Reaction of KMnO4 with alkynes
The correct answer and explanation is:
The reaction of potassium permanganate (KMnO4) with alkynes typically leads to the oxidation of the triple bond. The general reaction depends on the conditions used.
- Cold, Dilute KMnO4:
When KMnO4 is used in cold and dilute conditions, the alkynes undergo partial oxidation, forming cis-2,3-diol (vicinal diol) through a hydroxylation reaction. In this reaction, the manganese in KMnO4 is reduced from +7 to +4, and the alkynes are added to two hydroxyl groups (-OH) across the triple bond. For example:
RC≡CR’+KMnO4 (cold,dilute)→RC(OH)-C(OH)R’\text{RC≡CR’} + \text{KMnO}_4 \ (cold, dilute) \rightarrow \text{RC(OH)-C(OH)R’}
This reaction adds hydroxyl groups on the same side of the molecule, leading to the formation of a cis diol. - Hot, Concentrated KMnO4:
If potassium permanganate is used under hot and concentrated conditions, a stronger oxidation occurs. In this case, the alkynes are oxidized to carboxylic acids. The KMnO4 breaks the triple bond and cleaves the carbon-carbon bond, resulting in the formation of two carboxyl groups (-COOH). For example:
RC≡CR’+KMnO4 (hot,concentrated)→RCOOH+R’COOH\text{RC≡CR’} + \text{KMnO}_4 \ (hot, concentrated) \rightarrow \text{RCOOH} + \text{R’COOH}
This reaction fully oxidizes the alkyne, converting it into two carboxylic acids.
Explanation of the Mechanism:
KMnO4, being a strong oxidizing agent, tends to add oxygen atoms to unsaturated compounds. For alkynes, the permanganate ion (MnO4⁻) can add two hydroxyl groups across the triple bond in dilute conditions, forming a diol. Under more vigorous conditions (hot and concentrated KMnO4), the alkyne undergoes cleavage and oxidation, breaking down into carboxylic acids. The manganese in KMnO4 serves as the electron acceptor, causing the oxidation of the alkyne.