Reactivity of Halogenoalkanes (AQA AS Chemistry)

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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

Halogenoalkane Precipitates Table

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

  • 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

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

The trend in reactivity of halogenoalkanes

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Francesca

Author: Francesca

Expertise: Head of Science

Fran studied for a BSc in Chemistry with Forensic Science, and since graduating taught A level Chemistry in the UK for over 11 years. She studied for an MBA in Senior Leadership, and has held a number of roles during her time in Education, including Head of Chemistry, Head of Science and most recently as an Assistant Headteacher. In this role, she used her passion for education to drive improvement and success for staff and students across a number of subjects in addition to Science, supporting them to achieve their full potential. Fran has co-written Science textbooks, delivered CPD for teachers, and worked as an examiner for a number of UK exam boards.