17.4 Reactivity of Halogenoalkanes

The reactivity of halogenoalkanes is primarily controlled by two main factors: the bond polarity and the bond energy of the carbon-halogen (C-X) bond. These factors are influenced by the electronic structure and periodic trends of the halogens.

Ionization Energy→

1. Bond Polarity

The carbon-halogen (C-X) bond in a halogenoalkane is polar due to the difference in electronegativity between the carbon and halogen atoms. Halogens are significantly more electronegative than carbon.

• This electronegativity difference causes the halogen atom to pull electron density away from the carbon atom.
• As a result, the carbon atom acquires a partial positive charge ($\delta +$), and the halogen atom acquires a partial negative charge ($\delta -$).
• The carbon atom becomes an electrophilic center, making it susceptible to attack by nucleophiles.

Based only on bond polarity, the reactivity of alkyl halides would be expected to decrease as the electronegativity of the halogen decreases. This is because a more polar bond (like C-F) would create a more electrophilic carbon, which should be more reactive towards nucleophiles.

The predicted order of reactivity based on bond polarity is:

$\mathrm{R}-\mathrm{F}>\mathrm{R}-\mathrm{Cl}>\mathrm{R}-\mathrm{Br}>\mathrm{R}-\mathrm{I}$

Table: Electronegativity Values (Pauling Scale)

2. Bond Energy

Bond energy (or bond enthalpy) is the energy required to break one mole of a specific type of bond in the gaseous state.

• The reactivity of halogenoalkanes is inversely proportional to the strength of the C-X bond.
• A stronger bond requires more energy to break, making the compound less reactive. Conversely, a weaker bond is easier to break, making the compound more reactive.
• The strength of the C-X bond decreases down the group from fluorine to iodine. The C-F bond is the strongest, while the C-I bond is the weakest.

This is because the atomic size of the halogen increases down the group, leading to a longer and weaker bond with carbon.

Table: C-X Bond Energies

The actual, experimentally observed order of reactivity is determined by bond energy:

$\mathrm{R}-\mathrm{I}>\mathrm{R}-\mathrm{Br}>\mathrm{R}-\mathrm{Cl}>\mathrm{R}-\mathrm{F}$

3. Dominant Factor: Bond Energy vs Bond Polarity

Conclusion: Although the C-F bond is the most polar, the bond energy is the dominant factor in determining the reactivity of halogenoalkanes in nucleophilic substitution reactions. The C-F bond is so strong that alkyl fluorides are generally unreactive under ordinary conditions.

Halogens→

Summary

• Significance: This reactivity trend is fundamental to organic synthesis, allowing chemists to choose the appropriate halogenoalkane for a desired reaction rate in processes like nucleophilic substitution and elimination.