Class 12 Chemistry – Haloalkanes and Haloarenes

1. Introduction

• Haloalkanes are compounds in which one or more hydrogen atoms of an alkane are replaced by halogen atoms (F, Cl, Br, I).

• Haloarenes are compounds in which halogen atoms are attached to an aromatic ring (like benzene).

• These compounds are also called alkyl halides and aryl halides.

2. Classification

a) Haloalkanes

• Primary (1°): Halogen attached to a carbon bonded to one other carbon.

• Secondary (2°): Halogen attached to a carbon bonded to two other carbons.

• Tertiary (3°): Halogen attached to a carbon bonded to three other carbons.

b) Haloarenes

• Halogen is directly bonded to the aromatic ring.

• Example: Chlorobenzene, Bromobenzene.

3. Nomenclature

• Name the parent hydrocarbon and replace the -H with halogen name as prefix.

• Number the carbon chain so that the halogen gets the lowest possible number.

• For multiple halogens, use di-, tri-, tetra- prefixes.

4. General Properties

Physical Properties

• State: Usually liquids or solids at room temperature.

• Solubility: Insoluble in water but soluble in organic solvents.

• Boiling Point: Higher than corresponding alkanes due to polarity.

Chemical Properties

• Nucleophilic substitution: Halogen is replaced by nucleophiles.

• Elimination reactions: Haloalkanes can lose HX to form alkenes.

• Reactivity:

  • F < Cl < Br < I for reactivity in haloalkanes.

  • Haloarenes are less reactive due to resonance stabilization.

5. Important Reactions

• Haloalkanes:

  • React with NaOH to form alcohols.

  • React with CN⁻ to form nitriles.

  • React with NH₃ to form amines.

• Haloarenes:

  • React under electrophilic substitution rather than nucleophilic substitution.

  • Halogen is less reactive due to electron delocalization in the aromatic ring.

6. Applications

• Haloalkanes:

  • Used as solvents, refrigerants, anesthetics, and intermediates in organic synthesis.

• Haloarenes:

  • Used in dyes, pharmaceuticals, pesticides, and polymers.

7. Key Points to Remember

• Haloalkanes: Halogen attached to saturated carbon (sp³).

• Haloarenes: Halogen attached to aromatic carbon (sp²).

• Reactivity of haloalkanes depends on type of carbon (1°, 2°, 3°) and halogen.

• Haloarenes are less reactive than haloalkanes due to resonance in aromatic ring.

• Both classes are important in industrial and medicinal chemistry.