Potassium Permanganate - Reactions - Organic Chemistry

Organic Chemistry

Dilute solutions of KMnO₄ convert alkenes into diols (glycols). This behaviour is also used as a qualitative test for the presence of double or triple bonds in a molecule, since the reaction decolorizes the permanganate solution. It is sometimes referred to as Baeyer's reagent. However, bromine serves better in measuring unsaturation (double or triple bonds) quantitatively, since KMnO₄, being a very strong oxidizing agent, can react with a variety of groups.

Under acidic conditions, the alkene double bond is cleaved to give the appropriate carboxylic acid:

CH₃(CH₂)₁₇CH=CH₂ + 2 KMnO₄ + 3 H₂SO₄ → CH₃(CH₂)₁₇COOH + CO₂ + 4 H₂O + K₂SO₄ + 2 MnSO₄

Potassium permanganate oxidizes aldehydes to carboxylic acids, such as the conversion of n-heptanal to heptanoic acid:

5 C₆H₁₃CHO + 2 KMnO₄ + 3 H₂SO₄ → 5 C₆H₁₃COOH + 3 H₂O + K₂SO₄ + 2 MnSO₄

Even an alkyl group (with a benzylic hydrogen) on an aromatic ring is oxidized, e.g. toluene to benzoic acid.

5 C₆H₅CH₃ + 6 KMnO₄ + 9 H₂SO₄ → 5 C₆H₅COOH + 14 H₂O + 2 K₂SO₄ + 6 MnSO₄

Glycols and polyols are highly reactive toward KMnO₄. For example, addition of potassium permanganate to an aqueous solution of sugar and sodium hydroxide produces the "chemical chameleon" reaction, which involves dramatic color changes associated with the various oxidation states of manganese. A related vigorous reaction is exploited as a fire starter in survival kits. For example, a mixture of potassium permanganate and glycerol or pulverized glucose ignites readily. Its sterilizing properties are another reason for inclusion of KMnO₄ in a survival kit.

By itself, potassium permanganate does not dissolve in many organic solvents. If an organic solution of permanganate is desired, "purple benzene" may be prepared, either by treating a two phase mixture of aqueous potassium permanganate and benzene with a quaternary ammonium salt, or by sequestering the potassium cation with a crown ether.