carbon that is further attached to two neighbouring carbon atoms. For example- \(2\)-Bromopropane \(\left( {{\rm{C}}{{\rm{H}}_3}{\rm{CH}}\left( {{\rm{Br}}} \right){\rm{C}}{{\rm{H}}_3}} \right)\)
Tertiary or \(\left(3^{\circ}\right)\) alkyl halides consist of the halogen atom attached to the carbon that is further attached to three neighbouring carbon atoms. For example-\(2\)-Chloro-\(2\)-methylpropane \(\left( {{\rm{C}}{{\rm{H}}_3}{\rm{C}}\left( {{\rm{Cl}}} \right)\left( {{\rm{C}}{{\rm{H}}_3}} \right){\rm{C}}{{\rm{H}}_3}} \right).\)

Q.4. How do you classify primary, secondary, and tertiary haloalkanes?
Ans: Haloalkanes are classified as primary \(\left(1^{\circ}\right)\), secondary \(\left(2^{\circ}\right)\), and tertiary \(\left(3^{\circ}\right)\) haloalkanes based on which carbon atom is bonded to the halide group. In all these types, the carbon atom is \(\text {sp}^{3}\) hybridised.
Primary or \(\left(1^{\circ}\right)\) haloalkane consists of the halogen atom attached to the carbon that is further attached to only one adjacent carbon atom. For example- Ethyl chloride \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}\right)\).
Secondary or \(\left(2^{\circ}\right)\) haloalkane consists of the halogen atom attached to the carbon that is further attached to two neighbouring carbon atoms. For example- \(2\)-Bromopropane \(\left( {{\rm{C}}{{\rm{H}}_3}{\rm{CH}}\left( {{\rm{Br}}} \right){\rm{C}}{{\rm{H}}_3}} \right).\)
Tertiary or \(\left(3^{\circ}\right)\) haloalkane consists of the halogen atom attached to the carbon that is further attached to three neighbouring carbon atoms. For example-\(2\)-Chloro-\(2\)-methylpropane \(\left(\mathrm{CH}_{3} \mathrm{C}(\mathrm{Cl})\left(\mathrm{CH}_{3}\right) \mathrm{CH}_{3}\right).\)

Q.5. Which is more reactive, haloalkanes or haloarenes?
Ans: Haloarenes are more stable than haloalkane; hence, these are less reactive than haloalkanes. The enhanced stability of haloarenes is dependent on the polarity (less than haloalkanes) of the carbon-halogen bond, \({\text{s}}{{\text{p}}^2}\)  hybrid state of the carbon atom, and the presence of resonance in haloarenes.

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