Trends in Halogenoalkanes (Edexcel International AS Chemistry): Revision Note

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

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

• 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