Question

Which of the following react with KMnO₄ but does not react with AgNO₃?
A. C₂H₆
B. CH₄
C. C₂H₄
D. C₂H₂

Answer 1

Hint: KMnO₄ and AgNO₃ are strong oxidising agents and alkanes are saturated hydrocarbons in a saturated hydrocarbon carbons valency is completely fulfilled sigma bonds and maximum number of Hydrogen is attached with it. Thus, saturated hydrocarbons are highly stable and to break the C-H bond large amounts of energy is required.

Complete Step by Step Answer:
As we can see that,
- C₂H₆ is the formula of ethane
- CH₄ is the formula of methane
- C₂H₄ is the formula of ethene
- C₂H₂ is the formula of acetylene

Now since methane and ethane are saturated hydrocarbons, they will not react with either KMnO₄ or AgNO₃.
When ethene is mixed with KMnO₄ diol is formed.
CH₂=CH₂ KMnO₄ → CH₂(OH)-CH₂(OH)
But, when ethene is mixed with AgNO₃
CH₂=CH₂ AgNO₃ →No reaction
Now acetylene when mixed with KMnO₄ and AgNO₃. It gives a reaction with both

When it reacts with KMnO₄ the product will be acid-
CH≡CH KMnO₄ /H₂O → O=CH-CH=O [O] → HOOC-COOH
At first Ethane 1,2-dial is formed which on further oxidation gives oxalic acid.
When acetylene reacts with ammoniacal AgNO₃ the product formed will be-
CH≡CH AgNO₃(NH₃) → AgC≡CAg + 2NH₄NO₃ + 2NH₃

The product formed in this reaction is disilver acetylide. So, we can see that alkanes that is methane and ethane does not react with either KMnO₄ or AgNO₃ and acetylene reacts with both KMnO₄ and AgNO₃ only ethene is the one that react with KMnO₄ but does not react with AgNO₃. So, the correct answer is ethene.
Thus, Option (C) is correct

Note: As we can see that ethene and acetylene both react with KMnO₄ during the reaction it decolourises its pink colour. KMnO₄ is a very good oxidising agent, with exhaustive oxidation it proceeds in the formation of carboxylic acid. It oxidises aldehydes and ketones into carboxylic acid.