Ozone



 

Learning Objective

  • Discuss the properties of ozone.

Key Points

    • Ozone is formed from O2 by the action of ultraviolet light and also atmospheric electrical discharges. It is present in low concentrations throughout the Earth’s atmosphere.
    • Ozone is slightly soluble in water, and much more soluble in inert nonpolar solvents such as carbon tetrachloride (CCl4) or fluorocarbons, where it forms a blue solution.
    • Ozone will oxidize most metals (except gold, platinum, and iridium) to oxides of the metals in their highest oxidation state.
    • Alkenes can be oxidatively cleaved by ozone, in a process called ozonolysis. With reductive workup (e.g., zinc in acetic acid or dimethyl sulfide), ketones and aldehydes will be formed. With oxidative workup (e.g. aqueous or alcoholic hydrogen peroxide), carboxylic acids will be formed.
    • Ozone, along with reactive forms of oxygen such as superoxide, singlet oxygen, hydrogen peroxide, and hypochlorite ions, is naturally produced by white blood cells and other biological systems as a means of destroying foreign bodies.

Terms

  • diamagneticExhibiting diamagnetism; repelled by a magnet.
  • ozoneA triatomic molecule, also called trioxygen, consisting of three oxygen atoms (O3).
  • AlkenesIn organic chemistry, an alkene, olefin, is an unsaturated chemical compound containing at least one carbon-to-carbon double bond.

Properties of Ozone

Ozone (O3), or trioxygen, is a triatomic molecule consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope (O2), breaking down with a half life of about half an hour in the lower atmosphere to O2. Ozone is diamagnetic, which means that its electrons are all paired. In contrast, O2 is paramagnetic, containing two unpaired electrons.

Resonance Structures of OzoneThe two resonance structures of O3 are shown.

Ozone in the Atmosphere

Ozone is formed from dioxygen by the action of ultraviolet light and also atmospheric electrical discharges. It is present in low concentrations throughout the Earth’s atmosphere. In total, ozone makes up only 0.6 parts per million of the atmosphere. Ozone’s odor is sharp, reminiscent of chlorine, and detectable by many people at concentrations of as little as 10 parts per billion in air. In standard conditions, ozone is a pale blue gas that condenses at progressively cryogenic temperatures to a dark blue liquid and finally a violet-black solid. Ozone is a powerful oxidant (far more so than dioxygen) and has many industrial and consumer applications related to oxidation. However, this same high oxidizing potential causes ozone to damage mucus and respiratory tissues in animals as well as tissues in plants, when it exists in concentrations above 100 parts per billion. This makes ozone a potent respiratory hazard and pollutant near ground level. However, the so-called ozone layer (a portion of the stratosphere with a higher concentration of ozone, from two to eight ppm) is beneficial. It prevents damaging ultraviolet light from reaching the Earth’s surface, which benefits all living organisms.

Structure of OzoneOzone is a triatomic molecule with no unpaired electrons and a bent molecular shape. The bond lengths and angle formed by the three O atoms are shown.

Physical Properties of Ozone

Ozone is slightly soluble in water and much more soluble in inert nonpolar solvents such as carbon tetrachloride or fluorocarbons, where it forms a blue solution. At 161 K (−112 °C), it condenses to form a dark blue liquid. It is dangerous to allow this liquid to warm to its boiling point because both concentrated gaseous ozone and liquid ozone can detonate. At temperatures below 80 K (−193 °C), it forms a violet-black solid. It is also unstable at high concentrations, decaying to ordinary diatomic oxygen (with a half-life of about half an hour in atmospheric conditions):

[latex]2O_3 \rightarrow 3O_2[/latex]

This reaction proceeds more rapidly with increasing temperature and increased pressure.

Chemical Reactivity of Ozone

Ozone will oxidize most metals (except gold, platinum, and iridium) to oxides of the metals in their highest oxidation state. For example: [latex]{ 2Cu }^{ + }+2{ { H }_{ 3 }O }^{ + }+{ O }_{ 3 }\rightarrow 2{ Cu }^{ 2+ }+3{ H }_{ 2 }O+{ O }_{ 2 }[/latex]

Alkenes can be oxidatively cleaved by ozone in a process called ozonolysis, giving alcohols, aldehydes, ketones, and carboxylic acids, depending on the second step of the workup.

OzonolysisThe cleavage of carbon-carbon double bonds by O3 is shown in this figure.

Usually, ozonolysis is carried out in a solution of dichloromethane at a temperature of -78oC. After a sequence of cleavage and rearrangement, an organic ozonide is formed. With reductive workup (e.g., zinc in acetic acid or dimethyl sulfide), ketones and aldehydes will be formed. With oxidative workup (e.g., aqueous or alcoholic hydrogen peroxide), carboxylic acids will be formed.

Ozone’s Role in Biological Processes

Ozone, along with reactive forms of oxygen such as superoxide, singlet oxygen, hydrogen peroxide, and hypochlorite ions, is naturally produced by white blood cells and other biological systems (such as the roots of marigolds) as a means of destroying foreign bodies. Ozone reacts directly with organic double bonds.

When ozone breaks down to dioxygen, it produces oxygen free radicals, which are highly reactive and capable of damaging many organic molecules. Moreover, it is believed that the powerful oxidizing properties of ozone may be a contributing factor of inflammation. The cause-and-effect relationship of how the ozone is created in the body and what it does is still under consideration and still subject to various interpretations, since other body chemical processes can trigger some of the same reactions.