Explain why iodine (I2) does not react with ethane, even...

Key Concepts

Free Radical Mechanism

This concept involves a chain reaction process that includes initiation, propagation, and termination steps. In the context of halogenation, a halogen molecule first undergoes homolytic cleavage to generate radicals that can then abstract hydrogen from an alkane, creating a new radical which subsequently reacts with another halogen molecule. The efficiency of these steps is crucial for the reaction to proceed.

Bond Dissociation Energy

Bond dissociation energy (BDE) refers to the energy required to break a bond homolytically. While the I–I bond in iodine is weak and cleaves easily, the overall reaction also involves breaking strong C–H bonds and forming new bonds, such as the H–I bond. The relative energies of breaking and forming these bonds determine the feasibility of the reaction.

Thermodynamic Favorability

Thermodynamic favorability examines whether a reaction is exothermic or endothermic overall. Even though the weak I–I bond can be cleaved easily, the subsequent steps—such as forming a weak H–I bond from a strong C–H bond breaking—result in an overall reaction that is not energetically favorable. This lack of a driving force prevents the reaction from occurring under normal conditions.

Radical Reactivity

The reactivity of radicals plays a key role in determining whether a chain reaction can propagate effectively. Iodine radicals, while easy to generate due to the low BDE of I2, are less reactive compared to radicals derived from other halogens such as chlorine. Their lower reactivity means that they are not efficient at abstracting hydrogen atoms from alkanes, further inhibiting the chain reaction.

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