Radical Halogenation Mechanisms and Reactivity

3. Termination

Eventually, the chain is broken by termination steps. This occurs when two radicals collide and combine to form a stable covalent bond, effectively "killing" the radical concentration. Examples include:

• $2\text{Cl}\cdot \rightarrow \text{Cl}_2$
• $\text{CH}_3\cdot + \text{Cl}\cdot \rightarrow \text{CH}_3\text{Cl}$
• $\text{CH}_3\cdot + \text{CH}_3\cdot \rightarrow \text{CH}_3\text{CH}_3$

Exam Tip: When drawing mechanisms, use "fishhook" arrows (single-barbed arrows) to represent the movement of a single electron. Using standard double-barbed arrows for single electrons is a common error in organic chemistry grading.

Energetics and the Hammond Postulate

To understand why different halogens behave differently, we must look at the enthalpy of reaction ($\Delta H^\circ$) and the nature of the transition state. The reactivity of halogens follows the trend: $\text{F}_2 > \text{Cl}_2 \approx \text{Br}_2 > \text{I}_2$.

The Hammond Postulate

The Hammond Postulate provides a bridge between the energy of a reaction and the structure of its transition state. It states that the transition state of a reaction step will more closely resemble the species (reactant or product) to which it is closer in energy.

• Exothermic Steps (e.g., Fluorination): The transition state is "early." It resembles the reactants and occurs very quickly because the energy barrier is low.
• Endothermic Steps (e.g., Bromination): The transition state is "late." It resembles the products (the radical intermediate) and is much more sensitive to the stability of that radical.

Selectivity and Radical Stability

A crucial concept in halogenation is selectivity—the ability of a reagent to prefer one type of hydrogen over another. This is heavily influenced by the stability of the radical intermediate formed during the first propagation step.

Carbon Environments and Radical Stability

Not all C–H bonds are equal. The bond dissociation energy ($\Delta H^\circ$) decreases as we move from primary to tertiary carbons: $\text{CH}_3\text{--H} > \text{R}_{\text{prim}}\text{--H} > \text{R}_{\text{sec}}\text{--H} > \text{R}_{\text{tert}}\text{--H}$ . Tertiary radicals are the most stable due to hyperconjugation, where the p-orbital containing the single electron overlaps with the bonding molecular orbitals of neighboring C–H bonds.

Reactivity vs. Selectivity

Because bromine ($\text{Br}_2$) is less reactive and its first propagation step is endothermic, it has a "late" transition state that looks very much like the radical product. Consequently, bromine is highly selective for the most stable radical (tertiary > secondary > primary). Chlorine ($\text{Cl}_2$), being more reactive and having an exothermic first step, has an "early" transition state and is much less selective, often resulting in a complex mixture of constitutional isomers.

Summary Table: Halogen Reactivity

Predicting Products: Constitutional Isomers

When predicting the products of a halogenation reaction, you must identify the different carbon environments in the molecule. Any three identical groups on an $sp^3$ carbon are considered to be in the same environment.

For example, in a molecule like pentane, there are three distinct carbon environments. If you perform a chlorination, you might get a mixture of several constitutional isomers because chlorine is unselective. If you perform a bromination, you will primarily get the product corresponding to the most stable (tertiary or secondary) carbon environment.