What a radical is

A radical is a molecular entity with at least one unpaired electron. The definition is deliberately broad: a radical may be a single atom, a molecule, a cation or an anion, as long as the species contains an unpaired electron somewhere.

Radicals are shown with a dot, •, next to the atom carrying the unpaired electron. For example, $Cl\cdot$ is a chlorine atom with an unpaired electron, while $\cdot CH_3$ is a methyl radical with the unpaired electron on carbon. In a larger species, the dot matters; it shows where the radical character is located.

A molecular entity is a distinct atom, molecule, ion or radical that can be treated as an individual chemical species. So “radical” doesn’t automatically mean “neutral”. Examples include:

• atom radical: $Cl\cdot$
• molecular radical: $\cdot CH_3$
• radical cation: $NH_4\cdot^+$
• radical anion: $O_2\cdot^-$

The unpaired electron makes radicals highly reactive, since pairing that electron in a new covalent bond is usually favourable. In many organic reactions, radicals are short-lived intermediates: species formed during a reaction mechanism but not usually isolated as final products.

Why electron sharing is the theme

In ordinary covalent bonding, two atoms share a pair of electrons. Radical chemistry works differently because single electrons move around. A radical can collide with a stable molecule, take part in bond breaking and bond making, then leave a new radical behind. That is the basis of a chain reaction: one reactive radical generates another reactive radical, so the process carries on.

Two radicals can also meet and form a normal covalent bond. This removes unpaired electrons from the system and stops that particular chain. We’ll call that termination later, but the main idea is simple: radicals are reactive because they have unpaired electrons, and their formulae must show those unpaired electrons clearly.

Homolytic fission

Homolytic fission is bond breaking where each bonded atom takes one electron from the shared pair. Two radicals form. This is not the same as heterolytic fission, where both electrons move to one atom and ions are formed.

For a halogen molecule, the general equation is:

$X_2 \to 2X\bullet$

For chlorine:

$Cl_2 \to 2Cl\bullet$

The bond breaks when enough energy is supplied, commonly by ultraviolet light or heat. For halogens, UV light matters because it can provide the energy needed to break the X–X bond.

Fish-hook arrows

A single-barbed arrow, often called a fish-hook arrow, is a curved arrow showing the movement of one electron. Use it in radical mechanisms. A normal double-barbed curly arrow shows the movement of an electron pair, so it is not the correct arrow for homolytic fission.

In homolytic fission of $Cl_2$, draw one fish-hook arrow from the Cl–Cl bond to the left chlorine atom, and another from the same bond to the right chlorine atom. The two electrons in the covalent bond split evenly.

Be precise with these arrows. The tail starts at the electron source, such as the covalent bond or unpaired electron. The head ends where that single electron goes. In radical chemistry, the arrowhead is single-barbed because only one electron is moving.

Initiation in a chain reaction

An initiation step is the first step in a radical chain reaction that makes radicals from non-radical reactants. In halogenation reactions, initiation is usually homolytic fission of the halogen molecule:

$Cl_2 \to 2Cl\bullet$

or, for bromine:

$Br_2 \to 2Br\bullet$

A chain reaction is a reaction sequence where a reactive intermediate produced in one step causes further steps of the same type to occur. The initiation step gets the chain going by making the first radicals.

Reverse process and bond strength links

The reverse of homolytic fission is two radicals joining to form a covalent bond. In this reverse process, two single electrons pair up and make a bond, for example:

$Cl\bullet + Cl\bullet \to Cl_2$

During a radical chain mechanism, this also counts as a termination reaction.

CFCs in the atmosphere can release chlorine radicals more readily than fluorine radicals because the relevant C–Cl bonds are weaker than C–F bonds. A weaker bond needs less energy for homolytic fission, so UV radiation in the atmosphere is more able to break C–Cl bonds than C–F bonds.

Chlorine radicals can break down ozone, $O_3$, in the stratosphere, but they typically do not break down oxygen, $O_2$, in the same way. This tells us the bonding in $O_2$ is stronger and less readily disrupted than the bonding arrangement in $O_3$. Ozone is therefore more vulnerable to radical attack than oxygen.

Why alkanes need radical conditions

A substitution reaction is a reaction in which one atom or group in a molecule is replaced by another atom or group. In radical substitution of alkanes, a halogen atom takes the place of a hydrogen atom in the alkane.

Alkanes do not react much under ordinary conditions. Their $C-C$ and $C-H$ bonds are strong, and the molecules are essentially non-polar, so many polar reagents are not attracted to them. For that reason, alkanes are often described as kinetically stable: reactions may be possible, but the activation energy is high, so they are slow unless suitable conditions are used.

At the same time, alkanes are thermodynamically unstable with respect to combustion. Reaction with oxygen to form carbon dioxide and water is energetically favourable. A bottle of hexane does not burst into flames at room temperature because combustion has a high activation energy, not because combustion is unfavourable.

Radical substitution lets alkanes enter more reactive organic chemistry. If a non-polar $C-H$ bond is replaced by a polar $C-Cl$ or $C-Br$ bond, the product is a halogenoalkane, which is much more useful for further reactions.

Overall reaction

For methane reacting with chlorine under UV light or heat, the overall substitution is:

$$CH_4 + Cl_2 \to CH_3Cl + HCl$$

The product $CH_3Cl$ is chloromethane. In practice, radical substitution usually gives a mixture, since more than one substitution can occur and different radical combinations can terminate the chain.

The three stages

Radical substitution has three stages: initiation, propagation and termination.

Initiation produces radicals. For chlorination:

$$Cl_2 \to 2Cl\bullet$$

This is homolytic fission of the chlorine molecule, caused by UV light or heat.

Propagation is a chain step in which a radical reacts with a non-radical molecule to form a new radical. Radicals are regenerated, so the chain keeps going.

First propagation step:

$$Cl\bullet + CH_4 \to HCl + \bullet CH_3$$

The chlorine radical removes a hydrogen atom from methane, forming hydrogen chloride and a methyl radical.

Second propagation step:

$$\bullet CH_3 + Cl_2 \to CH_3Cl + Cl\bullet$$

The methyl radical reacts with chlorine to form chloromethane, while a chlorine radical is regenerated. That chlorine radical can then repeat the first propagation step.

For a general alkane, $R-H$, and a halogen, $X_2$, the propagation pattern is:

$$X\bullet + R-H \to H-X + R\bullet$$ $$R\bullet + X_2 \to R-X + X\bullet$$

Here R represents an alkyl group and X represents a halogen atom, usually chlorine or bromine in this syllabus context.

Termination

Termination is a step in which two radicals combine to form a non-radical product. Since it removes radicals, termination slows the chain reaction and eventually stops it.

For methane chlorination, possible termination steps include:

$$Cl\bullet + Cl\bullet \to Cl_2$$ $$\bullet CH_3 + Cl\bullet \to CH_3Cl$$ $$\bullet CH_3 + \bullet CH_3 \to C_2H_6$$

These equations help explain why a mixture forms. Some termination produces useful halogenoalkane, some reforms chlorine, and some produces a larger alkane such as ethane.

Product mixtures and choice of halogen

Radical substitution with chlorine or bromine can halogenate alkanes, but it is not usually perfectly selective. Once a halogenoalkane forms, further substitution may happen, giving products with more than one halogen atom. Larger alkanes may also contain more than one type of $C-H$ bond, so substitution can occur at different carbon atoms.

Fluorine is generally too reactive for controlled radical substitution of alkanes, often causing extensive bond breaking and complex mixtures. Iodine is not reactive enough for the corresponding radical iodination to proceed effectively. For IB purposes, chlorine and bromine are the sensible halogens to think about for alkane radical substitution.