There may be organic peroxides such as diakyl peroxides (R 1 -OOR 2 ), hydroperoxides (ROOH), peracids (RCO-OOH), and peresters (R 1 CO-OOR 2 ). We also have inorganic peroxides, such as metal peroxides (M n + O 2 2- ) and hydrogen peroxide (HOOH). The latter is the best known of all, receiving the name of hydrogen peroxide.
All peroxides share in common the property of being highly reactive, oxidizing and, in some cases, explosive substances. Unless synthesized, they are considered unwanted and dangerous impurities for many industrial processes and for laboratory synthesis.
Properties of peroxides
The structures of peroxides are centered around the OO group. Each oxygen atom has an sp 3 hybridization , so the -OO- bonds do not lie in the same plane. For example, for hydrogen peroxide, HOOH, the angle formed between the two Hs is 115.5º, which shows that the four atoms are not on a plane.
The distortions of this and other angles will depend on the identity of the other molecular fragments linked to the OO group.
On the other hand, organic and inorganic peroxides (with the exception of metallic ones) can be recognized with the naked eye if the OO group is detailed in their structure, like two glasses with a “mouth”. In this way, any peroxide is immediately identified with a single glance of its structure.
The OO bond is relatively weak compared to other bonds such as CH or CO. That is why it tends to break up or fragment to produce highly reactive oxygenated radicals.
It is because of these radicals that peroxides are classified as reactive and dangerous substances, as they can damage tissues and any organic matter in general.
Peroxides are powerful oxidizing agents, seeking to gain electrons from the environment to transform into water, oxygen, or hydroxides. For example, hydrogen peroxide is a stronger oxidizing agent than potassium permanganate itself.
This oxidizing action is used to detect them through their reaction with ferrous sulfate and potassium thiocyanate:
Peroxide + Fe 2+ + SCN – → Fe (SCN) n (3-n) – (n = 1 to 6).
The complex formed between Fe 3+ and SCN – is blood red in color, so the observation of this color corresponds to a positive test for the presence of peroxides in the sample.
Peroxides are substances that are sensitive to high temperatures and sunlight. Therefore, they should be stored in safe places and in opaque plastic containers, in such a way that light does not shine directly on them.
The nomenclature of peroxides depends on which type they belong to. In general, many are named by mentioning ‘peroxide’ followed by the R substituents in alphabetical order. For example, the compound CH 3 C (O) OOC (O) CH 3 (lower image) is called diacetyl or acetyl peroxide, because it has an acetyl group at each end.
The compound C 6 H 5 C (O) OOC (O) C 6 H 5 (lower image), on the other hand, is called dibenzoyl or benzoyl peroxide, for the same reasons discussed in the previous paragraph.
Meanwhile, the compound C 6 H 5 C (O) OOC (O) CH 3 is called acetyl benzoyl peroxide. This time the acetyl substituent is mentioned first by starting with the letter ‘a’. Another example is CH 3 CH 2 OOC 6 H 5 , called ethyl phenyl peroxide.
Similarly, we proceed with the hydroperoxides. For example, CH 3 OOH is called methyl hydroperoxide.
Some peroxides can be formed directly by the exposure of the substance in question with oxygen in the air, assisted or not with metallic catalysts. They can also be formed by exposing the substance to ozone under certain conditions, carrying out the ozonolysis reaction.
Another synthetic route is to react hydrogen peroxide with an alkyl sulfate:
R 2 SO 4 + H 2 O 2 → ROOR + H 2 SO 4
In order to thus synthesize or form dialkyl peroxides or even cyclic peroxides.
Meanwhile, several of the methods to form metal peroxides are based on oxygenating their respective metal oxides, so that they “oxygenate” and go from being MO (M n + O 2- ) to MO 2 (M n + O 2 2- ) .
Uses of peroxides
Peroxides, especially organic peroxides, are added to the reaction media for the synthesis of infinities of plastics and materials such as polystyrene, silicone, epoxy and acrylic resins, polyvinyl chloride, fiberglass, among others.
This is because they undergo thermal decompositions to become oxygenated free radicals, which in turn trigger and catalyze polymerizations. Therefore, they are desirable in the polymer industry.
Some of the peroxides used as initiators are methyl ethyl ketone (MEK) peroxide, and acetone peroxide.
Bleaches and disinfectants
Peroxides are bleaching agents, used for the bleaching of paper pulp and flour. Likewise, they are disinfectant agents, totally effective although aggressive in terms of the extermination of bacteria. The classic example corresponds to hydrogen peroxide, used in dilute solutions (3%) to disinfect wounds.
Peroxides are also oxidizing agents, which is why they find many uses in the organic syntheses of highly oxygenated substances.
Although it is not among its main uses, peroxides can also serve as sources of oxygen in some of its decompositions. This is the case, again, of hydrogen peroxide.
Examples of peroxides
Finally, some examples of peroxides, whether organic or inorganic, with their formulas and respective names will be listed:
-H 2 O 2 : hydrogen peroxide or hydrogen peroxide
-Na 2 O 2 : sodium peroxide
-BaO 2 : barium peroxide
-C 6 H 5 C (CH 3 ) 2 OOH: cumene hydroperoxide
– (NH 4 ) 2 S 2 O 8 : ammonium persulfate
– HO 3 SOOSO 3 H: peroxydisulfuric acid
– (CH 3 ) 3 COOC (CH 3 ) 3 : ditert-butyl peroxide