Free Radical Substitution of Alkanes

1. Initiation

• This step involves the photodissociation of a halogen molecule (like $Cl_2$ or $Br_2$) using energy from ultraviolet (UV) light. The energy from the UV light is sufficient to break the covalent bond homolytically, resulting in two halogen radicals.

2. Propagation

• This is a self-sustaining chain reaction consisting of two main parts. First, a halogen radical attacks an alkane, stripping a hydrogen atom to form a hydrogen halide and an alkyl radical (e.g., $\cdot CH_3$). Second, this new alkyl radical attacks a stable halogen molecule, forming the halogenoalkane product and regenerating a halogen radical to continue the cycle.

3. Termination

• The reaction ends when two radicals collide and combine their unpaired electrons to form a stable covalent bond. Because this step removes radicals from the system without creating new ones, it effectively stops the chain reaction.

• Energy Requirements: Alkanes are generally unreactive because their $C-H$ and $C-C$ bonds are strong and non-polar. The reaction requires an external energy source (UV light) to create the initial radicals that are reactive enough to overcome the activation energy of the alkane bonds.

• Chain Reaction Logic: The propagation steps are designed such that the number of radicals remains constant. One radical is consumed, but another is produced, allowing a single initiation event to result in thousands of product molecules.

• Collision Theory: Termination is statistically less likely than propagation at the start of the reaction because the concentration of radicals is very low compared to the concentration of stable reactant molecules.

• Further Substitution: Once a halogenoalkane is formed, it can be attacked by remaining halogen radicals. This leads to a mixture of products, such as dichloromethane, trichloromethane, and tetrachloromethane, making it difficult to synthesize a pure single product.

• Structural Isomers: In alkanes with three or more carbons, the halogen can substitute at different positions. For example, reacting propane with bromine can produce both 1-bromopropane and 2-bromopropane, depending on which carbon radical was formed during propagation.

• Impurity Formation: During termination, different types of radicals can collide. If two methyl radicals collide, they form ethane ($C_2H_6$), which acts as an impurity in a reaction intended to produce chloromethane.

• The Radical Dot: Always place the radical dot on the specific atom that has the unpaired electron. In an alkyl radical like $\cdot CH_2CH_3$, the dot must be on the carbon atom, not floating in the middle of the formula.

• Balanced Equations: Ensure that every propagation step is balanced for both atoms and radicals. A common mistake is forgetting to include the regenerated radical in the second propagation step.

• Condition Check: If a question asks for the conditions of an alkane-halogen reaction, 'UV light' or 'sunlight' is the mandatory answer. Without it, the initiation step cannot occur at room temperature.

• Predicting Impurities: To identify potential impurities, look at the radicals formed in propagation and imagine them colliding with each other in the termination step.