Several factors influence the rate of the substitution reaction of alkanes with chlorine:

• Concentration of Alkane: The rate is directly proportional to the concentration of the alkane. Higher alkane concentration means more molecules available to react with chlorine radicals, leading to a faster rate.
• Concentration of Chlorine: The rate is directly proportional to the concentration of chlorine. Higher chlorine concentration means more chlorine radicals are available, leading to a faster rate.
• Intensity of UV Light: The rate is directly proportional to the intensity of the UV light. More intense light provides more photons, leading to more Cl-Cl bond breaks and therefore more chlorine radicals, resulting in a faster rate.
• Temperature: Increasing the temperature generally increases the rate of the reaction. Higher temperatures increase the kinetic energy of the molecules, leading to more frequent collisions and a higher probability of successful radical reactions. However, very high temperatures can also lead to the decomposition of reactants and products, potentially decreasing the overall rate.
• Radical Inhibitors: Radical inhibitors are substances that react rapidly with free radicals, effectively removing them from the reaction mixture. This slows down the chain reaction and therefore reduces the rate of the substitution reaction. Common radical inhibitors include oxygen (O₂) and aromatic compounds like benzene. These inhibitors react with the highly reactive radicals, forming stable, unreactive products. For example, oxygen reacts with alkyl radicals to form peroxy radicals, which then abstract hydrogen atoms from other alkane molecules, terminating the chain reaction.

These factors affect the concentration of free radicals as follows:

• Increasing alkane or chlorine concentration increases the number of radicals formed in the initiation and propagation steps.
• Increasing UV light intensity increases the rate of Cl-Cl bond breaking, leading to a higher concentration of chlorine radicals.
• Increasing temperature increases the rate of radical formation and propagation steps.
• Radical inhibitors remove radicals from the reaction mixture, decreasing their concentration and slowing down the chain reaction.

The mixture of products is formed because the substitution of hydrogen atoms in propane by chlorine is a free radical reaction. This reaction is non-selective, meaning that chlorine can substitute any hydrogen atom in the propane molecule. The products are formed through a series of successive substitution reactions.

The proportion of each product changes as the reaction proceeds as follows:

As the reaction continues and more chlorine is added, the proportion of methyl chloride decreases, while the proportions of dichloro propane and trichloro propane increase. This is because the reaction is statistically more likely to substitute a second and then a third hydrogen atom as the reaction progresses. Eventually, the reaction will produce a mixture of all possible chlorinated propane products, but the relative proportions will be determined by the ratio of chlorine to propane used in the reaction.

This is a substitution reaction. Specifically, it's a free radical substitution reaction.

Why is direct sunlight required?

Direct sunlight provides the energy (photons) needed to break the Cl-Cl bond in chlorine (Cl₂). This homolytic cleavage generates two chlorine radicals (Cl•). These radicals are highly reactive and initiate a chain reaction, where a chlorine radical abstracts a hydrogen atom from methane, forming a methyl radical (CH₃•). The methyl radical then reacts with another chlorine molecule to form chloromethane and regenerate a chlorine radical, continuing the chain. Without the UV light, the Cl₂ molecule would not be broken down into radicals, and the reaction would not proceed effectively. The UV light acts as an initiator for the reaction.