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Chloroalkane Synthesis Reactions

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

How does the radical chlorination of methane occur? Let's investigate, as well as see how halogenated organic compounds can be effective fire-extinguishing agents, and how chloroalkanes find use as general organic solvents but have a negative role in atmospheric chemistry.

Putting Out Fires

Have you ever wondered why certain materials can extinguish a fire more effectively than just water alone? In its simplest form, fire is a combustion reaction in which organic compounds are converted to CO2 and water, with both light and heat being released as well. But combustion is actually a chain (self-perpetuating) process and proceeds via free-radical intermediates.

Let's learn about free-radicals in chemical reactions and use the radical chlorination of methane as our model system. We'll see that carefully designed chemicals are able to destroy free-radicals and thus halt the combustion process and extinguish the fire.

Radicals Review

A radical is some sort of chemical entity with an unpaired electron. In order to signify a radical, we usually use a single dot on the atom that bears the unpaired electron.

Many types of organic radicals exist, including:

  • methyl radical - when a single electron is on a methyl carbon
  • allylic radical - a single electron on a carbon directly adjacent to an alkene
  • benzylic radical - a single electron on a carbon directly bonded to a benzene ring

Common organic radicals; note the dot indicating the atom with the unpaired electron
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Common Patterns in Radical Mechanisms

Fortunately, drawing radical reaction mechanisms only involves a small number of steps. Remember that since we are talking about moving single electrons and not pairs, we must use single-headed arrows or 'fish-hooks' at all times during our mechanism.

Let's look at the three main components that any radical reaction will have:

  1. Initiation: the radicals are actually created.
  2. Propagation: the unpaired electron is moved from place to place (can involve multiple sub-steps).
  3. Termination: the end of the road for our radicals. This always involves two radicals coming together to make a chemical bond.

Chlorination of Methane

Let's take a look at the radical chlorination of methane and use the reaction as our model system to see how we can apply the individual steps outlined above.

Initiation

In the initiation step, chlorine radicals are generated (usually by exposing them to UV light) from molecular chlorine.


Initiation step to generate chlorine radicals
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Propagation

In the first propagation step, our shiny new chlorine radical reacts with a molecule of methane to pull off a hydrogen atom. This is also called hydrogen abstraction.


First propagation step to generate carbon radical
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The second propagation step is when the real magic happens! This is when our carbon radical steals a chlorine atom from a molecule of chlorine, generating our product as well as another chlorine radical.


Second propagation step to form product plus another chlorine radical
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Notice how the first propagation step consumes a chlorine radical, and the second propagation step actually regenerates a chlorine radical. In theory, one chlorine radical could ultimately cause thousands or even millions of methane molecules to be chlorinated (providing enough chlorine was present). This is why the reaction is called a chain reaction!

Termination

In the termination part of the journey, any radicals we have left could combine in a variety of ways. Chlorine radicals could react to form molecular chlorine, methyl and chlorine radicals could combine to form the desired product, or two methyl radicals could combine to form ethane (an unwanted side product).


Termination steps and all possible outcomes
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Halons to Put Out Fire

As we mentioned previously in our opening discussion, the combustion process is thought to involve radicals and occurs as a chain reaction. In order for a fire to thrive, it needs fuel (such as wood, an organic compound), oxygen, and heat.

Some of the most common and powerful chemicals for fighting fires are halons, organic compounds that contain halogens. Halons are great at their job of extinguishing fires because:

  • Halons are usually gases, thus a quick discharge of a halon gas can deprive the fire of oxygen.
  • Halons are very good at absorbing heat, another thing the fire needs.
  • Halons can actually react with the radicals produced by the fire, thus terminating the chain reaction and stopping the fire.

Chloroalkanes: A Love/Hate Relationship

Use as Solvents

Chloroalkanes, namely chloroform (CHCl3) and dichloromethane (CH2 Cl2) find wide use in a plethora of organic reactions. One reason these types of compounds are used frequently is their ability to effectively solvate a wide variety of organic compounds, both polar and non-polar alike.

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