IB DP Chemistry: SL复习笔记10.2.12 Halogenoalkanes

Reactions of Halogenoalkanes

  • Halogenoalkanes are much more reactive than alkanes due to the presence of the electronegative halogens
    • The halogen-carbon bond is polar causing the carbon to carry a partial positive and the halogen a partial negative charge
  • nucleophilic substitution reaction is one in which a nucleophile attacks a carbon atom which carries a partial positive charge
  • An atom that has a partial negative charge is replaced by the nucleophile

3.3-Halogen-Compounds-Polarity-of-the-C-X-bond

Due to large differences in electronegativity between the carbon and halogen atom, the C-X bond is polar

Reaction with NaOH

  • The reaction of a halogenoalkane with aqueous alkali results in the formation of an alcohol
  • The halogen is replaced by the OH-
  • The aqueous hydroxide (OH- ion) behaves as a nucleophile by donating a pair of electrons to the carbon atom bonded to the halogen
  • Hence, this reaction is a nucleophilic substitution
    • For example, bromoethane reacts with aqueous alkali when heated to form ethanol

3.3-Halogen-Compounds-Electrophilic-Substitution-by-NaOH

The halogen is replaced by a nucleophile, OH-

  • The reaction is slow at room temperature so to ensure a high yield it is heated under reflux
  • Since haloalkanes are not usually soluble in water, a polar solvent such as ethanol is often used as it will dissolve haloalkanes as well as sodium hydroxide

 

 

 

 

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