Hydrogen Peroxide : Hydrogen Peroxide, Formula

The vapour-density of hydrogen peroxide at 90° is 17; the molecular weight of the substance has been found from the freezing point of its aqueous solution (Carrara, 1892) to be 34; hence its formula is H₂O₂. In the pure state hydrogen peroxide, like water, is probably associated.

The constitutional formula may be H-O-O-H, i.e., dihydroxyl, HO•OH. This is in accordance with the instability of compounds which contain chains of directly linked oxygen atoms.

In order to account for the instability of one oxygen atom, which suggests that it is linked differently from the other, Kingzett (1884) wrote the formula Kingzett H2O2

, in which one oxygen is quadrivalent. In the modern theory of valency the maximum covalency of oxygen is three, and this formula would have to be written as [O=O-H]^-H⁺, a weak acid. The concentrated peroxide evolves carbon dioxide from a solution of sodium carbonate added to it drop by drop: H₂O₂ + Na₂CO₃ = Na₂O₂ + H₂O + CO₂. (If the peroxide is added to the carbonate, pure oxygen is evolved by catalytic decomposition.)

By the action of hydrogen peroxide on diethyl sulphate, (C₂H₅)₂SO₄, Baeyer and Villiger (1900) obtained diethyl peroxide (C₂H₅)₂O₂, and ethyl hydroperoxide, C₂H₅HO₂. The former boils at 65° and is stable; the latter is violently explosive. By the action of zinc and acetic acid on diethyl peroxide it is reduced to ethyl alcohol, C₂H₅OH. This agrees with the formula C₂H₅OOC₂H₅:

= C₂H₅OH +HOC₂H₅

Kingzett's formula, on the contrary, would require that ether, (C₂H₅)₂O, should be formed:

[png:19_05.png text:Kingzetts equation]

The formula of ethyl peroxide is therefore C₂H₅OOC₂H₅ and it is probable that the formula of hydrogen peroxide is HOOH. The two hydrogen atoms are probably fixed in two perpendicular planes passing through the O-O axis: probably peroxide structure

H₂O₂ is a true peroxide containing two singly linked oxygen atoms: H2O2

. Sodium peroxide is

and barium peroxide

. True peroxides, which give hydrogen peroxide with dilute acids, differ in constitution from the dioxides of lead, manganese, etc., which give oxygen with concentrated sulphuric acid, and chlorine with concentrated hydrochloric acid. Their formulae are of the type:

. This is confirmed by the formation of unstable higher chlorides on treatment with cold concentrated hydrochloric acid; e.g., MnCl4 and PbCl4. These form complex salts, e.g., (NH₄)₂PbCl6, ammonium chloro-plunibate. With hot concentrated hydrochloric acid, however, barium peroxide evolves chlorine (Brodie, 1863):

BaO₂ + 4HCl = BaCl₂ + Cl₂ + 2H₂O.

Protein crystallography
Chemistry guide
Hydrogen Peroxide
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