Free Radical Substitution

• Bond Dissociation Energy: The initiation step requires energy to break the $X-X$ bond (where $X$ is a halogen). UV light provides photons with sufficient energy to overcome this bond enthalpy.

• Radical Reactivity: Because radicals have an incomplete octet, they are extremely unstable and seek to pair their single electron by abstracting atoms from stable molecules.

• Thermodynamic Driving Force: The propagation steps are generally exothermic, meaning the energy released by forming new bonds (like $H-Cl$ or $C-Cl$) is greater than the energy required to break existing bonds ($C-H$ or $Cl-Cl$).

1. Initiation

• The halogen molecule absorbs UV light and undergoes homolytic fission: $X_2 \xrightarrow{UV} 2X\cdot$
• This step increases the number of radicals in the system from zero to two.

2. Propagation

• Step A: A halogen radical attacks the alkane, abstracting a hydrogen atom to form a hydrogen halide and an alkyl radical: $R-H + X\cdot \rightarrow R\cdot + HX$
• Step B: The alkyl radical then reacts with a halogen molecule to form the halogenoalkane and regenerate the halogen radical: $R\cdot + X_2 \rightarrow R-X + X\cdot$
• This cycle continues as long as reactants are available, maintaining a constant number of radicals (one in, one out).

3. Termination

• Two radicals collide and combine to form a stable molecule, effectively removing radicals from the cycle: $X\cdot + X\cdot \rightarrow X_2$ or $R\cdot + X\cdot \rightarrow R-X$ or $R\cdot + R\cdot \rightarrow R-R$
• This step reduces the number of radicals from two to zero.

• It is vital to distinguish between the different stages based on the change in the number of radicals present in the reaction mixture.

• Homolytic vs. Heterolytic: In free radical substitution, bonds break homolytically (one electron each). In nucleophilic substitution, bonds break heterolytically (one atom takes both electrons).

• Identify the Step: If a question asks to identify a mechanism step, look at the radicals. If there is a radical on both sides, it is Propagation. If radicals are only on the product side, it is Initiation. If radicals are only on the reactant side, it is Termination.

• Predicting Byproducts: Always check for the 'dimer' product in termination (e.g., ethane forming during the chlorination of methane). This is a common exam question regarding evidence for the radical mechanism.

• Radical Placement: Ensure the radical dot ($\cdot$) is placed on the correct atom. In an alkyl radical like $\cdot CH_3$, the dot must be on the Carbon atom, as that is where the unpaired electron resides.

• Reaction Control: To favor mono-substitution (e.g., $CH_3Cl$), use an excess of the alkane. To favor multi-substitution (e.g., $CCl_4$), use an excess of the halogen.