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Ch. 17 - Reactions at the Alpha-Carbon
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Non è quello che usi tu?Cambia libro di testo
Tutti i libri di testoBruice 8th EditionCh. 17 - Reactions at the Alpha-CarbonProblema 85
Capitolo 17, Problema 85

The following reaction is known as the benzoin condensation. The reaction does not take place if sodium hydroxide is used instead of sodium cyanide. Propose a mechanism for the reaction and explain why the reaction does not occur if hydroxide ion is the base.
Benzoin condensation reaction diagram showing reactants, sodium cyanide, and product benzoin.

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The benzoin condensation is a reaction between two aldehydes (typically aromatic aldehydes like benzaldehyde) in the presence of a cyanide ion (CN⁻) as a catalyst. The reaction forms a benzoin product, which is a hydroxy ketone. The mechanism involves nucleophilic addition and subsequent rearrangement steps.
Step 1: The cyanide ion (CN⁻) acts as a nucleophile and attacks the carbonyl carbon of one benzaldehyde molecule. This forms a tetrahedral intermediate, which is stabilized by the resonance of the cyanohydrin structure.
Step 2: The intermediate formed in Step 1 generates a carbanion at the alpha-carbon (adjacent to the carbonyl group) due to the electron-withdrawing effect of the cyanide group. This carbanion is stabilized by resonance, making it a strong nucleophile.
Step 3: The carbanion attacks the carbonyl carbon of a second benzaldehyde molecule, forming a new carbon-carbon bond. This step results in the formation of another tetrahedral intermediate.
Step 4: The intermediate undergoes proton transfer and elimination of the cyanide ion, regenerating the catalyst and forming the final benzoin product. The reaction does not occur with hydroxide ion (OH⁻) because hydroxide lacks the ability to stabilize the intermediate carbanion through resonance, unlike cyanide, which has a strong electron-withdrawing effect.

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Benzoin Condensation

Benzoin condensation is a reaction between two molecules of benzaldehyde in the presence of a base, leading to the formation of benzoin. This reaction involves nucleophilic addition where one benzaldehyde acts as a nucleophile, attacking the carbonyl carbon of another benzaldehyde. The reaction is typically catalyzed by cyanide ions, which facilitate the formation of the intermediate that ultimately leads to the product.
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Condensation Reactions

Role of Base in Organic Reactions

In organic chemistry, the choice of base can significantly influence the outcome of a reaction. Bases can deprotonate compounds, generating nucleophiles or stabilizing intermediates. In the case of benzoin condensation, sodium cyanide acts as a strong nucleophile, while sodium hydroxide, being a stronger base, does not provide the necessary nucleophilic character required for the reaction to proceed effectively.
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Recognizing Acid-Base Reactions.

Nucleophilicity vs. Basicity

Nucleophilicity refers to the ability of a species to donate an electron pair to form a chemical bond, while basicity is a measure of a species' ability to accept protons. In the context of the benzoin condensation, cyanide ion is a good nucleophile, allowing it to effectively attack the carbonyl carbon of benzaldehyde. In contrast, hydroxide ion, while a strong base, is less nucleophilic in this context, leading to the failure of the reaction when it is used.
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Understanding the difference between basicity and nucleophilicity.
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