Reaction of ethyne with HCN in presence of Ba(CN)2 is an example of Nucleophilic addition.

Nucleophilic addition is a type of reaction where a nucleophile attacks an electrophile, resulting in the addition of a nucleophile to a molecule. In this case, the nucleophile is the cyanide ion (CN-) and the electrophile is the carbon atom in ethyne (C2H2). The reaction takes place in the presence of barium cyanide (Ba(CN)2) as a source of cyanide ions.

Reaction mechanism:

The reaction proceeds through a stepwise mechanism involving the following steps:

1. Formation of the nucleophile: Ba(CN)2 dissociates in water to give CN- ions. These ions act as the nucleophile in the reaction.

Ba(CN)2 → 2 CN-

2. Activation of the electrophile: The carbon atom in ethyne is electron-deficient due to the presence of the triple bond. This makes it susceptible to attack by nucleophiles.

HC≡CH + CN- → HC≡C-CN + H+

3. Nucleophilic attack: The CN- ion attacks the carbon atom of ethyne, resulting in the formation of a carbon-carbon bond.

HC≡C-CN + CN- → HC≡C-CN-CN

4. Protonation: The reaction is carried out in an aqueous medium, so the product is protonated to give the final product.

HC≡C-CN-CN + H2O → HC≡C-CN-C(OH)(CN)

Significance of nucleophilic addition in this reaction:

The nucleophilic addition of cyanide ions to ethyne in the presence of Ba(CN)2 is significant for the following reasons:

1. Formation of a new carbon-carbon bond: The reaction leads to the formation of a new carbon-carbon bond, resulting in the conversion of the triple bond in ethyne to a double bond.

2. Addition of functional groups: The addition of the cyanide group (-CN) to ethyne introduces a new functional group (nitrile) into the molecule, which can be further modified or used in various synthetic applications.

3. Selectivity: The reaction is selective for the addition of cyanide ions to the carbon atom of ethyne, rather than other possible sites of nucleophilic attack. This selectivity is due to the electron-deficient nature of the carbon atom in the triple bond.

In conclusion, the reaction of ethyne with HCN in the presence of Ba(CN)2 is an example of nucleophilic addition. This reaction involves the attack of the cyanide ion on the carbon atom of ethyne, resulting in the formation of a new carbon-carbon bond and the introduction of a nitrile functional group.