Bihar Board - Class 12 Chemistry - Chapter 10: Haloalkanes and Haloarenes Handwritten Notes

Haloalkanes and haloarenes are organic compounds in which one or more hydrogen atoms are replaced by halogen atoms (fluorine, chlorine, bromine, or iodine). These compounds play an important role in both industrial applications and environmental science. While haloalkanes contain halogens attached to carbon atoms in an alkane chain, haloarenes have halogens attached to a benzene ring. Understanding their structure, properties, and reactions is essential for studying organic chemistry.

Key Points:

Definition:

• Haloalkanes (Alkyl Halides): Organic compounds where one or more halogen atoms replace hydrogen atoms in an alkane chain (e.g., chloroform, bromoethane).
• Haloarenes (Aryl Halides): Organic compounds where a halogen atom is attached to a carbon atom of an aromatic (benzene) ring (e.g., chlorobenzene, bromobenzene).

Nomenclature:

• Haloalkanes are named by prefixing the halogen (e.g., chloro-, bromo-) to the name of the alkane.
• Haloarenes are named by adding the halogen to the benzene ring, and their position is indicated by numbers (e.g., 1,2-dichlorobenzene).

Methods of Preparation:

• Haloalkanes: Prepared by reactions like halogenation of alkanes (free radical substitution) or from alcohols using reagents like PCl₃ or SOCl₂.
• Haloarenes: Prepared by halogenation of aromatic compounds in the presence of a catalyst like FeCl₃ or by direct halogenation of the benzene ring.

Reactions:

• Haloalkanes:
  • Nucleophilic Substitution Reactions: When the halogen is replaced by a nucleophile (e.g., alcohols, amines).
  • Elimination Reactions: When a hydrogen atom and halogen atom are eliminated, resulting in the formation of alkenes.
• Haloarenes:
  • Electrophilic Substitution Reactions: The halogen atom is replaced by another electrophile (e.g., nitro group).
  • Nucleophilic Aromatic Substitution: Occurs when a halogen is replaced by a nucleophile under certain conditions (like the presence of a strong electron-withdrawing group).

Physical Properties:

• Both haloalkanes and haloarenes tend to be non-polar, but haloarenes are generally less reactive than haloalkanes.
• They are usually colorless and have a higher boiling point compared to their parent alkanes or arenes due to the presence of halogen atoms.

Uses and Applications:

• Haloalkanes: Used as solvents, refrigerants, pesticides, and in pharmaceuticals.
• Haloarenes: Used in the production of dyes, pharmaceuticals, and in the manufacture of various chemicals.

Environmental Impact:

• Haloalkanes: Some, like CFCs (chlorofluorocarbons), are harmful to the ozone layer and contribute to global warming.
• Haloarenes: Often toxic and persistent in the environment, making them pollutants.

Conclusion:

Haloalkanes and haloarenes are important classes of organic compounds with a wide range of applications. Their preparation, reactions, and environmental impact are key areas of study in organic chemistry. While they offer several industrial benefits, their environmental risks highlight the need for sustainable alternatives and careful management in industrial processes. Understanding these compounds helps in developing safer, more eco-friendly chemical technologies.