CIE AS Chemistry (9701) 2019-2021

Revision Notes

3.3.3 Relative strength of the C–Hal bond

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

Halogen Compounds Table 1_Reactivity of Halogenoalkanes, downloadable AS & A Level Chemistry revision notes

  • 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

Halogenalkane precipitates table

Halogen Compounds Table 2_Reactivity of Halogenoalkanes, downloadable AS & A Level Chemistry revision notes

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

 

Halogen Compounds Reactivity of Halogenoalkanes, downloadable AS & A Level Chemistry revision notes

The trend in reactivity of halogenoalkanes

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