•OH    The Hydroxyl Radical

Nature's most

reactive Free Radical

Oxidized water, the hydroxyl radical contains one hydrogen and one oxygen.

The hydroxyl radical is electrically neutral and the oxygen atom has an unpaired outer valence electron.


This makes it highly reactive. It is a strong electrophile and wants to steal an electron from any molecule it meets.


Removing an electron is oxidation. Many times the oxidized molecule becomes itself, a radical that can enter a chain reaction of repeated electron losses and molecular disintegration

 The hydroxyl's electron attractive force, which is its oxidizing power, is second only to Flourine.

 Hydroxyl radicals are short lived, usually finding an electron within milliseconds



Advanced Oxidation




The Hydroxyl Radical:


(1) Adds oxygen to inorganic molecules


(2) "Burns" organics producing CO2, water, and

 Fully oxidized inorganic residues


These reactions take place at low temperature and low pressure.

Depending on what compound and whether in gaseous or liquid phase, the hydroxyl radical performs differently.

Some examples are given below. In all cases, the hydroxyl radical has the ability to "incinerate" the compounds it comes into contact with.

Inorganic Oxides

The oxidation states of sulfur and nitrogen are increased and they join with the oxygen from the hydroxyl radical, which is reduced

Organic comounds are "incinerated" in a radical oxidation chain reaction, with end products water, carbon dioxide, gaseous oxides insoluble oxide precipitants (a kind of ash).

For Aliphatics


Hydrogen is removed and hydroxyl is reduced to water. Intermediates are themselves radicals, which further breakdown.


(1) RH  +  •OH  →  H2O  +  •R (Alkyl Radical)


(2) •R  +   O→  ROO• (Peroxy Radical)


(3) ROO•  +  RH  →  ROOH  +  •R



For Aromatics  (shown as Ar)


•OH is added on creating unstable compounds that further breakdown in a chain reaction.


(1) ArH  +  •OH  →  ArH(OH)•


(2) ArH(OH)•  + •O→  [ArH(OH)OO]•


(3) [ArH(OH)OO]•  →  Ar(OH)  +  HO2



For Anions (in water)


Electrons are removed, forming reactive species that combine with dissolved cations to form stable oxidant precipitants.


Water contains hydrogen and oxygen in various stable forms.

Energy in the form of heat, light, or electric field must be applied in order to form the highly energetic but unstable •OH radical.

These reactions are more likely to occur in the presence of a catalyst,  which holds the hydrogen and oxygen in favorable energy states for •OH formation.



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