Radicals
- Reaction equations tell you about the amount of reactants and products, including their stoichiometry, in a reaction
- Reaction mechanisms tell you about how the reaction actually takes place
- Mechanisms involve the movement of electrons and a variety of chemical species
- One group of chemical species involved in reaction mechanisms are radicals
What are radicals?
- A radical is a chemical species that has an unpaired electron
- They can be described as:
- Atomic - a single atom with an unpaired electron
- Polyatomic / molecular - a group of atoms bonded together with no overall charge, that contains an unpaired electron
- Anionic - an atom or molecule that gains one electron to become an anion AND has one atom with an unpaired electron
- Cationic - an atom or molecule that loses one electron to become a cation AND has one atom with an unpaired electron
- The sole requirement for a radical is the unpaired electron
- They can exist independent of the charge on the chemical species, e.g. cations must have a corresponding anion and vice versa
Examples of radicals table
| name | formula | atomic | polyatomic / molecular |
anionic / cationic |
| benzene | •C6H6– | ✓ | anionic | |
| bromine | Br• | ✓ | ||
| ethanol | C2H5OH+• | ✓ | cationic | |
| hydrogen | H• | ✓ | ||
| hydroxyl | OH• | ✓ | ||
| methyl | •CH3 | ✓ | ||
| nitric oxide | NO• | ✓ | ||
| propane | •C3H8+ | ✓ | cationic | |
| superoxide | O2–• | ✓ | anionic |
- Radicals are indicated by the dot (•) in the formula of the chemical species
- When the radical is made of several atoms, the radical dot should be shown on the atom with the unpaired electron
- This is most obviously seen in displayed formulae
The methyl radical
Worked example
Which of the following species are radicals?
- Cl–
- F
- Mg2+
- NH3
- NO2
Answer:
- Draw the Lewis formulas, including all valence electrons, of each species:
|
Cl– |
F |
Mg |
NH3 |
NO2 |
- Identify the species that have an unpaired electron
- The species that are radicals are:
- F
- NO2
- The species that are radicals are:
Reactivity of radicals
- The unpaired electron of a radical makes them highly reactive
- It causes them to have a high enthalpy
- In terms of energetics, it is favourable for radicals to react and form products with a lower enthalpy which can be achieved by:
- Taking an electron from another species - although this creates other radical species because the resulting other species will have an unpaired electron
- Combining with another radical to form a covalent bond
- Their high reactivity means that radicals are, typically, not long lasting