Oxidation of Alkynes and Alkenes using Potassium Permanganate (KMnO4)

When 1 ml of hexane is mixed with 2-3 drops of KMnO4, there is no observable change. This is due to the fact that hexane does not react with KMnO4 under these conditions. However, the oxidative behavior of alkynes and alkenes in the presence of KMnO4 is quite interesting and involves complex transformations worth exploring.

Oxidation of Alkynes with KMnO4

By using hot alkaline solutions of KMnO4, the alkyne molecule undergoes oxidative cleavage at the position of the triple bond, resulting in the formation of two carboxylic acids. If the triple bond is involved with a terminal carbon on the chain, the oxidation of that carbon continues up to CO2.

For 3-hexyne, the reaction can be represented as follows:

C6H10 2 KMnO4 → 2 C2H5COOK 2 MnO2

For 2-hexyne, the equation is only slightly different:

C6H10 2 KMnO4 → C3H7COOK CH3COOK 2 MnO2

For 1-hexyne, the equation is more complex due to the involvement of a terminal carbon:

3 C6H10 8 KMnO4 H2O → 3 C4H9COOK 8 MnO2 3 CO2 5 KOH

From a redox standpoint, the first two equations do not present significant complexity. The two carbon atoms of the triple bond are oxidized from 0 to 3, while Mn is reduced from 7 to 4, requiring a 1:2 molar ratio between the alkyne and KMnO4. In the third equation, the second inner carbon of the triple bond goes from 0 to 3, and the first outer carbon goes from -1 to 4, requiring a 3:8 molar ratio between 1-hexyne and KMnO4.

Oxidation of Alkenes with KMnO4

Your revised question referred to 1-hexene, not 1-hexane, as 1-hexane does not exist. Potassium permanganate reacts with 1-hexene just like it reacts with any alkene. The product in this case would be a vic-diol, also known as a glycol, specifically 1,2-hexanediol.

Under more vigorous conditions, the vic-diol can undergo additional oxidation, breaking the bond between the two alcohol carbons and forming carbonyl groups on each. If the carbonyl is an aldehyde, both would be here. The final products would be pentanoic acid, carbon dioxide, and water. The formic acid from the end carbon would be oxidized to carbonic acid, i.e., H2O CO2.

Conclusion

The behavior of alkynes and alkenes in the presence of KMnO4 can lead to complex and intriguing transformations, depending on the specific structure and conditions of the reactants. Understanding these reactions is crucial for various applications in organic synthesis and materials science.

For further reading, explore the detailed mechanisms and chemical pathways involved in these oxidations to gain a deeper understanding of the underlying chemistry.