Problem 25 - A Complete Resource Book in Chemistry for JEE Main

Question

Addition of water to acetylene compounds is catalysed by ........... and ........... (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) \(\mathrm{H}_{2} \mathrm{O} / \mathrm{H}^{+}\)

Step-by-Step Solution

Answer

(c) Hg^{+2} salt and dil. acid.

Step 1: Understand the Reaction in Question

The addition of water to an acetylene compound is a type of hydration reaction. In this particular chemical process, a catalyst is often required to facilitate the addition of water across the carbon-carbon triple bond of an acetylene.

Step 2: Recognize Catalysts in Acetylene Hydration

In the hydration of alkynes, especially acetylene, the typical catalysts used are mercury (II) salts (Hg^+2) in combination with a dilute acid. This is because mercury salts help in forming an intermediate with acetylene upon which water adds.

Step 3: Evaluate Given Options

Looking at the options provided: - Option (a) uses barium salt which is not commonly used for such a reaction. - Option (b) mentions concentrated acid, which is not ideal for the typical addition reaction conditions. - Option (c) mentions mercury (II) salt with dilute acid, which is standard for acetylene hydration. - Option (d) mentions just water and acid, without the mercury catalyst, making it unlikely in standard conditions.

Step 4: Confirm the Correct Choice

Given that the addition of water to acetylene requires catalysts like mercury salts and a dilute acid to proceed efficiently, the most fitting option is (c) mercury salt and dilute acid.

Key Concepts

Catalysis in Organic Chemistry

In organic chemistry, catalysis is crucial for enhancing reaction rates and ensuring desired outcomes. A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process. Catalysts provide an alternative reaction pathway with a lower activation energy. This means that reactions can occur more easily and often under milder conditions.

When talking about acetylene hydration, the role of catalysts like mercury (II) salts becomes evident. Mercury catalysts, specifically, help in forming transient intermediates that facilitate the addition of water across the carbon-carbon triple bond, an otherwise challenging task. Here, the catalyst plays a pivotal role in breaking the robust triple bond, enabling hydration to proceed at a feasible rate.

Overall, the concept of catalysis underscores the importance of certain substances, which, though they don't alter the equilibrium of a reaction, dramatically affect the speed and efficiency of the process.

Hydration Reactions

Hydration reactions are a fundamental type of chemical reaction, wherein water is added to a molecule. In the context of alkyne chemistry, hydration specifically refers to the process of adding water across a carbon-carbon triple bond, converting it into a carbonyl compound.

The reactivity of alkynes in hydration reactions is influenced by the triple bond, which is quite strong due to its sp hybridization. Therefore, catalysts are necessary to facilitate the reaction. In acetylene hydration, the presence of mercury (II) salts and a dilute acid is crucial, as they lower the activation barrier, allowing for the water to "add across" the alkyne effectively.

This type of reaction is of significant importance as it allows for the transformation of alkynes into more complex functional groups like ketones or aldehydes, which are useful in various organic synthesis pathways.

Alkyne Chemistry

Alkynes are hydrocarbons characterized by their carbon-carbon triple bonds, represented generally as \(-C \equiv C-\). These compounds are unsaturated, meaning they can undergo additional chemical reactions, such as hydration, to achieve saturation.

The chemistry of alkynes is rich and diverse, largely due to the potential of their triple bonds to participate in various addition reactions. Their unique molecular geometry and bonding create opportunities for specific reaction pathways, like hydration, which are less prominent in other types of hydrocarbons.

In the case of acetylene, the simplest alkyne, its hydration is a starting point for many synthesizing processes in organic chemistry. By understanding how alkynes like acetylene can be hydrated into carbonyl compounds, chemists can manipulate these reactions to produce desired materials or intermediates for pharmaceuticals, polymers, and other useful chemicals.