Chlorination

How do we chlorinate alkanes?

To chlorinate alkanes, we use a mechanism known as free radical substitution.

Let’s break that definition down a little.

What is a free radical?

You should know that electrons like to be in pairs, and orbiting by themselves makes them very likely to interact with other molecules. Therefore, free radicals are highly reactive and generally short-lived. They don’t tend to hang around long.

The unpaired electron in a free radical is represented by a small dot at the appropriate position on the molecule. For example, a chlorine free radical is written as shown below:

$$ Cl\space \cdot $$

Another example is an ethyl free radical:

$$ \cdot \space CH_2CH_3 $$

What is a substitution reaction?

In the case of chlorinating alkanes, we take off a hydrogen atom from the alkane and replace it with a chlorine atom. This changes the functional group, making the molecule a halogenoalkane. We'll look at these a little later on.

Fig. 1 - A substitution reaction. A hydrogen atom has been swapped with a chlorine atom, forming a halogenoalkane. This molecule is chloromethane

The process of chlorination

As described above, in the chlorination of alkanes by free radical substitution, one or more hydrogen atoms in an alkane is replaced by a chlorine free radical. UV light is used as a catalyst and hydrochloric acid is also given off.

For example, the substitution of methane using chlorine produces chloromethane. The overall equation is as follows:

\(CH_4+Cl_2\rightarrow CH_3Cl+HCl\)

Free radical substitution has four steps:

• Initiation.
• Propagation I.
• Propagation II.
• Termination.

We’ll take a look at them in turn below.

Initiation

UV light is used to break the bond in a chlorine molecule. Taking our example of methane and chlorine, the Cl-Cl bond is broken to form two chlorine free radical atoms, each with one unpaired electron. These free radicals are extremely reactive. We call this homolytic fission.

Homo comes from the Greek homos, meaning one and the same; we call this type of bond-breaking 'homolytic' fission because the two molecules produced are identical:

\(Cl_2\rightarrow Cl\cdot +Cl\cdot\)

Propagation I

One of the chlorine free radicals takes a hydrogen atom from methane by breaking one of the C-H bonds. It forms the stable compound hydrochloric acid. A methyl free radical is left behind. Once again, this radical is extremely reactive.

Study tip: Notice how the radical is represented. The small dot is located on the carbon, as this is where we find the unpaired electron.

\(Cl\cdot \space + CH_4\rightarrow HCl+\space \cdot CH_3\)

Propagation II

The methyl free radical reacts with a second chlorine molecule. One of the chlorine atoms adds onto the methyl radical to form chloromethane, and the other chlorine atom is left behind as the regenerated radical. This means the reaction can happen again and again until the radical is destroyed in the final step, termination. We call this a chain reaction.

Termination

Termination removes the remaining free radicals. This can occur in multiple different ways, but in each case, two radicals react together to form a stable compound. For example:

• Two chlorine radicals react together to form a chlorine molecule:

\(Cl\cdot \space +\space Cl\cdot \rightarrow Cl_2\)

• Two methyl radicals react together to form ethane:

\(\cdot \space CH_3+\space \cdot CH_3\rightarrow C_2H_6\)

• One methyl free radical and one chlorine free radical react together to form chloromethane:

\(\cdot \space CH_3+Cl\space \cdot \rightarrow CH_3Cl\)

The products of chlorination

Our example of methane and chlorine initially produces chloromethane and hydrochloric acid. Chloromethane is an example of a halogenoalkane.

Fig. 2 - Chloromethane

Halogenoalkanes all contain a carbon bonded to a halogen atom. This bond makes them a lot more reactive than alkanes. Halogenoalkanes are a great starting point for all manner of organic molecules, including alkenes, alcohols, nitriles, and amines. To explore their properties, check out Halogenoalkanes.

However, free radical substitution reactions are chaotic. There are radicals bouncing around everywhere with their unpaired electrons, crashing into hydrocarbons again and again, sometimes reacting with molecules that have already been previously substituted. They are hard to control and produce a mixture of products. This means that they aren’t very useful industrially.

Let’s look at some further molecules that can be produced in our above example using chlorine and methane:

• If two methane radicals react in the termination step, ethane (C₂H₆) can be formed.
• If a chlorine free radical instead reacts with chloromethane in the propagation step, a second hydrogen is substituted. This produces dichloromethane and we know this type of process as a chain reaction. The equation is given below:

\(CH_3Cl+Cl_2\rightarrow CH_2Cl_2+HCl\)

• Similarly, trichloromethane and carbon tetrachloride can be produced:

\(CH_2Cl_2+Cl_2\rightarrow CCl_4+HCl\)

The reaction becomes even more complicated with longer-chain alkanes, and many isomers are formed. This is because the chlorine free radical isn’t fussy about where it attacks on the carbon chain, and can replace any of the hydrogens present. For example, the primary reaction between 2-methylbutane and chlorine can produce four different products:

Fig. 3 - A table showing the isomeric products produced in the chlorination of 2-methylbutane