Problem 4 - A Complete Resource Book in Chemistry for JEE Main

Question

Addition of HCN to ethyne in the presence of \(\mathrm{Ba}(\mathrm{CN})_{2}\) as a catalyst gives (a) vinyl cyanide (b) ethyl cyanide (c) 1,1 -dicyanoethane (d) divinyl cyanide

Step-by-Step Solution

Answer

The product is (a) vinyl cyanide.

Step 1: Understanding the Reaction Conditions

The problem involves the addition of hydrogen cyanide (HCN) to ethyne (acetylene, C₂H₂) in the presence of barium cyanide, Ba(CN)₂, as a catalyst. The key chemical reaction here is an addition reaction where the unsaturated bond, C≡C, in ethyne will be targeted to form a new single bond configuration, commonly leading to a more stable product.

Step 2: Determine the Product Formation

Ethyne, or acetylene, has a triple bond. Adding HCN to this alkyne without further reagents like excess HCN typically forms a vinyl group by transforming the triple bond into a double bond. This results in the formation of vinyl cyanide, also known as acrylonitrile, where the addition of the cyano group, CN, to the unsaturated hydrocarbon framework yields CH₂=CHCN.

Step 3: Identify Given Options

Given the choices, (a) vinyl cyanide corresponds to the expected product formation of CH₂=CHCN following the mechanism of addition. Other options like ethyl cyanide, 1,1-dicyanoethane, and divinyl cyanide do not match the expected mechanism of this reaction, since these products would require different conditions or starting compounds.

Key Concepts

Ethylene Derivatives

Ethylene derivatives play a significant role in the landscape of organic chemistry. Ethylene itself, with the formula C₂H₄, is a vital building block in creating various compounds. The reaction of ethylene derivatives generally involves breaking a π-bond and forming new bonds, making it possible to transform simple alkenes into more complex molecules.

• These transformations are often achieved through addition reactions, where reagents add to the carbon-carbon double bond.
• Common derivatives include polyethylene, halogenated compounds, and compounds with substituted groups, like cyanides or alcohols.

Addition reactions with ethylene serve as a model for understanding similar reactions in alkenes. The transformation typically results in more stable and complex structures, desirable in industrial and synthetic applications.

Hydrogen Cyanide Reactions

Hydrogen cyanide (HCN) is an important reagent in organic synthesis, particularly for addition reactions. In the context of ethyne (acetylene) reactions, HCN acts as an electrophile due to the partial positive charge on the hydrogen atom.

• In these reactions, HCN typically adds across unsaturated carbon-carbon bonds, such as the triple bond in acetylene, leading to interesting derivatives.
• This type of reaction is crucial in producing compounds like acrylonitrile (vinyl cyanide), which serves as a precursor for polymers and other industrial substances.

The addition of HCN to alkynes like ethyne not only changes the bond structure but also adds significant functionality to the molecule, enabling it to participate in further reactions.

Catalysis with Barium Cyanide

Catalysis with barium cyanide is a fascinating aspect of this reaction, introducing a different dynamic for the addition of cyanide to alkynes. As a catalyst, barium cyanide, Ba(CN)₂, is not consumed in the reaction but facilitates the process by lowering the activation energy required for the reaction to proceed.

• It helps in stabilizing the transition state, making the addition of HCN more efficient and selective.
• The process ensures that the reaction proceeds under milder conditions and produces the desired product more cleanly.

Utilizing barium cyanide as a catalyst leads to higher yields of vinyl cyanide, reflecting the key role of catalysts in enhancing both the rate and outcomes of organic reactions.