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