Radical reactions

A radical (or free radical) is a species with at least one unpaired electron. Because electrons normally prefer to be paired, a radical is highly reactive and tends to react quickly to gain or share an electron.

We show a radical by writing a single dot next to the chemical symbol, representing the lone unpaired electron:

• $\text{Cl}\cdot$ - a chlorine radical (a chlorine atom)
• $\cdot\text{CH}_3$ - a methyl radical
• $\text{H}\cdot$ - a hydrogen radical (atom)

The dot is not a charge: radicals are usually neutral, unlike ions. This is what distinguishes radical chemistry from the ionic chemistry of nucleophiles and electrophiles.

Key ideas to remember:

• Radicals form when a bond breaks homolytically (one electron to each atom).
• They have an odd (unpaired) electron and so readily react with stable molecules and with each other to pair it up.
• When two radicals meet, their unpaired electrons can pair up to form a new covalent bond, giving a stable molecule.

Radicals are central to processes such as the halogenation of alkanes, combustion, the action of CFCs on ozone, and polymerisation - all of which rely on their unpaired-electron reactivity.

Homolytic fission breaks a covalent bond so that each atom keeps one of the two shared electrons. This produces two radicals:

$$\text{X-Y} \rightarrow \text{X}\cdot + \text{Y}\cdot$$

Contrast this with heterolytic fission, where one atom takes both electrons, giving a pair of ions. The 'homo' (same) refers to the symmetrical, even split of the electron pair.

When does it happen?

• For non-polar or weakly polar bonds (like $\text{Cl-Cl}$ or $\text{C-H}$), an even split is favoured because there is no strong pull of electrons to one side.
• It needs an energy input: usually UV light or high temperature. A UV photon carries energy $E = hf$; high-frequency (short-wavelength) UV has enough energy per photon to break the bond. This is why $\text{Cl}_2$ splits under UV but not in the dark.

Curly arrows: in radical chemistry we use single-barbed 'fish-hook' arrows ($\curvearrowright$ with one barb), because each arrow moves only one electron. A normal double-barbed arrow moves a pair of electrons. For homolysis of $\text{Cl-Cl}$, two fish-hook arrows point away from the bond, one to each chlorine, showing one electron going to each atom.

Alkanes are unreactive (non-polar, strong $\sigma$ bonds), but they undergo radical substitution with halogens such as $\text{Cl}_2$ or $\text{Br}_2$ in the presence of UV light. A hydrogen atom is replaced by a halogen atom:

$$\text{CH}_4 + \text{Cl}_2 \xrightarrow{\,UV\,} \text{CH}_3\text{Cl} + \text{HCl}$$

The mechanism has three stages:

• Initiation - UV light causes homolytic fission of the halogen, making radicals: $\;\text{Cl}_2 \xrightarrow{h\nu} 2\,\text{Cl}\cdot$
• Propagation - radicals are used up and regenerated, sustaining a chain:
• $\text{Cl}\cdot + \text{CH}_4 \rightarrow \cdot\text{CH}_3 + \text{HCl}$
• $\cdot\text{CH}_3 + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \text{Cl}\cdot$
• Termination - two radicals combine, removing radicals and ending the chain:
• $\text{Cl}\cdot + \cdot\text{CH}_3 \rightarrow \text{CH}_3\text{Cl}$
• $\cdot\text{CH}_3 + \cdot\text{CH}_3 \rightarrow \text{C}_2\text{H}_6$

Because every propagation step makes a fresh radical, one initiation can trigger thousands of reactions (a chain reaction). A downside is a mixture of products: further substitution gives $\text{CH}_2\text{Cl}_2$, $\text{CHCl}_3$ and $\text{CCl}_4$, plus chain-combination products like ethane - so this is a poor route to a single pure compound.