OCR A Level Chemistry

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4.5.3 Hydrolysis of Primary Haloalkanes

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Hydrolysis of Primary Haloalkanes

Bond Enthalpy

  • 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

Haloalkane 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-

                                                                                        halooalkane          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

Exam Tip

  • Bond energy / enthalpy values can vary in published literature despite theoretically being completed under the same conditions
  • Another reason for variation in bond energy / enthalpy values is due to the published values being the mean average of the specific bond from different substances
  • The key point is that you are able to use the data given to you and apply it in your answers

Aqueous silver nitrate

  • Reacting haloalkanes 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 haloalkane

Haloalkane 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 haloalkanes

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

The trend in reactivity of haloalkanes

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Philippa

Author: Philippa

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.