CIE AS Chemistry (9701) exams from 2022

Revision Notes

3.3.5 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

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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Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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