Classifying of Haloalkanes and Haloarenes

Haloalkanes and haloarenes are hydrocarbons in which halogen atoms replace one or more hydrogen atoms. Haloalkanes are derived from open chained hydrocarbons, also known as alkanes. Haloarenes are derived from aromatic hydrocarbons.

The Modern Periodic Table’s group 17(VIIA) elements are halogens, which are made up of five chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). In haloalkanes, the halogen atoms are bonded to the carbon atom of an alkyl group. On the other hand, haloarenes have halogen atoms bonded to aryl group carbon atoms.

What are Haloalkanes?

• Haloalkanes are elements that contain hydrocarbons composed of aliphatic alkanes with one or more hydrogen atoms replaced by halogens.

• It is an alkane that contains one or more halogens bonded to the alkyl group’s sp3 hybridised carbon atom.
• These are commonly represented as R–X, where R denotes the alkyl group (CH3, C2H5), and X denotes the halogen atom (Cl, F, Br).
• Example: CH3Cl (Methyl Chloride)
• These are saturated organic compounds with single bonds connecting all chemical bonds to the carbon atom and a single carbon atom to the halogen atom.
• Aliphatic alkanes undergo free radical halogenation to create haloalkanes.
• They have no odour and form due to SN2 substitution reactions.

Classification of Haloalkanes

We can group haloalkanes in different types based on the number of halides group attached to the carbon atom in the chain and the carbon to which the halide is attached:

Classification based on which carbon atom the halide is attached to

Primary halide (1°)

The compound is a primary halide if the halide (X) group is attached to a primary carbon atom or one carbon atom bonded to only one other carbon atom. In this, the halogen group is attached to a carbon atom with only one alkyl group nearby. Primary halides include the following compounds: CH3CH2-Br (Bromoethane), CH3Cl (Chloromethane) etc.

Secondary halide (2°)

A compound is classified as a secondary halide if the halide (X) group is attached to a secondary carbon atom, i.e., the X group is attached to a carbon atom attached to the other two carbon atoms. In this, the halogen group is attached to a carbon with two same or different alkyl groups in its neighbour. Examples are CH3CHBrCH3 (2-Bromo Propane), CH3CHBrCH2CH3 (2-BromoButane), etc.

Tertiary halide (3°)

A compound is classified as a tertiary halide if the halide (X) group is attached to a tertiary carbon atom, i.e., the X group is attached to a carbon atom attached to the other three carbon atoms. The halogen group is attached to a carbon with three same or different alkyl groups in its neighbour. Examples are (CH₃)₃CCl (2-Chloro 2-Methylpropane), (CH₃)₃CBr (2-Bromo-2methylbutane), etc.

Classification based on the number of halogen atoms attached to the chain of carbon

Mono haloalkane

These are haloalkanes with only one halogen group attached to the carbon atom. Mono haloalkanes are further grouped based on the type of halogen-carbon bond:

C(sp3)-X bond

• Alkyl halide: In this type of haloalkane, the halogen (X) group is attached to the carbon of the alkyl group, which is sp3-hybridised. It is represented by R–X.
• Allylic halide: In this type of haloalkane, the halogen (X) group is attached to the sp3-hybridised carbon atom next to the carbon-carbon double bond.

C(sp2) bond

In this type of haloalkane, the halogen (X) group is attached to carbon of the alkyl group, which is sp2-hybridised.

Dihaloalkane

These are haloalkanes with two halogen groups attached to the carbon atom. Dihaloalkanes are further grouped based on the type of halogen-carbon bond:

Geminal dihalides: The halogen atom is attached to the same carbon atom.

Vicinal dihalides: The halogen atom is attached to carbon atoms which are neighbours.

Trihaloalkane

These are haloalkanes with two halogen groups attached to the carbon atom.

What are Haloarenes?

• Haloarenes are hydrocarbons that contain an aromatic ring or rings and one or more hydrogen atoms that halogens have replaced.
• The halogen atom is attached to the alkyl group’s sp3-hybridised carbon atom.
• Examples: chlorobenzene and bromobenzene
• Direct halogenation of aromatic rings results in haloarenes.
• They have a pleasant fragrance.

Classification of Haloarenes

We can group halogen atoms based on the following factors:

Number of halogen groups attached to the benzene ring

Mono haloarene

When only one halogen group is attached to the benzene ring, it is called a mono haloarene.

Benzylic halide: In this type of haloalkane, the halogen (X) group is attached to carbon next to an aromatic ring which is sp3-hybridised.

Aryl halide: In this type of haloalkane, the halogen (X) group is attached to the carbon of an aromatic ring which is sp2-hybridised.

Dihaloarene

When two same or different halogen groups are attached to the benzene ring, it is called mono haloarene.

Trihaloarene

When three same or different halogen groups are attached to the benzene ring, it is called mono haloarene.

Conclusion

Haloalkanes and haloarenes are organic molecules with numerous applications in industry and daily life. They are used as starting materials in synthesising a wide range of organic compounds and solvents for nonpolar molecules. The halogen atom’s bonding to the aryl or alkyl chain allows these compounds to be classified into numerous classes.