Question

In the free radical chlorination of methane, the chain initiating step involves the formation of (1) chlorinc atom (2) hydrogen chloride (3) methyl radical (4) chloromcthyl radical

Short answer

The chain initiating step involves the formation of a chlorine atom.

Step-by-step solution

Understanding Chain Initiation

The chain initiating step in a free radical reaction involves the creation of reactive radicals from stable molecules due to the presence of energy (often in the form of heat or light).

Initiation in Chlorination of Methane

In the free radical chlorination of methane, the initiation step generally involves the homolytic cleavage of chlorine molecules \(Cl_2\) under UV light, producing two chlorine atoms.

Chemical Equation Representation

The reaction can be represented as follows: \[ Cl_2 \rightarrow 2Cl. \] Here, the chlorine molecule breaks down into two chlorine radicals (atoms).

Identifying the Correct Option

Considering the initiation step and the radicals formed, the correct answer is the chlorine atom (option 1).

Key concepts

reaction mechanism

In chemistry, a reaction mechanism demonstrates the step-by-step sequence of elementary reactions by which an overall chemical change occurs. The free radical chlorination of methane is a classic example of such a mechanism. It involves several steps, starting from the initial absorption of energy which breaks bonds and ultimately leads to product formation. The free radical chlorination of methane proceeds through a series of steps: initiation, propagation, and termination. Each of these steps involves specific reactions and intermediates, leading to the final product, chloromethane.

• **Initiation**: This is where radicals are formed.
• **Propagation**: Involves the radicals reacting with the stable molecules to form new radicals.
• **Termination**: Radicals combine to form stable molecules, ending the chain reaction.

By understanding each part of the mechanism, students can grasp how reactions progress and how different intermediates play a role in bringing about the chemical transformation.

chain initiation

The chain initiation step is the first crucial part of a chain reaction in chemical processes like free radical chlorination. This step is responsible for producing the necessary reactive species that drive the rest of the reaction forward. In the case of free radical chlorination of methane, the initiation step involves the homolytic cleavage of a chlorine molecule.
Energy, often provided by UV light, breaks the bond in the chlorine molecule \(Cl_2 \), producing two chlorine radicals (\(Cl\cdot\)): \[ Cl_2 \rightarrow 2Cl. \] These highly reactive chlorine radicals are essential for the subsequent steps in the reaction mechanism. Without them, the chain reaction could not proceed.

homolytic cleavage

Homolytic cleavage is a process in which a chemical bond breaks evenly, with each of the bonded atoms receiving one electron from the bond pair. This is represented as:
\[ AB \rightarrow A. + B. \] In free radical chlorination of methane, the homolytic cleavage of the chlorine molecule under UV light initiates the reaction. When the bond in the chlorine molecule (\(Cl_2\)) breaks, each chlorine atom takes one electron, forming two chlorine radicals (free radicals). These radicals are extremely reactive due to their unpaired electrons. The importance of homolytic cleavage lies in its ability to generate these highly reactive species that drive the chain reactions forward. This step sets the stage for the rest of the mechanism by providing the radicals necessary for subsequent reactions.

chlorine radicals

Chlorine radicals are highly reactive species formed during the initiation step of the free radical chlorination reaction. These radicals are denoted by the symbol \(Cl\cdot\) and possess an unpaired electron, making them extremely reactive. Once formed, chlorine radicals serve as the primary agents for the propagation steps in the reaction mechanism.
When a chlorine radical collides with a methane molecule \((CH_4)\), it abstracts a hydrogen atom, forming hydrochloric acid (HCl) and a methyl radical \((CH_3\cdot)\): \[ Cl. + CH_4 \rightarrow HCl + CH_3. \] The methyl radical can then react with another chlorine molecule to form chloromethane (CH3Cl) and regenerate the chlorine radical: \[ CH_3. + Cl_2 \rightarrow CH_3Cl + Cl. \] This propagation cycle continues, allowing the chain reaction to proceed. The role of chlorine radicals in these reactions highlights their importance in driving the overall mechanism of free radical chlorination.