AQA A Level Chemistry复习笔记3.3.1 Reactivity of Halogenoalkanes

Reactivity of Halogenoalkanes

 

  • The halogenoalkanes have different rates of substitution reactions
  • Since substitution reactions involve breaking the carbon-halogen bond the bond energies can be used to explain their different reactivities

Halogenoalkane Bond Energy Table

3.3-Halogen-Compounds-Table-1_Reactivity-of-Halogenoalkanes

  • The table above shows that the C-I bond requires the least energy to break, and is therefore the weakest carbon-halogen bond
  • During substitution reactions the C-I bond will therefore heterolytically break as follows:

R3C-I + OH-     →    R3C-OH + I-

halogenoalkane          alcohol

 

  • The C-F bond, on the other hand, requires the most energy to break and is, therefore, the strongest carbon-halogen bond
  • Fluoroalkanes will therefore be less likely to undergo substitution reactions

Aqueous silver nitrate

  • Reacting halogenoalkanes with aqueous silver nitrate solution will result in the formation of a precipitate
  • The rate of formation of these precipitates can also be used to determine the reactivity of the halogenoalkanes

Halogenoalkane Precipitates Table

 

3.3-Halogen-Compounds-Table-2_Reactivity-of-Halogenoalkanes

  • The formation of the pale yellow silver iodide is the fastest (fastest nucleophilic substitution reaction) whereas the formation of the silver fluoride is the slowest (slowest nucleophilic substitution reaction)
  • This confirms that fluoroalkanes are the least reactive and iodoalkanes are the most reactive halogenoalkanes

3.3-Halogen-Compounds-Reactivity-of-Halogenoalkanes

The trend in reactivity of halogenoalkanes

 

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