## 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. ### 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 (δ+), and the halogen atom acquires a partial negative charge (δ-). - 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: $$\mathrm{R-F > R-Cl > R-Br > R-I}$$ **Table: Electronegativity Values (Pauling Scale)** | Atom | Electronegativity | | :--- | :----------------- | | F | 3.98 | | Cl | 3.16 | | Br | 2.96 | | I | 2.66 | | C | 2.55 | | H | 2.2 | ### 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. The actual, experimentally observed order of reactivity is determined by bond energy: $$\mathrm{R-I > R-Br > R-Cl > R-F}$$ **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. - Reactivity of halogenoalkanes is governed by two competing factors: **bond polarity** and **bond energy**. - **Bond Polarity** suggests the reactivity order should be $\mathrm{R-F > R-Cl > R-Br > R-I}$ because the C-F bond is the most polar. - **Bond Energy** suggests the reactivity order should be $\mathrm{R-I > R-Br > R-Cl > R-F}$ because the C-I bond is the weakest and easiest to break. - Experimentally, **bond energy is the decisive factor**. - The overall reactivity order of halogenoalkanes is: $$\mathrm{Iodoalkane > Bromoalkane > Chloroalkane > Fluoroalkane}$$ - **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.