Trends in Halogenoalkanes (Edexcel International A Level Chemistry)

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Reactivity & Bond Enthalpy

  • Halogenoalkanes contain at least one halogen atom covalently bonded to a carbon atom within the molecules structure
  • Due to the difference in electronegativity between the halogen and carbon atoms:
    • The carbon-halogen, C-X, bond is polar 
    • The carbon atom has a δ+ charge
    • The halogen atom has a δ– charge

Halogen Compounds Polarity of the C-X bond, downloadable AS & A Level Chemistry revision notes

Due to large differences in electronegativity between the carbon and halogen atom, the C-X bond is polar

  • As the electronegativity down the group decreases, so does the polarity of the bond
    • This suggests that fluoroalkanes would be the quickest to react but the strength of the bond is another important and greater factor

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

Halogenoalkane Bond Energy

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 of the halogenoalkane will therefore heterolytically break to form the alcohol
    • For example

CH3CH2I + OH→ CH3CH2OH + I-

  • 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

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Richard

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Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.