Nucleophilic addition with cyanideAQA A-Level Chemistry: Revision notes

Aldehydes and ketones react with hydrogen cyanide, HCN, to form hydroxynitriles: compounds containing both an OH group and a C≡N group on the same carbon. In the laboratory the reagent is KCN followed by dilute acid (the acid supplies the H⁺ needed for the last step), or equivalently HCN in the presence of cyanide ions.

Overall equations (name the product from the carbon chain including the nitrile carbon):

• CH₃CHO + HCN → CH₃CH(OH)CN (2-hydroxypropanenitrile)
• CH₃COCH₃ + HCN → (CH₃)₂C(OH)CN (2-hydroxy-2-methylpropanenitrile)

The reaction increases the carbon chain by one carbon, so it is useful in synthesis.

The mechanism is nucleophilic addition. The C=O bond is polar, so the carbon is δ+, and the cyanide ion CN⁻ is the nucleophile.

1. CN⁻ attacks the δ+ carbon using the lone pair on its carbon atom (curly arrow from the lone pair to the carbon of C=O).
2. The C=O π bond breaks and the electron pair moves to oxygen (second curly arrow), forming an intermediate with a negative charge on oxygen.
3. The O⁻ is protonated by H⁺ from the dilute acid (curly arrow from the O⁻ lone pair to H⁺), giving the hydroxynitrile.

Draw the curly arrows from the lone pair on the carbon of CN⁻ and show the charge on the intermediate.

The carbonyl group is trigonal planar, so the cyanide ion can attack the δ+ carbon from either side of the plane. Both directions of attack are equally likely. If the carbon ends up bonded to four different groups, the two directions of attack give the two enantiomers, which are formed in equal amounts as a racemic mixture. A racemic mixture does not rotate plane-polarised light because the rotations cancel.

Examples:

• Aldehydes (except methanal) give a chiral product: carbon bonded to H, OH, CN and R. Ethanal gives CH₃CH(OH)CN.
• Unsymmetrical ketones such as butanone give a chiral product: carbon bonded to OH, CN, CH₃ and C₂H₅.
• Symmetrical ketones such as propanone or pentan-3-one, and methanal, do not give a chiral product, because two groups on the carbon are identical.

Potassium cyanide is highly toxic: it can poison by ingestion, inhalation or skin absorption. Cyanide ions react with acid to release hydrogen cyanide gas, which is toxic and volatile. Safe practice therefore includes:

• use a fume cupboard
• wear gloves, eye protection and a lab coat
• keep cyanide solution away from acid until the reaction is begun, and add the acid slowly
• treat and dispose of cyanide waste carefully, following a risk assessment

In industry, the hazard is a reason to control quantities and use closed systems.

For any carbonyl compound with HCN:

1. Write the overall equation: carbonyl + HCN → hydroxynitrile.
2. Check whether the product carbon has four different groups.
3. If it does, state that both enantiomers form in equal amounts (racemic mixture), because CN⁻ attacks the planar C=O group from either side.

Worked example: butanal, CH₃CH₂CH₂CHO + HCN → CH₃CH₂CH₂CH(OH)CN, 2-hydroxypentanenitrile, which has a chiral carbon (H, OH, CN, C₃H₇).