permanganate

The structure of the manganate(VII) anion

A permanganate is the general name for a chemical compound containing the manganate(VII) ion, (MnO₄⁻). Because manganese is in the +7 oxidation state, the manganate(VII) ion is a strong oxidizing agent. The ion has a tetrahedral geometry ^[1] Permanganate solutions are purple in color and are stable in neutral or slightly alkaline media.

In an acidic solution, manganate(VII) is reduced to the colourless +2 oxidation state of the manganese(II) (Mn²⁺) ion.

8 H₃O ⁺ + MnO₄⁻ + 5 e⁻ → Mn²⁺ + 4 H₂O

In a strongly basic solution, manganate(VII) is either reduced to the brown +4 oxidation state of manganese dioxide MnO₂ or to the green +6 oxidation state of the manganate MnO₄²⁻.

2 H₂O + MnO₄⁻ + 3 e⁻ → MnO₂ + 4 OH⁻

MnO₄⁻ + e⁻ → MnO₄²⁻

Production

Permanganates may be produced by oxidation of manganese compounds by strong oxidizing agents, for instance, sodium hypochlorite or lead dioxide.

2 MnCl₂ + 5 NaClO + 6 NaOH → 2 NaMnO₄ + 9 NaCl+ 3 H₂O

2 MnSO₄ + 5 PbO₂+ 3 H₂SO₄ → 2 HMnO₄+ 5 PbSO₄ + 2 H₂O

Or by dismutation of manganates:

3 Na₂MnO₄ + 2 H₂O → 2 NaMnO₄ + MnO₂ + 4 NaOH

Properties

Permanganates(VII) are salts of permanganic acid. Permanganate(VII) is a strong oxidizer, and similar to perchlorate. It is therefore in common use in qualitative analysis that involves redox reactions (permanganometry). Besides this, it is stable.

It is a useful reagent, though with organic compounds, not very selective.

Manganates(VII) are not very thermally stable. For instance, potassium permanganate decomposes at 230 °C:

2 KMnO₄ → K₂MnO₄ + MnO₂ + O₂

A permanganate can oxidize an amine to a nitro compound ^[2][3], an alcohol to a ketone ^[4], an aldehyde to a carboxylic acid ^[5][6], a terminal alkene to a carboxylic acid ^[7], oxalic acid to carbon dioxide ^[8] or an alkene to a diol ^[9] (list not exhaustive)

In alkene oxidations one intermediate is a cyclic Mn(V) species:

Compounds

• Ammonium permanganate
• Calcium permanganate, Ca(MnO₄)₂
• Potassium permanganate, KMnO₄
• Sodium permanganate, NaMnO₄

References