Lecture Eight: Reaction with Potassium Permanganate PDF

# Lecture Eight ## Reaction with Potassium Permanganate ### Principle - Alkenes can be oxidized to geminal diols by diluted aqueous solutions of potassium permanganate. - Diols refer to two hydroxyl groups and geminals refer to the adjacent carbons. - Alkynes can also be oxidized but to germinal d...

# Lecture Eight ## Reaction with Potassium Permanganate ### Principle - Alkenes can be oxidized to geminal diols by diluted aqueous solutions of potassium permanganate. - Diols refer to two hydroxyl groups and geminals refer to the adjacent carbons. - Alkynes can also be oxidized but to germinal diketones. - In this process, a brown precipitate of manganese oxide is formed by the reduction of purple potassium permanganate. - This reaction occurs at the junction between water and hydrocarbons because neither water nor potassium permanganate is soluble in hydrocarbons. - Several minutes are required for the appearance of the brown precipitate because this reaction is slow. #### Example * 3H3C-CH=CH-CH3 + 2KMnO4 + 4H2O → 3 H3C-CH-CH-CH3 + 2MnO2 + 2KOH * **Purple** **a geminal diol** **Brown** * 3 H3C-CC- CH3 + 4KMnO4 + 2H2O → 3 H3C-C-CH3 + 4 MnO2 + 4KOH * **Purple** **a geminal diketone** **Brown** - Geminal diols yielded from alkenes and germinal diketones yielded from alkynes can be further oxidized by permanganates in hard conditions to form much more manganese dioxide. - There is no reaction between alkanes and aromatic rings with diluted aqueous solutions of potassium permanganate. ## Procedure 1. Label four test tubes with the name of a different hydrocarbon (hexane, cyclohexene, 1-hexyne, and toluene) 2. Place 10 drops of the suitable hydrocarbon in each of these test tubes. 3. Put 10 drops of 1% aqueous potassium permanganate into each test tube. 4. Mix the test tube as above several times after the addition. 5. You will observe that: - Hexane and toluene do not react with aqueous potassium permanganate solution and the purple color of potassium permanganate will persist. - There is a small solubility between the cyclohexene and 1-hexyne with aqueous potassium permanganate. - The permanganate and hydrocarbon can react at the interface between the liquids. - After 5 minutes of intermittent agitation, brown precipitate of manganese dioxide is produced in the tube. - The image shows a negative test (left) and a positive test (right).