Haloalkanes and Haloarenes

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Haloalkanes and Haloarenes are an important class of organic compounds containing halogen atoms. This chapter from NCERT Class 12 Chemistry covers their structure, properties, reactions, and uses. This tutorial provides a comprehensive guide to the chapter along with NCERT solutions for key questions.

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Haloalkanes and Haloarenes

Haloalkanes and Haloarenes are halogen-containing organic compounds where one or more hydrogen atoms in a hydrocarbon are replaced by halogen atoms (F, Cl, Br, I).

1. Haloalkanes (Alkyl Halides)

  • Definition: Organic compounds where a halogen atom (X) is attached to an aliphatic carbon chain.
  • General Formula: R-X (where R = alkyl group, X = halogen)
  • Example:
    • Methyl chloride (CH₃Cl)
    • Ethyl bromide (C₂H₅Br)

Classification Based on Carbon Attachment:

  1. Primary (1°): Halogen attached to a carbon bonded to one other carbon. (Example: CH₃CH₂Cl)
  2. Secondary (2°): Halogen attached to a carbon bonded to two other carbons. (Example: CH₃CHClCH₃)
  3. Tertiary (3°): Halogen attached to a carbon bonded to three other carbons. (Example: (CH₃)₃CCl)

2. Haloarenes (Aryl Halides)

  • Definition: Organic compounds where a halogen atom (X) is attached to an aromatic ring (benzene or derivatives).
  • General Formula: Ar-X (where Ar = Aryl group, X = halogen)
  • Example:
    • Chlorobenzene (C₆H₅Cl)
    • Bromobenzene (C₆H₅Br)

Key Differences from Haloalkanes:
✔ Less reactive in nucleophilic substitution due to resonance.
✔ Undergo electrophilic substitution reactions like nitration and sulphonation.

Classification and Nomenclature

Types of Halogen Compounds

  1. Haloalkanes (Alkyl Halides): Halogen atom attached to an aliphatic carbon chain.
    Example: CH₃Cl (Methyl chloride)
  2. Haloarenes (Aryl Halides): Halogen atom directly attached to an aromatic ring.
    Example: C₆H₅Cl (Chlorobenzene)

IUPAC Naming Rules

  • Identify the longest carbon chain containing the halogen.
  • Number the chain so that the halogen gets the lowest possible number.
  • Use prefixes like fluoro-, chloro-, bromo-, or iodo-.

Example: 2-Bromopropane (CH₃-CHBr-CH₃)

Nature of the C–X Bond

  • The carbon-halogen (C–X) bond is polar due to the electronegativity difference between carbon and halogen.
  • Bond strength decreases down the group: C-F > C-Cl > C-Br > C-I.

Conclusion

Haloalkanes and Haloarenes are crucial in organic chemistry and industrial applications. Understanding their reactions, properties, and environmental effects helps in solving NCERT problems effectively.