How to Identify Ethyne and Propyne? | Alkynes with Acidic Hydrogen

Ethyne and propyne are alkynes. Alkyne is one of the type of organic compounds. Ethyne and propyne have two and three carbon atoms respectively. Each molecule has one triple bond in their structure.

ethyne and propyne

In this tutorial, we are going to learn, how to identify ethyne and propyne and discuss why they are unable to identify from acidic hydrogen reactions

How to identify ethyne and propyne?

If we need to identify ethyne and propyne, we have to see a different characteristics of both compounds. Such as,

  • A colour difference
  • physical state (gas, liquid or solid)

These differences can be observed after a reaction if we cannot see a difference in initial compound.

Ethyne and propyne are gases at room temperature. So we cannot separate them from looking their physical state.

Our next step is, doing some reactions to both ethyne and propyne. This means we add same reagent to ethyne and propyne and see whether, is there a visible change?

Acidic hydrogen in alkynes

When there is a hydrogen atom in the carbon atom which is connected to the triple bond, is defined as acidic hydrogen. Both ethyne and propyne have acidic hydrogen atoms.

  • Ethyne has two acidic hydrogen atoms
  • Propyne has one acidic hydrogen atom.

Reactions of alkynes with acidic hydrogen

There are some specified reactions for alkynes with acidic hydrogen than typical alkynes and they are listed below.

  • With sodium, sodamide (NaNH2)
  • With ammoniacal silver nitrate - Tollens' reagent
  • With ammonical Cuprous chloride

Both ethyne and propyne have acidic hydrogen atoms. So we cannot identify ethyne and propyne from these specified reactions

Alkyne hydration

Alkyne hydration gives aldehydes and ketones as products. Dilute H2SO4 and HgSO4 are used as reagents. Ethyne and propyne are hydrated to give acetaldehyde and propanone respectively. Acetaldehyde and propanone are carbonyl compounds. But they have significant differences in reactions.

Ethyne hydration: Acetaldehyde is given as the product

ethyne hydration

Propyne hydration: Propanone is given as the product

propyne hydration

Both acetaldehyde and propanone are liquids at room temperature. So we cannot separate acetaldehyde and propanone from looking theie colour or physical state.

Identify acetaldehyde and propanone

Acetaldehyde and propanone have differences on reactions. Acetaldehyde can be oxidized easily. But propanone cannot be oxidized. We use that characteristic to identify acetaldehyde and propanone.

Acidic potassium permanganate

Acidic potassium permanganate (H+ / KMnO4) solution is added to acetaldehyde and propanone separately to see, what will oxidize. When oxidation reduction reaction occur, purple colour of H+ / KMnO4 is changed to colourless or light pink.

Acetaldehyde oxidation

Acetaldehyde is oxidized by H+ / KMnO4. We can see the colour change.

Propanone is not oxidized by H+ / KMnO4.

So, we can identify acetaldehyde and propanone by H+ / KMnO4. Acetaldehyde and propanone are related to ethyne and propyne respectively. Therefore ethyne and propyne can identify from this method.

How to identify ethyne and propyne