Chemical Properties of Potassium Permanganate

Potassium permanganate is a strong oxidising agent whose specific oxidising behaviour differs markedly depending on the reaction medium in which it is used -- neutral, alkaline (basic), or acidic -- with the acidic-medium pathway delivering by far the most powerful (most electron-hungry) oxidation of the three.

Before turning to its oxidising chemistry, two further reactions of KMnO₄ are worth noting. On simple heating (thermal decomposition), potassium permanganate disproportionates: some of the manganese is reduced from +7 down to +6 (giving green potassium manganate, K₂MnO₄), while some is reduced further still, down to +4 (giving black manganese dioxide, MnO₂), with oxygen gas evolved in the process: 2KMnO₄ →(Δ) K₂MnO₄ + MnO₂ + O₂ -- this thermal decomposition is, in fact, the standard simple laboratory method for generating oxygen gas from a solid reagent in a test-tube demonstration. Separately, on treatment with concentrated sulphuric acid, the outcome depends sharply on temperature: with COLD concentrated H₂SO₄, potassium permanganate decomposes to give manganese heptoxide (Mn₂O₇), an unstable oily liquid that subsequently decomposes further, explosively, to manganese dioxide and oxygen (2KMnO₄ + 2H₂SO₄ →(cold) Mn₂O₇ + 2KHSO₄ + H₂O, followed by 2Mn₂O₇ →(Δ) 4MnO₂ + 3O₂); but with HOT concentrated H₂SO₄, the product is instead manganese(II) sulphate, MnSO₄, with the manganese reduced all the way down to the +2 state (4KMnO₄ + 6H₂SO₄ →(hot) 4MnSO₄ + 2K₂SO₄ + 6H₂O + 5O₂).

Turning to its central oxidising chemistry: in NEUTRAL medium, permanganate is reduced only as far as manganese dioxide, MnO₂ -- a comparatively mild, three-electron reduction: MnO₄⁻ + 2H₂O + 3e⁻ → MnO₂ + 4OH⁻. Two worked examples of this neutral-medium pathway are given: (i) permanganate oxidises hydrogen sulphide (H₂S) to elemental sulphur, 2MnO₄⁻ + 3H₂S → 2MnO₂ + 3S + 2OH⁻ + 2H₂O; and (ii) permanganate oxidises thiosulphate ion to sulphate ion, 8MnO₄⁻ + 3S₂O₃²⁻ + H₂O → 6SO₄²⁻ + 8MnO₂ + 2OH⁻.

In ALKALINE medium, in the presence of added alkali-metal hydroxide, the permanganate ion is first converted (reduced by one electron) into manganate ion, MnO₄⁻ + e⁻ → MnO₄²⁻; this manganate intermediate is then further reduced, by whatever reducing agent is present, down to manganese dioxide, MnO₄²⁻ + 2H₂O + 2e⁻ → MnO₂ + 4OH⁻. Combining these two steps gives an overall three-electron equation, MnO₄⁻ + 2H₂O + 3e⁻ → MnO₂ + 4OH⁻, which is numerically identical to the neutral-medium equation already given above -- the alkaline pathway simply proceeds via a distinguishable two-step manganate intermediate rather than a single concerted step, but arrives at the same overall reduction. A specific and practically important reagent built on this alkaline-medium chemistry is Baeyer's reagent: cold, dilute, alkaline potassium permanganate solution, used to oxidise alkenes into vicinal diols (for example, converting ethylene into ethylene glycol) -- a colour-fading reaction (from purple permanganate to brown MnO₂ precipitate) that serves as the classic Baeyer test for detecting carbon-carbon double-bond unsaturation in an organic compound.

In ACIDIC medium, specifically in the presence of dilute sulphuric acid, potassium permanganate acts as a genuinely very strong oxidising agent, with the permanganate ion reduced all the way down to the pale pink Mn²⁺ ion -- a substantial five-electron reduction: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O. This acidic-medium pathway underlies a wide range of worked oxidation examples: (i) permanganate oxidises ferrous (Fe²⁺) salts to ferric (Fe³⁺) salts, 2MnO₄⁻ + 10Fe²⁺ + 16H⁺ → 2Mn²⁺ + 10Fe³⁺ + 8H₂O; (ii) permanganate oxidises iodide ion to iodine, 2MnO₄⁻ + 10I⁻ + 16H⁺ → 2Mn²⁺ + 5I₂ + 8H₂O; (iii) permanganate oxidises oxalic acid to carbon dioxide, 2MnO₄⁻ + 5(COOH)₂ + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O -- the basis of the well-known standardisation of KMnO₄ solutions against a primary-standard oxalic acid or sodium oxalate solution; (iv) permanganate oxidises sulphide ion to sulphur, 2MnO₄⁻ + 5S²⁻ + 16H⁺ → 2Mn²⁺ + 5S + 8H₂O; (v) permanganate oxidises nitrite ion to nitrate ion, 2MnO₄⁻ + 5NO₂⁻ + 6H⁺ → 2Mn²⁺ + 5NO₃⁻ + 3H₂O; (vi) permanganate oxidises ethanol to acetaldehyde (a milder, PARTIAL oxidation, contrasting directly with acidified dichromate's complete oxidation of ethanol all the way to acetic acid, section 4.4.2.4), 2KMnO₄ + 3H₂SO₄ + 5CH₃CH₂OH → K₂SO₄ + 2MnSO₄ + 5CH₃CHO + 8H₂O; and (vii) permanganate oxidises sulphite ion to sulphate ion, 2MnO₄⁻ + 5SO₃²⁻ + 6H⁺ → 2Mn²⁺ + 5SO₄²⁻ + 3H₂O. …