Halogenoalkanes Flashcard Notes

Production of Halogenoalkanes

• Free-Radical Substitution:

  • Alkanes react with $Cl<em>2$ or $Br</em>2$ in the presence of UV light.
  • Example: Reaction of ethane.

• Electrophilic Addition:

  • Alkenes react with $X_2$ or $HX(g)$ at room temperature.

• Reaction with Alcohols:

  • React alcohols with $HX$ or $KBr + H<em>2SO</em>4/H<em>3PO</em>4$.
  • React alcohols with $PCl_3$ and heat.
  • React alcohols with $PCl_5$.
  • React alcohols with $SOCl_2$.

Classification of Halogenoalkanes

• Primary, Secondary, Tertiary: Based on the carbon atom to which the halogen is attached.

Reactions of Halogenoalkanes

• With Aqueous NaOH and Heat:

  • Product: Alcohol.

• With KCN in Ethanol and Heat:

  • Product: Nitrile.

• With $NH_3$ in Ethanol Under Pressure and Heat:

  • Product: Amine.

• With Aqueous Silver Nitrate in Ethanol:

  • Used to identify the halogen present.
  • Example: Bromoethane.

Elimination Reactions

• With NaOH in Ethanol and Heat:
  • Product: Alkene (e.g., from bromoethane).

Nucleophilic Substitution Mechanisms

• SN1 Mechanism:

  • Two-step mechanism.
  • Involves carbocation formation.
  • Typically occurs with tertiary halogenoalkanes.

• SN2 Mechanism:

  • One-step mechanism.
  • Involves a transition state.
  • Typically occurs with primary halogenoalkanes.

Mechanism Preference

• Primary Halogenoalkanes:

  • Usually follow the SN2 mechanism.

• Tertiary Halogenoalkanes:

  • Usually follow the SN1 mechanism.

• Secondary Halogenoalkanes:

  • Typically follow a mixture of SN1 and SN2, depending on structure.

Factors Affecting Reactivity

• C-X Bond Strength:
  • Weaker bonds react faster (e.g., C-I).
  • Observed in reactions with aqueous silver nitrate.