Question

Benzaldehyde is heated with alcoholic KCN. The product formed is (A) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHOHCN}\) (B) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCHOHC}_{6} \mathrm{H}_{5}\) (C) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCOC}_{6} \mathrm{H}_{5}\) (D) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCN}\)

Short answer

(A) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHOHCN}\)

Step-by-step solution

Identify the reactants

In this reaction, the reactants are Benzaldehyde and alcoholic KCN. The structure of Benzaldehyde is given by the formula \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}\), and the KCN is a source of cyanide ion (CN⁻).

Identify the nucleophile

The nucleophile in this reaction is the cyanide ion (CN⁻) provided by KCN. It will attack the carbonyl carbon of Benzaldehyde.

Nucleophilic addition

The cyanide ion, being a nucleophile, attacks the carbonyl carbon of Benzaldehyde. The double bond between the carbonyl carbon and oxygen breaks and forms a single bond with the oxygen atom. This results in the addition of the cyanide ion across the carbonyl bond.

Find the product

After the nucleophilic addition, we now have a molecule with a hydroxy and a cyano group attached to the former carbonyl carbon. The resulting product's structural formula is \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHOHCN}\). By comparing this result to the four options given, we find it corresponds to the choice (A). Therefore, the correct answer is: (A) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHOHCN}\).

Key concepts

Benzaldehyde Reactions

Benzaldehyde, typically represented by the formula \( \text{C}_6\text{H}_5\text{CHO} \), is a crucial starting material in organic syntheses. The reactivity of benzaldehyde primarily stems from its carbonyl group, which is the site of attack for various nucleophiles. In educational contexts like the organic chemistry syllabus for JEE, understanding the behavior of benzaldehyde is important because it forms the basis for grasping reactions of aldehydes and ketones.

A prominent reaction is the nucleophilic addition, where nucleophiles, such as the cyanide ion, add to the carbon atom of the carbonyl group. This can result in the formation of compounds like hydroxynitriles. The nucleophilic addition of benzaldehyde serves as a useful example when teaching reaction mechanisms, and its reactions are often included in JEE advanced organic chemistry problems.

Alcoholic KCN Reaction

The reaction of benzaldehyde with alcoholic KCN is a classical example of a nucleophilic addition. In this reaction, KCN dissolves in alcohol, and the cyanide ion (CN⁻) acts as the nucleophile. Why is alcoholic KCN used specifically? The alcohol serves as a solvent that enhances the reactivity of the cyanide ion.

During the reaction, the cyanide ion is able to effectively attack the electrophilic carbonyl carbon of benzaldehyde. This attack is facilitated by the solvent's polarity which also tends to stabilize the intermediate species that form. Students prepping for competitive exams such as JEE need to know that the product of this nucleophilic addition is usually a hydroxynitrile, a compound that contains both a hydroxy group and a nitrile group.

Cyanide Ion Nucleophile

In the context of nucleophilic addition reactions, the cyanide ion (CN⁻) acts as a powerful nucleophile due to its negative charge and the presence of a lone pair of electrons on the carbon atom. This allows it to form a new bond with an electrophilic carbon, typically found in carbonyl compounds like aldehydes and ketones.

The mechanism of attack involves the lone pair of electrons on the cyanide ion approaching the partial positive charge on the carbonyl carbon. It's this fundamental nucleophilic characteristic of the cyanide ion that leads to the formation of carbon-carbon bonds, greatly influencing the synthesis of a wide array of organic compounds. For students, understanding the behavior of common nucleophiles like the cyanide ion is essential when studying organic chemistry for exams like JEE.

Organic Chemistry JEE

The Joint Entrance Examination (JEE) is an engineering entrance assessment in India, recognized for its challenging advanced level of questions, particularly in organic chemistry. In this subject, students must grasp reaction mechanisms, predict products, and understand the reactivity of various functional groups.

Important JEE Concepts

• Understanding the reactivity of functional groups like carbonyl compounds in aldehydes and ketones.
• Grasping the mechanisms of nucleophilic addition reactions, including the role of nucleophiles like the cyanide ion.
• Ability to identify reaction solvents and their effects on reaction mechanisms, such as the use of alcoholic KCN for nucleophilic additions.
• Problem-solving that involves predicting reaction products accurately, as in the case of benzaldehyde's reactions in organic synthesis.

Students studying for JEE must be adept at applying concepts to diverse problem sets, which includes reactions of benzaldehyde and understanding the roles of reagents like KCN.