2.8.5 Trends in Halogenoalkanes

• 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