Radicals (HL IB Chemistry)

• 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

• 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

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