Tools of the trade: understanding free radicals
Free radicals are species that have one or more unpaired valence electrons and are typically very reactive. The picture below shows all the valence electrons on the oxygen atom in the hydroxyl radical – notice that there is one unpaired one, making this a free radical (or sometimes simply called a radical).
Free radicals are usually represented with a single dot next to the atom with the unpaired electron (like a lone pair, but with a single electron instead of two). An example of the common representation of the hydroxyl radical is shown below.
In the mechanisms that we studied in week one, any bonds broken occurred via heterolytic cleavage (also known as heterolysis) and we used double-headed curly arrows. A double-headed curly arrow indicates that the two electrons from the covalent bond are both moving onto one atom and results in the formation of a negatively-charged anion and a positively-charged cation.
Conversely, free radicals are formed by homolytic cleavage (also known as homolysis); the process by which a covalent bond is broken symmetrically with one electron going to each atom, a single headed curly arrow is used to represent the movement of a single electron.
Homolysis generates two free radicals, as shown in the following example. The covalent O–O bond in hydrogen peroxide undergoes homolysis to form two hydroxyl radicals. H2O2 contains a very weak O–O bond, which can undergo homolysis at temperatures below 150 °C, or on irradiation with UV light. The process of forming free radicals is called initiation.
Radical reactions that produce new radicals are called propagation reactions. H-O• is a reactive radical and can abstract a hydrogen atom from an organic compound to form a strong O-H bond in water.
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