Question

When treated with KCN in methyl alcohol, benzaldehyde is converted into a molecule called benzoin. Write a mechanism for this change. O=C(COC(=O)c1ccccc1)c1ccccc1 2\. \(\mathrm{H}_{2} \mathrm{O} / \mathrm{HCl}\) Caution! This problem is hard. Do not succumb to the temptation to remove that aldehydic proton-it is not acidic. Something else must happen. Hints: What will the nucleophile cyanide do to benzaldehyde? How does that reaction alter the nature of the formerly "aldehydic" hydrogen?

Short answer

Benzaldehyde is converted to benzoin through nucleophilic attack by CN⁻, enolate formation, and a second nucleophilic attack on an additional benzaldehyde, followed by protonation.

Step-by-step solution

Nucleophilic Attack by Cyanide Ion

Identify the carbonyl carbon in benzaldehyde. The nucleophile, CN⁻ (cyanide ion), will attack the electrophilic carbon of the carbonyl group in benzaldehyde, forming a cyanohydrin intermediate.

Formation of Cyanohydrin

The nucleophilic attack by CN⁻ results in the formation of a tetrahedral alkoxide intermediate. Protonation of this intermediate yields the cyanohydrin structure.

Enolate Formation

Under the basic conditions provided by CN⁻, the alpha-hydrogen adjacent to the carbonyl group in the cyanohydrin is relatively acidic and gets deprotonated to form an enolate ion.

Nucleophilic Attack on a Second Benzaldehyde Molecule

The enolate formed in the previous step will perform a nucleophilic attack on the carbonyl carbon of another benzaldehyde molecule, leading to the formation of a new C-C bond.

Protonation of the Intermediate

The intermediate formed from the nucleophilic attack will be protonated by the solvent (methyl alcohol) or another proton source to give the final benzoin product.

Final Mechanism Overview

Combine all the steps to present the overall mechanism for the conversion of benzaldehyde to benzoin using KCN in methyl alcohol.

Key concepts

Cyanohydrin formation

Cyanohydrin formation is an important step in several organic reactions, including the benzoin condensation mechanism. When benzaldehyde is treated with KCN (potassium cyanide) and methyl alcohol, it forms a molecule called benzoin. The key to this process is the transformation of benzaldehyde into a cyanohydrin. The cyanide ion (CN⁻) is a nucleophile, meaning it has a high affinity for electrophilic centers, such as the carbonyl carbon in benzaldehyde. The nucleophilic attack involves the cyanide ion attacking the electrophilic carbon of the carbonyl group. This results in the formation of a tetrahedral alkoxide intermediate. This intermediate is then protonated, leading to the formation of the cyanohydrin structure.

The essential steps in cyanohydrin formation are:

• The nucleophilic attack by CN⁻
• The creation of a tetrahedral alkoxide intermediate
• Protonation of the intermediate to form the cyanohydrin

Understanding these steps is crucial for comprehending the benzoin condensation mechanism.

Enolate ion

An enolate ion is formed through the deprotonation of a proton located alpha to a carbonyl group. In the context of benzoin condensation, this occurs in the basic conditions provided by the cyanide ion. After the cyanohydrin is formed, the alpha-hydrogen adjacent to the carbonyl group becomes relatively acidic. The basic environment allows for the deprotonation of this hydrogen, resulting in the formation of an enolate ion. The process of forming an enolate ion includes the following steps:

• The formation of a cyanohydrin intermediate
• Deprotonation of the alpha-hydrogen by the base (CN⁻) to form the enolate ion

This enolate ion is then poised to react further, playing a critical role in the subsequent steps of the benzoin condensation mechanism.

Nucleophilic attack

Nucleophilic attack is a core concept in many organic chemistry reactions, including the benzoin condensation mechanism. After the enolate ion forms, it acts as a nucleophile due to the electron densities on the carbon atom. In this mechanism, the enolate ion performs a nucleophilic attack on the carbonyl carbon of another benzaldehyde molecule. This step establishes a new carbon-carbon bond. Key points about nucleophilic attack in the context of benzoin condensation are:

• The enolate ion, formed due to deprotonation, acts as the nucleophile
• The target is the electrophilic carbonyl carbon of another benzaldehyde molecule
• This reaction results in the formation of a new C-C bond

Understanding this nucleophilic attack is pivotal for grasping the benzoin condensation mechanism.

Carbonyl chemistry

Carbonyl chemistry is an extensive field in organic chemistry dealing with compounds containing the carbonyl group (C=O). In the benzoin condensation mechanism, understanding the behavior of carbonyl compounds is essential. The carbonyl carbon is typically electrophilic, making it susceptible to nucleophilic attack. The process starts with the carbonyl carbon in benzaldehyde, which is attacked by a nucleophile, CN⁻, forming the cyanohydrin intermediate. The next stages involve the enolate ion interacting with another carbonyl group in a second benzaldehyde molecule. Key aspects of carbonyl chemistry relevant to this mechanism include:

• The electrophilic nature of the carbonyl carbon
• The susceptibility of carbonyl groups to nucleophilic attack
• The role of carbonyl compounds in forming key intermediates like cyanohydrins and enolate ions

Each of these points is crucial for the formation of the final benzoin product in the benzoin condensation mechanism.