IB DP Chemistry: HL复习笔记10.2.12 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
• A 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

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

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