Free-Radicals in Chemistry

Oxygen radical with one unpaired electron.

What are Free-Radicals?

Free-radicals are highly reactive atoms, compounds, or ions. Chlorine radicals help to synthesize more reactive organic compounds. The atmosphere naturally produces oxygen radical intermediates in chemical reactions between oxygen and ozone. Artificial chemicals called chlorofluorocarbons (CFCs) produce chlorine monoxide radical intermediates in the atmosphere. And the human body naturally produces superoxide anion radicals.

The image below shows the Lewis dot structures for free-radicals of a chlorine atom, oxygen atom, chlorine monoxide molecule, and oxygen molecule. All free-radicals have one unpaired valence electron.

Free-radicals of chlorine, oxygen, chlorine monoxide, and oxygen. The red dots represent the unpaired electron.
Free-radicals with one unpaired valence electron

Atoms, compounds, and ions commonly contain pairs of valence electrons. The Lewis Dot Structures below, compares an oxygen molecule with the superoxide free-radical. The oxygen molecule contains paired electrons and the superoxide radical contains one unpaired electron.

An oxygen molecule with paired electrons. A superoxide radical with one unpaired electron.

Radicals are highly reactive and undergo consecutive chain reactions. Let’s take a closer look at chemical reactions involving free-radicals in organic chemistry, the Earth’s atmosphere, and the human body.

Free-Radicals in Organic Chemistry

In organic chemistry, alkanes are chemically unreactive. Organic chemists use chlorine free-radicals in a series of chain reactions to form a more reactive halogenoalkane species. Chlorine free-radicals convert methane into the more reactive halogenoalkane, chloromethane, in a process called free-radical substitution. This reaction mechanism occurs in three stages: 1) Initiation 2) Propagation 3) Termination.

  1. Initiation: Formation of free-radicals

In the presence of UV light, chlorine atoms separate by homolytic fission. Two electrons in the covalent bond split evenly amongst two chlorine atoms. In the image below, the chlorine (Cl2) molecule splits into two chlorine free-radicals (Cl·).

A chlorine molecule absorbing energy and splitting into two chlorine radicals.
Chlorine molecule Chlorine free-radicals
  1. Propagation: Free-radical chain reactions

Propagation 1: A chlorine free-radical reacts with methane to form hydrogen chloride (HCl) and a methyl free-radical (·CH3).

A chlorine radical reacting with methane to form hydrogen chloride and a methyl free radical.
Chlorine free-radical + MethaneHydrogen chloride + Methyl free-radical

Propagation 2: A methyl free-radical reacts with a chlorine molecule to form the desired product, chloromethane, and a chlorine free-radical. Termination of free-radicals begins in stage 3.

The methyl free-radical reacts with a chlorine molecule to form chloromethane and a chlorine free-radical.
Methyl free-radical + Chlorine Chloromethane + Chlorine free-radical
  1. Termination: Free-Radical “mop up”

The concentration of methane starts to decrease. In the image below, chlorine radicals and methyl radicals react to form molecules. At the end of the reaction, no more radicals are present in the reaction mixture.

In step 1, chlorine radicals react to form a chlorine molecule. In step 2, chlorine and methyl radicals react to form chloromethane. In step, 3 two methyl radicals react to form ethane.
Free-radicals Molecules

Free-Radicals in the Atmosphere: Ozone Protection and Depletion

Ozone Protection & Oxygen Free-Radical Intermediates

Ozone (O3) is present the stratosphere of the Earth’s atmosphere. It protects organic life by absorbing harmful ultraviolet (UV) radiation from the sun. In step 1, UV radiation breaks down ozone to produce an oxygen molecule and an oxygen free-radical intermediate. In step 2, an oxygen molecule and an oxygen free-radical react to reform ozone. Ozone absorbs UV radiation energy and converts it into heat energy which is less harmful.

  1. Ozone absorbs energy (hv) from UV radiation to form oxygen (O2) and an oxygen free-radical (O·).
  2. An oxygen molecule reacts with the oxygen free-radical to reform ozone.
In step 1, Ozone absorbs UV energy to form oxygen and an oxygen radical. In step 2, the oxygen and oxygen radical react to form ozone and heat energy.
Oxygen Free-Radicals intermediates in the Atmosphere

Alternatively, oxygen may absorb UV light to form ozone in a two step reaction mechanism producing oxygen free-radicals intermediates.

  1. Oxygen absorbs UV light to form two oxygen radicals.
  2. An oxygen free-radical reacts with an oxygen molecule to form ozone.
In step 1, oxygen absorbs UV energy to forms two oxygen radicals. In step 2, oxygen and the oxygen radical react to form ozone.
Oxygen Free-Radicals in the Atmosphere

Ozone Depletion – CFC Catalysts & Free-radical Intermediates

The Lewis dot structure of chlorofluoromethane.

Chlorofluorocarbons, CFCs, contain carbon (C), Fluorine (F), and Chlorine (Cl). Refrigerants and aerosol propellants contain CFCs which are non-toxic close on the Earth’s surface. CFCs absorb UV radiation in the stratosphere and chlorine atoms break away from the compound to react with ozone. In step 1, ozone and chlorine atoms react to from a chlorine monoxide (ClO·) free-radical intermediate and an oxygen molecule. In step 2, the chlorine monoxide (ClO·) free-radical reacts with ozone to produce a chlorine atom and oxygen.

  1. A chlorine atom reacts with ozone to form a chlorine monoxide free-radical and oxygen.
  2. The Chlorine monoxide free radical reacts with an ozone molecule to form a chlorine atom and oxygen.
In step 1, a chlorine atom reacts with ozone to form a chlorine monoxide radical and oxygen. In step 2, the chlorine monoxide radical reacts with ozone to form a chlorine atom and oxygen molecule.
Ozone depletion by chlorine atoms produce free-radical intermediates
The concentration of ozone in the Earth's stratosphere. The purple area contains low amounts of ozone. The green area represents high amounts of ozone.
Earth’s Ozone Hole

These chlorine atoms act as catalyst by increasing the rate at which ozone decomposes into oxygen. The use of CFCs caused the ozone layer to diminish. The blue and purple areas represent the “ozone hole” which indicate low concentrations of ozone. The green and yellow areas represent high concentrations of ozone.

Free-Radicals in the Body

Nicotinamide adenine dinucleotide phosphate (NADPH) metabolizes oxygen in the body producing a free-radical, superoxide (O2•–). NADPH transfers one electron to oxygen to quickly form a proton (H+), superoxide anion, and NADP+.

NADPH reacts with oxygen to form a proton (H+), a superoxide radical, and NADP+. An electron is transferred from NADPH to an oxygen molecule to form the superoxide radical.

Superoxides are free-radicals or reactive oxygen species (ROS) that support immunity, cell growth, and cell signaling. Overproduction of ROS are lethal to cells. Various foods and drinks contain antioxidants that protect cells from high concentration of free-radicals. Substances that contain antioxidants have become increasingly popular.