Question

Addition of water to acetylene compounds is catalysed by .......... and \(\ldots \ldots \ldots .\) (a) \(\mathrm{Ba}^{+2}\) salt and \(\mathrm{HgSO}_{4}\) (b) \(\mathrm{Hg}^{+2}\) salt and conc. acid (c) \(\mathrm{Hg}^{+2}\) salt and dil. acid (d) \(\mathrm{H}_{2} \mathrm{O} / \mathrm{H}^{+}\)

Short answer

The correct answer is (c) \(\mathrm{Hg}^{+2}\) salt and dil. acid.

Step-by-step solution

Understanding the Problem

We are asked to identify the catalysts used in the hydration (addition of water) of acetylene compounds. This process is typically used to convert alkynes to ketones, particularly in the presence of catalysts.

Reviewing the Given Options

Given are four options with different catalysts: (a) uses \(\mathrm{Ba}^{+2}\) and \(\mathrm{HgSO}_{4}\), (b) uses \(\mathrm{Hg}^{+2}\) and concentrated acid, (c) uses \(\mathrm{Hg}^{+2}\) and dilute acid, and (d) uses \(\mathrm{H}_{2}\mathrm{O} / \mathrm{H}^{+}\).

Identifying the Known Catalysts

The hydration of acetylene compounds is commonly catalyzed by mercuric ion \(\mathrm{Hg}^{+2}\) and either dilute acid or water in acidic medium, which facilitate the reaction.

Comparing with Reaction Requirements

Compare this with the given options: (b) and (c) both involve \(\mathrm{Hg}^{+2}\), but only (c) mentions dilute acid, which aligns with the typical conditions of the reaction.

Selecting the Correct Option

Option (c) is correct as it involves \(\mathrm{Hg}^{+2}\) with dilute acid, which matches the typical catalytic conditions for the hydration of acetylene compounds.

Key concepts

Catalysis

Catalysis is the process that enhances the rate of a chemical reaction by using a substance called a catalyst. This catalyst itself does not undergo any permanent chemical change. In the context of converting alkynes to ketones, catalysis plays a crucial role.

Catalysts function by providing an alternative reaction pathway with a lower activation energy. This means that the reactants can transform into products more efficiently and at a faster rate without the catalyst being consumed or altered in the process.

In many reactions, including the hydration of alkynes, catalysts are essential for practical reasons. They allow the transformation to occur under milder conditions that might otherwise require more energy or extreme temperatures. In organic chemistry, such catalysts not only speed up the process but also influence the selectivity of the reaction, leading to the desired product such as a ketone directly.

Alkyne to Ketone Conversion

When transforming alkynes into ketones, the process of hydration is key. Hydration of alkynes involves the addition of a water molecule to an alkyne, converting it into a ketone. This is typically achieved through the presence of a catalyst.

The reaction mechanism can be broken down into a few essential steps:

• First, the alkyne undergoes an addition of the catalyst complex, which helps to activate the carbon-carbon triple bond.
• Next, the water molecule adds across the triple bond, resulting in a temporary enol intermediate.
• Finally, this enol rearranges to form a more stable ketone product through a process called tautomerization.

The overall reaction converts a typically less reactive alkyne into a more chemically versatile ketone, which is valuable in many synthetic applications.

Mercuric Ion Catalysis

Mercuric ion (\(\mathrm{Hg}^{+2}\)) is frequently employed in the catalysis of alkyne hydration to ketones. Known as mercuric ion catalysis, it is a widely used method due to its effectiveness. In this process, mercuric ions serve as the catalyst, working alongside dilute acid to facilitate the hydration reaction.

Mercuric ion catalysis works by interacting with the alkyne, forming a mercuric complex that activates the triple bond. This activation lowers the energy barrier for water addition.

During the reaction, the \(\mathrm{Hg}^{+2}\) binds temporarily with the alkyne to enhance its reactivity. With dilute acid present, the water molecule can then add across the triple bond to form the enol intermediate. The enol is subsequently rearranged into a ketone via tautomerization.

• Mercuric ions are effective because they stabilize positive charges and facilitate the addition of water to the alkyne.
• The dilute acid ensures that the reaction conditions are favorable for the addition and subsequent rearrangement processes.

This method is favored for its reliability and efficiency in producing ketones from alkynes.