Alkanes such as methane, propane, and decane have a range of uses. We most commonly use them as fuels because they have a high negative enthalpy of combustion, meaning they release lots of energy to the environment as heat and light when burnt. We also use them in aerosols and to pave roads, and split longer-chain alkanes into shorter-chain alkanes and alkenes in a process known as cracking.
However, alkanes are generally quite unreactive. This is because they only contain nonpolar C-C and C-H single bonds, as explored in Alkanes. By reacting them with chlorine, we can change their functional group and make them a lot more reactive. This is called chlorination.
Chlorination is the process of adding chlorine atoms to a molecule.
How do we chlorinate alkanes?
To chlorinate alkanes, we use a mechanism known as free radical substitution.
Free radical substitution is a type of reaction, in which a free radical replaces another atom or functional group in a molecule.
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.
A free radical is an atom, molecule, or ion with an unpaired outer shell electron.
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:
$$ \cdot \space CH_2CH_3 $$
Remember how your parents always told you to eat your veggies? They weren’t wrong: fruits and vegetables are great sources of antioxidants. Our bodies are constantly exposed to free radicals caused by smoking, air pollution, stress, and simply just converting food into energy. Having too many free radicals in your body can lead to oxidative stress. This damages cells and is thought to play a role in a variety of conditions, such as Parkinson’s disease, diabetes and Alzheimer’s. Antioxidants fight free radicals and help keep their levels in check, reducing oxidative stress.
What is a substitution reaction?
A substitution reaction is a reaction that involves swapping one function group, atom or group of atoms with another.
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
A substitution reaction. A hydrogen atom has been swapped with a chlorine atom, forming a halogenoalkane. This molecule is chloromethane. Anna Brewer, StudySmarter Originals
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.
A chain reaction is a reaction that produces a by-product which then goes on to react further.
The equation is shown below:\(\cdot \space CH_3+Cl_2\rightarrow \space Cl\cdot + CH_3Cl\)
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.
Halogenoalkanes are organic molecules formed from alkanes, where halogen atoms have replaced one or more hydrogen atoms.
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:
\(CH_3Cl+Cl_2\rightarrow CH_2Cl_2+HCl\)
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