Problem 90 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Benzaldehyde and alcohol KCN reacts to produce: (a) ethyl benzoate (b) benzyl alcohol (c) benzoin (d) benzyl benzoate

Step-by-Step Solution

Answer

The reaction produces benzoin (option c).

Step 1: Identify Reactants and Reaction Type

Recognize the reactants: Benzaldehyde and alcohol with KCN. Note that this is a typical setup for a cyanohydrin formation followed by a benzoin condensation.

Step 2: Determine the Likely Reaction Pathway

Benzaldehyde reacts with KCN in the presence of alcohol. The KCN provides catalytic CN⁻ ions that initiate the benzoin condensation reaction through a nucleophilic addition mechanism.

Step 3: Analyze the Benzoin Condensation

In the presence of a cyanide catalyst, benzaldehyde forms a dimer through nucleophilic addition to another benzaldehyde molecule, resulting in a benzoin. This step is crucial as benzoin is a diketone or hydroxyketone product derived from two aldehyde molecules.

Step 4: Identify the Product

The product of benzaldehyde reacting with KCN and alcohol is benzoin. The characteristic feature of benzoin is the presence of a hydroxyl group adjacent to a carbonyl group due to the condensation reaction.

Key Concepts

Cyanohydrin Formation

Cyanohydrin formation is an intriguing chemical reaction in which an aldehyde or ketone reacts with hydrogen cyanide (HCN) to form a cyanohydrin. This reaction is significant because it introduces a nitrile group adjacent to an alcohol group. In the context of benzaldehyde reacting with KCN, particularly relevant here, the cyanide ion provided by KCN acts as a nucleophile.

• **Nucleophilic Cyanide Ion:** The CN⁻ ion attacks the carbonyl carbon of the benzaldehyde. The carbonyl carbon is electrophilic due to the electron-withdrawing nature of the oxygen in the carbonyl group.
• **Formation of Cyanohydrin:** This initial attack results in the formation of a tetrahedral intermediate that ultimately leads to the production of benzaldehyde cyanohydrin. This cyanohydrin can undergo further reactions, such as benzoin condensation.

Cyanohydrin formation is an essential step in organic synthesis and provides a means to diversify molecules through further reactions. Understanding this reaction is crucial when studying reactions involving benzaldehyde and other similar organic compounds.

Nucleophilic Addition

Nucleophilic addition is a cardinal concept in organic chemistry, especially important in the context of reactions involving carbonyl compounds like aldehydes and ketones. During this process, a nucleophile, which is an electron-rich species, donates a pair of electrons to the electrophilic carbon in the carbonyl group.

• One of the common scenarios is when cyanide ions (CN⁻) act as the nucleophile, as seen in the reaction between benzaldehyde and KCN.
• This mode of addition is crucial for the formation of new carbon-carbon bonds, marking the transformation of functional groups and contributing significantly to synthetic organic chemistry processes.

In this reaction, nucleophilic cyanide ions initiate the attack on benzaldehyde's carbonyl carbon atom. The result is an inversion of hybridization on the carbon from a trigonal planar to a tetrahedral geometry.
Nucleophilic addition reactions form the backbone of many key transformations in organic chemistry, enabling the creation of more complex molecules and facilitating the development of an immense variety of chemicals and pharmaceuticals.

Benzaldehyde Reaction

Benzaldehyde, a simple aromatic aldehyde, engages in a myriad of reactions due to its reactive carbonyl group. One of its notable reactions is its interaction with cyanide ions in the presence of alcohol to undertake benzoin condensation. In this reaction mechanism:

• Benzaldehyde acts as a substrate that reacts with a nucleophile, in this case, a cyanide ion.
• The presence of the cyanide ion catalyzes the reaction, facilitating the nucleophilic addition that leads to the dimerization of benzaldehyde molecules.

The benzoin condensation is an exceptional example of benzaldehyde's ability to form more complex molecules by linking two aldehyde molecules into one, producing a compound known as benzoin. Benzoin possesses a hydroxyl group (-OH) adjacent to a carbonyl group (C=O), a hallmark motif of this type of reaction.
Understanding benzaldehyde's reactivity through such mechanisms can illuminate the pathways for the synthesis of essential aromatic compounds, frequently encountered in both academic and industrial chemical research scenarios.