Combustion & Free Radical Substitution of Alkanes

Complete Combustion

• Process: Occurs in the presence of excess oxygen, ensuring all carbon atoms are fully oxidized to carbon dioxide ($CO_2$) and all hydrogen atoms to water ($H_2O$).
• General Equation: $C_nH_{2n+2} + \frac{3n+1}{2}O_2 \rightarrow nCO_2 + (n+1)H_2O$.
• Energy: This process releases the maximum amount of energy per mole of fuel, making it the ideal reaction for heating and propulsion.

Incomplete Combustion

• Process: Occurs when oxygen supply is limited, leading to the formation of carbon monoxide ($CO$) or solid carbon (soot/particulates) alongside water.
• Environmental Impact: Carbon monoxide is a toxic, colorless gas that binds to hemoglobin, while soot contributes to respiratory issues and global dimming.

1. Initiation

• This step involves the homolytic fission of a halogen molecule (e.g., $Cl_2$) into two radicals. This requires energy, usually provided by Ultraviolet (UV) light, to break the covalent bond.

2. Propagation

• These are 'chain-carrying' steps where a radical reacts with a stable molecule to produce a new radical and a new stable molecule. For example, a chlorine radical reacts with methane to form a methyl radical and $HCl$.

3. Termination

• The reaction ends when two radicals collide and combine to form a stable molecule. This removes the reactive species from the system, effectively stopping the chain reaction.

Chain Reaction Nature: Because propagation steps regenerate a radical, a single initiation event can lead to thousands of substitution cycles until termination occurs.

• Substitution vs. Addition: Alkanes undergo substitution (replacing an atom) because they are saturated. Alkenes, being unsaturated, prefer addition reactions where the double bond opens up to take in new atoms without losing any.

• Initiation vs. Propagation: Initiation creates radicals from non-radicals using external energy, whereas propagation maintains the radical count by converting one radical into another.

• Balancing Combustion: Always balance Carbon first, then Hydrogen, and finally Oxygen. If you end up with a fraction for Oxygen (e.g., $6.5 O_2$), you can either leave it as a fraction or double the entire equation.

• Identifying Mechanism Steps: Look at the number of radicals. Initiation: $0 \rightarrow 2$ radicals. Propagation: $1 \rightarrow 1$ radical. Termination: $2 \rightarrow 0$ radicals.

• The UV Requirement: In exam questions, if a reaction between an alkane and a halogen is described as occurring 'in the dark,' the answer is usually that no reaction occurs. Always check for the mention of UV light or sunlight.

• Multiple Substitutions: Remember that propagation can continue. Once $CH_3Cl$ is formed, it can react with another $Cl^{\bullet}$ to form $CH_2Cl_2$, and so on, until $CCl_4$ is reached.