Question

Which of the following is the correct sequence of steps in the halogenation of an alkanc? (1) Propagation, initiation, termination (2) Initiation, termination, propagation (3) Initiation, propagation, termination (4) Propagation, termination, initiation

Short answer

Option (3) Initiation, propagation, termination.

Step-by-step solution

- Identify Each Stage

In the halogenation of an alkane, there are three main stages: initiation, propagation, and termination.

- Understand 'Initiation'

Initiation is the stage where the halogen molecules are split into radicals. This usually requires energy, such as UV light, to break the bond and produce free radicals.

- Understand 'Propagation'

Propagation involves the radical reacting with the alkane to generate a new radical and a hydrogen halide. This new radical then reacts with another halogen molecule to form the halogenated alkane and another radical, continuing the chain reaction.

- Understand 'Termination'

Termination occurs when two radicals combine to form a stable molecule, thus ending the chain reaction. This stage reduces the number of radicals in the system.

- Correct Sequence

Based on the understanding of each stage, the correct sequence of steps in the halogenation of an alkane is: initiation, propagation, and then termination.

- Select the Correct Option

Review the given options to find the sequence that matches: initiation, propagation, termination.

Key concepts

initiation step

In the halogenation of alkanes, the process begins with the initiation step. This is key to starting the reaction.
During initiation, a halogen molecule, typically chlorine or bromine, absorbs energy, often from UV light.
This energy breaks the bond between the atoms in the halogen molecule, creating two halogen radicals (atoms with an unpaired electron).
It's important to note that these radicals are highly reactive.
The process can be represented by the equation: \[Cl_2 \rightarrow 2\text{\textsuperscript{·}} Cl\]
This means one molecule of chlorine splits into two chlorine radicals.
Without this step, the entire halogenation process wouldn’t proceed because free radicals are necessary for the next steps.

propagation step

After initiation, the reaction moves into the propagation step.
This is where the actual substitution happens, involving a series of two key reactions.
First, one of the halogen radicals produced in the initiation step reacts with an alkane molecule.

• This generates a new radical from the alkane (the \text{alkyl} radical) and a stable hydrogen halide molecule.

For example: \[ \text{\textsuperscript{·}} Cl + CH_4 \rightarrow \text{\textsuperscript{·}} CH_3 + HCl \] The second part involves the newly formed alkyl radical reacting with another halogen molecule to regenerate the halogen radical and form the halogenated alkane.

• This ensures the chain reaction continues as long as there are reactants available.

Example: \[ \text{\textsuperscript{·}} CH_3 + Cl_2 \rightarrow CH_3Cl + \text{\textsuperscript{·}} Cl \] These steps repeat and can rapidly produce a large number of halogenated alkanes.
The propagation steps are crucial for the chain reaction nature of the halogenation process.

termination step

The reaction concludes with the termination step.
In this stage, the free radicals in the reaction mixture combine to form stable molecules, stopping the reaction.
Several combinations can occur during termination:

• Two halogen radicals can combine: \[ \text{\textsuperscript{·}} Cl + \text{\textsuperscript{·}} Cl \rightarrow Cl_2 \]
• An alkyl radical can combine with a halogen radical: \[ \text{\textsuperscript{·}} CH_3 + \text{\textsuperscript{·}} Cl \rightarrow CH_3Cl \]
• Two alkyl radicals can combine: \[ \text{\textsuperscript{·}} CH_3 + \text{\textsuperscript{·}} CH_3 \rightarrow C_2H_6 \]

Each of these termination reactions helps to decrease the number of free radicals in the system, bringing an end to the chain reaction.
Understanding the termination step is essential as it caps off the entire halogenation process, ensuring the reaction doesn't go on indefinitely.