Which of the following is the final product of the addition reaction of water to ethyne in the presence of H2SO4 and HgSO4? (A) Ethanal, (B) ethene, (C) ethane, (D) ethyl alcohol, or (E) vinyl alcohol.
An addition reaction is a type of chemical reaction where two or more molecules combine to form a larger molecule without any by-products forming. In the addition reaction described in the question, water and ethyne are the molecules combining to form a larger product. Sulfuric acid and mercuric sulfate act as catalysts for this reaction. Ethyne is an alkyne, a class of substances which contain a carbon-carbon triple bond. When alkynes first undergo an addition reaction, one of the 𝜋 bonds between the carbon atoms can break and two new substituents can be added to the molecule. In the case of water addition, the two new substituents are a hydrogen atom and a hydroxy group. The product of this reaction contains a carbon-carbon double bond and a hydroxy group. This type of organic substance is an alkenol, but it’s more commonly known as an enol.
Enols also contain a 𝜋 bond, so we might expect that this 𝜋 bond breaks, allowing for the addition of two new substituents, as is the case with many alkyne addition reactions. But the addition of water to an alkyne does not produce a product with two hydroxy groups. The final product of this reaction is not the enol either. As it turns out, enols can undergo a reaction in the presence of the same acid catalyst. Over the course of this reaction, the hydroxy group becomes a carbonyl group and the carbon-carbon double bond becomes a carbon-carbon single bond. This product, the final product of the addition of water to an alkyne, may be an aldehyde or a ketone depending on the groups bonded to the carbonyl carbon.
Here is the overall reaction of a generic alkyne with water. We can use this generic equation to help us determine the product of the reaction of water with ethyne. The prefix eth- tells us that ethyne contains two carbon atoms. And since ethyne is an alkyne, the carbon atoms must be joined by a carbon-carbon triple bond. Let’s follow along with the generic equation to draw the product of the addition of water to ethyne. Over the course of this reaction, the carbon-carbon triple bond will become a carbon-carbon single bond. The original carbon substituents are still bonded to the same carbon atoms at the end of the reaction. One of the carbon atoms forms two new single bonds to hydrogen atoms, while the other carbon atom becomes double bonded to an oxygen atom.
Since ethyne is symmetrical, it doesn’t matter which of the carbon atoms forms which new set of bonds as the product will be the same either way. As one of the substituents bonded to the carbonyl carbon is a hydrogen atom, this product is an aldehyde. The name of this aldehyde is ethanal, eth- meaning two carbon atoms, A-N indicating that the carbon atoms are single-bonded together, and A-L for aldehyde. So the final product of the addition reaction of water to ethyne in the presence of sulfuric acid and mercuric sulfate is ethanal, answer choice (A).