Problem 16 - A Complete Resource Book in Chemistry for JEE Main

Question

Acetylene and HCHO react in the presence of copper acetylide catalyst to form (a) 2-butyne-1, 4 -diol (b) but-2-yne-1, 2 -diol (c) but-1-yne-1, 4 -diol (d) none of these

Step-by-Step Solution

Answer

The answer is (a) 2-butyne-1,4-diol.

Step 1: Identify the reactants

We need to identify the reactants in the reaction. The reactants given are acetylene (C_2H_2) and formaldehyde (HCHO). The reaction occurs in the presence of a copper acetylide catalyst.

Step 2: Determine the product formation

Acetylene can undergo nucleophilic addition reactions with carbonyl compounds like formaldehyde in the presence of a copper catalyst. The reaction forms a compound known as butyne-diol.

Step 3: Identify the formed compound

When acetylene reacts with formaldehyde, in the presence of copper acetylide catalyst, it specifically forms 2-butyne-1,4-diol as, traditionally, this reaction leads to a structure where two acetylene molecules add to two formaldehyde molecules creating a diol at the 1 and 4 positions, with a triple bond at the 2 position.

Step 4: Choose the correct option

Given the reactions and product analysis, the correct compound formed is (a) 2-butyne-1,4-diol, which aligns with the structure determined through the reaction mechanism.

Key Concepts

Nucleophilic Addition Reactions

Nucleophilic addition reactions are fundamental in organic chemistry, especially when dealing with carbonyl compounds. In these reactions, a nucleophile, which is rich in electrons and seeks to donate them, attacks an electron-deficient site on another molecule. Typically, this electron-deficient site is part of a carbonyl compound, characterized by a carbon atom double-bonded to an oxygen atom.
Acetylene (\(C_2H_2\)) acts as the nucleophile in the discussed exercise. Due to the presence of a triple bond, acetylene has electron-rich sites that can interact with electrophilic centers in carbonyl compounds like formaldehyde (\(HCHO\)).

• The nucleophile attacks the carbon in the carbonyl group, leading to the formation of an intermediate product.
• This reaction often requires catalysts, like copper acetylide, to proceed efficiently, ensuring a smooth electron flow from the nucleophile to the electrophile.
• After the addition, the typical outcome is the conversion of the carbonyl structure into a new functional group, such as a diol in this case.

Understanding nucleophilic addition is crucial as it allows chemists to synthesize a wide variety of organic compounds with desired functional groups.

Carbonyl Compounds

Carbonyl compounds are ubiquitous in organic chemistry. They include aldehydes, ketones, carboxylic acids, esters, and more, all characterized by the presence of a carbon atom double-bonded to an oxygen atom (\(C=O\)). This double bond creates a significant electrophilic site because of the electronegativity difference between carbon and oxygen.
In the specific reaction between acetylene and formaldehyde, formaldehyde acts as the carbonyl compound. It is the simplest aldehyde, containing one carbonyl group attached to a hydrogen atom.

• The carbon in the carbonyl group is electrophilic due to the partial positive charge it carries, making it susceptible to attacks from nucleophiles.
• A reaction with nucleophiles, such as acetylene here, can convert the carbonyl group into various functional groups, depending on the reaction conditions and the nucleophiles used.

Understanding the behavior of carbonyl compounds under different reaction conditions is pivotal for predicting the products in organic synthesis.

Reaction Mechanism

A reaction mechanism illustrates the step-by-step sequence of events at the molecular level that lead to the transformation of reactants into products. In the reaction between acetylene and formaldehyde, understanding the mechanism helps us comprehend how and why certain products are formed.
The reaction mechanism here involves several key steps:

• Initially, the copper acetylide catalyst activates acetylene, enhancing its nucleophilic potential.
• The nucleophilic acetylene molecule attacks the electrophilic carbonyl carbon in formaldehyde, creating a temporary intermediate.
• This intermediate then undergoes further rearrangements, specifically forming a linear chain structure with alcohol groups (diols) at the termini of the chain.
• Finally, the triple bond in acetylene is preserved by joining two acetylene molecules. This is evident as the chemical formula for the product, 2-butyne-1,4-diol, retains a triple bond at the second carbon.

Understanding the reaction mechanism not only helps in predicting product formation but also aids in optimizing reaction conditions for desired outcomes in synthetic chemistry.