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

Hydration of acetylene compounds is catalyzed by \(\mathrm{Hg}^{+2}\) salt and a dilute acid (option c).

Step 1: Understand the Reaction

The reaction in question is the addition of water to acetylene compounds, specifically known as hydration of alkynes. This reaction typically requires a catalyst to proceed.

Step 2: Identify the Reactants and Catalysts

The hydration of alkynes involves the addition of water (H₂O) in the presence of a catalyst. Commonly, the catalyst for this reaction includes mercury(II) salts (like Hg²⁺) along with an acid to promote the reaction.

Step 3: Recognize the Correct Catalyst Combination

From the provided options, the pair mentioned in option (c) which includes \(\mathrm{Hg}^{+2}\) salt and a diluted acidic solution, refers to the commonly used catalysts for alkyne hydration. This setup helps the reaction proceed effectively, making it the correct choice.

Key Concepts

Catalysts for Alkyne Reactions

In organic chemistry, reactions involving alkynes often require catalysts to proceed efficiently. These catalysts aid in breaking the bonds of reactants, allowing new compounds to form. Since alkynes are relatively unreactive due to their triple bonds, catalysts are especially crucial. One prominent reaction involving alkynes is their hydration, where water is added to the alkyne. This reactions’ successful completion typically hinges on the appropriate catalyst to enable the transformation into alcohols or ketones. Regular catalysts include:

• Mercury(II) salts
• Acids

Using a suitable catalyst ensures that the reaction occurs under milder conditions. This reduces the likelihood of unwanted side reactions, enhancing the yield and selectivity of the desired product. Catalysts make these reactions more practical for both academic studies and industrial applications.
It allows for controlled conditions, leading to higher precision and efficiency in chemical transformations.

Mercury(II) Salt Catalyst

Mercury(II) salts, such as mercury sulfate \( (\text{HgSO}_4)\), are often used in the hydration of alkynes. Their role is to facilitate the addition of water, a process that would otherwise be inefficient.These catalysts work by interacting with the alkyne to form a mercurinium ion intermediate, a critical step in the reaction. This intermediate is more reactive, allowing water to add across the carbon-carbon triple bond, eventually leading to the formation of ketones.Importantly, mercury(II) salts are particularly effective when paired with an acid. The acid helps stabilize the reaction intermediates, resulting in a smoother reaction process. While mercury-based catalysts are powerful, their toxicity requires careful handling. This prompts chemists to work in controlled environments with appropriate safety measures.
The combination of mercury(II) salt catalysts and acids is classic in organic chemistry for achieving efficient alkyne hydration.

Acid Catalysis in Organic Reactions

Acid catalysis is a fundamental concept in organic chemistry. It involves the use of acids to increase the reaction rate, a handy technique in various transformations.In the context of alkyne hydration, acids serve multiple purposes:

• Stabilizing reaction intermediates, like mercurinium ions, making subsequent reactions possible.
• Assisting in protolysis which helps in the rearrangement of reaction intermediates to form the desired end product.

By providing a proton source \((H^+)\), acids help in breaking down reactants' stable structures, enabling new bonds to form. Dilute acids are common as they offer better control over the reaction.This process highlights the importance of acid catalysis beyond hydration reactions. It extends to esterifications, acylations, and Friedel-Crafts reactions among others, showcasing how vital acids are in facilitating a wide array of chemical transformations. Without these catalysts, many reactions would require extreme conditions, making acid catalysis an invaluable tool in organic synthesis.