Introduction

Hydroxynitrile formation through nucleophilic addition of hydrogen cyanide is a key carbon–carbon bond-forming reaction that extends carbonyl chemistry and introduces useful synthetic functionality.

Understanding the Context of Nucleophilic Addition

Aldehydes and ketones undergo nucleophilic addition because the polar C=O bond creates an electron-deficient carbon atom. This allows negatively charged species, such as cyanide ions (CN–), to attack the electrophilic carbon and form new bonds efficiently. The reaction produces hydroxynitriles, molecules that contain both a nitrile group (–C≡N) and an alcohol group (–OH), which are important intermediates in organic synthesis.

This diagram shows the general structure of an α-cyanohydrin (hydroxynitrile), where –OH and –C≡N are attached to the same carbon. The R groups represent the substituents inherited from the original aldehyde or ketone. Source

Key Features of Hydrogen Cyanide in this Reaction

Hydrogen cyanide provides both components required for hydroxynitrile formation:

• The cyanide nucleophile, which attacks the carbonyl carbon

• A proton source to convert the intermediate into the hydroxynitrile

However, pure HCN is hazardous and weakly dissociated, so the reaction is typically carried out using acidified sodium cyanide or potassium cyanide, which generates CN– in situ.

Structure and Reactivity of Carbonyl Compounds

The reactivity of aldehydes and ketones toward nucleophilic attack depends on both steric and electronic factors:

• Aldehydes react more readily because they are less sterically hindered and the carbonyl carbon is more electrophilic.

• Ketones react more slowly due to increased steric hindrance and electron-donating alkyl groups.

When CN– attacks the carbonyl group, a tetrahedral intermediate forms, which then undergoes protonation to yield the final product.

Formation of Hydroxynitriles: Reaction Mechanism

Step 1: Generation of the Nucleophile

Acidified cyanide sources generate the cyanide ion, the active nucleophile.

The CN– ion attacks the electron-poor carbonyl carbon.

This reaction scheme illustrates cyanohydrin formation via CN⁻ attack on a carbonyl, producing a tetrahedral intermediate that is then protonated by HCN. The diagram also shows CN⁻ regeneration; the specific aromatic example and percentage yield shown go beyond the OCR syllabus. Source

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Step 2: Nucleophilic Attack

During nucleophilic attack, the carbonyl π-bond breaks and electron density shifts onto the oxygen atom. This produces a negatively charged alkoxide ion, a typical intermediate in nucleophilic addition reactions.

Step 3: Protonation

The alkoxide ion is protonated by an acidic species present in the reaction mixture (often HCN or dilute acid), producing the final hydroxynitrile.

Overall Reaction Description

The mechanism involves:

• Attack by CN– at the carbonyl carbon

• Formation of a tetrahedral intermediate

• Protonation to produce the hydroxynitrile

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This transformation increases molecular complexity and introduces two functional groups that can undergo further reactions, making hydroxynitrile formation synthetically valuable.

Importance of the Cyanide Ion

The cyanide ion is both a strong nucleophile and an important C–C bond-forming reagent. Its ability to attack carbonyl compounds allows the synthesis of molecules that can later be converted into:

• Carboxylic acids (via hydrolysis of –C≡N)

• Amines (via reduction)

• Other functional groups that extend carbon frameworks

Because the reaction involves a planar carbonyl group, the CN– ion may attack from either side of the plane. This leads to the possibility of racemic mixtures when the product contains a chiral centre.

Stereochemical Outcomes

Hydroxynitriles formed from aldehydes or asymmetrical ketones often create a new stereocentre at the carbonyl carbon. Since attack by CN– is equally likely from either side, students should understand that:

• A racemic mixture is usually produced

• The mechanism does not favour one enantiomer over the other

• Optical activity is not observed unless chiral conditions are used

These stereochemical considerations align with broader organic chemistry principles and explain why nucleophilic addition reactions are useful but not stereoselective under standard conditions.

Conditions Required for the Reaction

To ensure successful addition of HCN:

• The reaction mixture must contain a source of CN–, usually NaCN or KCN

• A weak acid is needed to protonate the intermediate

• Temperature is controlled to minimise hazards and improve yield

• Ventilation and safety measures are essential due to HCN toxicity

Safety Considerations in Laboratory Practice

Because HCN and cyanide salts are extremely toxic, school-level reactions often use alternative reagents or controlled microscale procedures. Although the specification focuses on understanding the mechanism rather than on practical execution, awareness of hazards reinforces why cyanide chemistry must be handled with care.

Summary of Mechanistic Features

Important mechanistic points include:

• Polar C=O bond activates carbonyl carbon

• CN– is the nucleophile that attacks

• Tetrahedral intermediate is formed

• Protonation yields the hydroxynitrile

• Racemic mixtures arise when a chiral centre is generated

These steps reflect the OCR requirement to understand cyanide addition and to outline the mechanism clearly.