Oxygen occurs in nature as oxygen gas, O2, and ozone, O3. Our lives depend on the 21% (by volume) of oxygen in the air, and our protection against the UV radiation from the sunlight is a belt of ozone at about 25km up in atmosphere. Ozone is produced in the atmosphere by the action of ultraviolet light on oxygen gas.
There are three natural isotopes of oxygen: 16O (99.76%), 17O (0.04%) and 18O (0.20%).
Oxygen gas reacts with almost all elements, except the noble gases and the noble earth metals, to give oxides. The earth's crust contains about 50% by weight of oxygen. Ozone is an even more powerful reagent.
Tutorial 1
The bond angle for ozone, O−O−O, is about 117�, and the all the bond lengths are similar with a value of 1.28�. The bond length of an O−O σ−bond is 1.49 A and an O=O π−bond is 1.27�. Propose the bonding between the oxygen atoms in ozone. Answer
Oxygen has the electron configuration [He] : 2s� , 2p4 , and so it can take two electrons to achieve the neon configuration. In this case it will become the oxide ion, O‾�. It can also share two of its electrons to form covalent bonds, to achieve the neon configuration.
Besides these primary mode of bonding the oxygen atom also has strong tendency to interact with electron deficient groups via its lone electron pair. One very good illustration is the hydroxonium ion.
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The π−bond in the oxygen gas can also interact with an empty orbital of the transition metals. One important reaction is with the heme unit. The heme is an organo-metallic complex with an iron (II) ion in the centre. This heme unit is attached to the protein globin to form the protein molecule of blood, haemoglobin. The ferrous ion in the haemoglobin absorbs the oxygen via this form of interaction and carried it to the required areas in our body. Other molecules with π-bond and small enough to fit into the haemoglobin can also attach itself onto the ferrous ion in a similar manner. A good example will be carbon dioxide, CO. Such molecules can deprive the haemoglobin of oxygen. In this case the person can die from carbon monoxide poisoning.
Oxygen gas is an oxidizing agent and dissolves readily in organic solvents. If you are conducting reactions in organic solvents that are sensitive to oxidation you must take precaution to exclude oxygen. One method is by flushing the solution with nitrogen. For more stringent requirement a freeze-dry technique will have to be used. In all cases the experiment must be conducted in an oxygen free enclosure (a box) normally filled with nitrogen or even with argon.
OXIDES & HYDROXIDES
Most of the metallic elements will form ionic oxides and they are basic anhydrides. If the oxide can dissolve in water it will form the basic hydroxide. The alkali elements and the alkaline earth elements form oxides that are soluble in water. For example sodium;
If the metallic oxides are not soluble in water, it will most likely dissolve in dilute acids to give a salt and water. Example: magnesium oxide.
Note that although it is an ionic oxide we do not get O‾� in solution.
Most non-metallic elements will form covalent oxides and they are usually acidic anhydrides. If it can dissolve in water it will give an acid. The most wellknown are: NO, NO2, and SO2. Such acids are referred to as oxo acids.
The insoluble covalent oxides generally dissolve in bases to give salts.
There is a special class of oxides, known as amphoteric oxides, which have both basic and acidic properties.
Some elements can form more than one type of oxide. Carbon gives carbon monoxide and carbon dioxide. While carbon dioxide is a covalent oxide that dissolves in water to give an acid, carbon monoxide will not dissolve in water, nor will it react with bases. Similarly there is nitrous oxide, N2O, nitric oxide, NO, and nitrogen dioxide, NO2. Nitrous oxide will not dissolve in water to give an acid, nor react with base.
HYDROGEN PEROXIDES, H2O2
Hydrogen peroxide is sold commercially as 20%, 35% or 65% (by weight per volume) solutions. The exact concentration can be determined using an iodine solution. Hydrogen peroxide is not very stable at room temperature.
So it is best to store the solutions in a refrigerator. The oxygen atom recombines instantaneously to form the O2 molecule. The oxygen atom is a very powerful oxidising agent. So wear a pair of rubber gloves when handling hydrogen peroxide solutions. The trace amount of hydrogen peroxide on the cap of the bottle is sufficient to irritate your skin by oxidation and turn it white.
The decomposition of hydrogen peroxide is catalysed in a basic solution or by traces of metal ions.
Hydrogen peroxide is also a weak acid.
PEROXIDES, -O-O-
Peroxides of the Group IA and IIA have also been prepared. For example sodium peroxide, Na2O2 is a stable solid. They are very powerful oxidizing agents and can even oxidise carbon dioxide to a carbonate.
These compounds when dissolve in water or dilute acid will give hydrogen peroxide. However commercially hydrogen peroxide is not prepared from the peroxides.
Other important inorganic peroxides are the persulphates (or peroxosulphates), like potassium and ammonium persulphates. The persulphate ion has the structure [OS(O2)-O-O-S(O2)-O]‾�. Needless to say they are also strong oxidizing agents in an acid solution.
Organic acids can also react with hydrogen peroxide to form the peracid. Example: peracetic acid,
The reaction is catalysed by hydrogen sulphate. Peracetic acid is not very stable. The more stable peracids are perbenzoic acid and cumyl hydroperoxide.
Organic peroxides can explode when heated. So in the distillation of ethers, or unsaturated hydrocarbons, it is very important that they are not heated to dryness. Such organic compounds can form peroxides with oxygen of the air.
The next time you visit a pharmacy you may want to look up the antiseptic section. Do not be surprise if you find 20% solutions of hydrogen peroxide on the shelf.
Tutorial 2
How does hydrogen sulphate increase the yield of peracid in the above react? Answer
