Question

Which of the following alkyne give aldehyde on reaction with dil. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) in the presence of \(\mathrm{HgSO}_{4}\) ? (1) propyne (2) 1 -butyne (3) ethyne (4) all of these

Short answer

Ethyne (option 3) gives an aldehyde.

Step-by-step solution

Identify the Reaction

The reaction involves an alkyne reacting with dilute sulfuric acid \(\mathrm{H}_2\mathrm{SO}_4\) in the presence of mercury sulfate \(\mathrm{HgSO}_4\). This specific combination of reagents typically causes the hydration of the alkyne to form a vinyl alcohol, which tautomerizes to a carbonyl compound (either a ketone or an aldehyde).

Assess the Products

For the given alkynes, determine whether the product formed will be an aldehyde or a ketone:1. Ethyne (acetylene) will give acetaldehyde.2. Propyne will give acetone (a ketone).3. 1-Butyne will give butan-2-one (a ketone).

Analyze Each Alkyne

Since aldehyde formation is required, check which alkyne forms an aldehyde:1. Ethyne (ethyne) forms acetaldehyde (an aldehyde).2. Propyne forms acetone (a ketone).3. 1-Butyne forms butan-2-one (a ketone).

Conclusion

From the analysis, only ethyne forms an aldehyde upon reaction with dilute \(\mathrm{H}_2\mathrm{SO}_4\) in the presence of \(\mathrm{HgSO}_4\).

Key concepts

alkyne reactions

Alkynes are hydrocarbons with at least one triple bond between carbon atoms. They are quite reactive and participate in a variety of chemical reactions. One important reaction involving alkynes is hydration, where water is added to the molecule. This reaction typically uses dilute sulfuric acid (H₂SO₄) in the presence of mercury sulfate (HgSO₄) as a catalyst.
When these conditions are met, the alkyne undergoes hydration to form a vinyl alcohol, or enol. However, enols are usually unstable and quickly convert into more stable carbonyl compounds such as ketones or aldehydes. This conversion is known as tautomerization.
Remember, the specific product formed (whether a ketone or an aldehyde) depends on the structure of the starting alkyne. For terminal alkynes (an alkyne with the triple bond at the end of the molecule), the result can often be an aldehyde.

tautomerization

Tautomerization is a chemical process where a compound converts from one isomer to another. In the context of alkynes, this typically refers to the conversion of an enol (vinyl alcohol) to a carbonyl compound (a ketone or an aldehyde). This change generally occurs through the migration of a hydrogen atom and a shift in the position of a double bond.
During the hydration of alkynes, the initial product is a vinyl alcohol. For instance, when ethyne reacts with dilute H₂SO₄ and HgSO₄, it initially forms vinyl alcohol. Vinyl alcohol is not stable and quickly converts to acetaldehyde through tautomerization.
This conversion happens because the oxygen in the enol donates electrons, stabilizing the formation of a carbonyl (C=O) group. Thus, tautomerization ensures that the more stable aldehyde or ketone forms.

aldehyde formation

Aldehydes are organic compounds containing a carbonyl group (C=O) bonded to at least one hydrogen atom. When considering alkyne reactions, certain conditions favor aldehyde formation, specifically with terminal alkynes.
When ethyne (acetylene) is hydrated using dilute H₂SO₄ and HgSO₄, it forms vinyl alcohol. This vinyl alcohol then tautomerizes to form acetaldehyde, which is an aldehyde.
It's crucial to note that not all alkynes will form aldehydes. For instance, propyne and 1-butyne in a similar reaction generally form ketones instead. Therefore, identifying the starting alkyne and the reaction conditions is key to predicting aldehyde formation. Terminal alkynes like ethyne are excellent candidates for aldehyde synthesis under the right conditions.