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ASChemistryCambridge (CIE)Revision Notes17. Carbonyl CompoundsAldehydes & KetonesReactions of Aldehydes & Ketones with HCN

Reactions of Aldehydes & Ketones with HCN (Cambridge (CIE) AS Chemistry): Revision Note

Exam code: 9701

Philippa Platt

Last updated

Reaction of Hydrogen Cyanide with Aldehydes & Ketones

  • The carbonyl group -C=O, in aldehydes and ketones is polarised

  • The oxygen atom is more electronegative than carbon drawing electron density towards itself

  • This leaves the carbon atom slightly positively charged and the oxygen atom slightly negatively charged

  • The carbonyl carbon is therefore susceptible to attack by a nucleophile, such as the cyanide ion

Nucleophilic addition

  • The nucleophilic addition of hydrogen cyanide to carbonyl compounds is a two-step process

  • In step 1 the cyanide ion attacks the carbonyl carbon to form a negatively charged intermediate

  • In step 2 the negatively charged oxygen atom in the reactive intermediate quickly reacts with aqueous H+ (either from HCN, water or dilute acid) to form 2-hydroxynitrile

The nucleophilic addition of HCN mechanism 

Diagram showing nucleophilic attack on ethanal by cyanide ion, forming a reactive intermediate, followed by protonation to yield 2-hydroxypropanenitrile.
The cyanide ion attacks the carbonyl carbon to form a negatively charged intermediate which quickly reacts with a proton to form a 2-hydroxynitrile compound

Examiner Tips and Tricks

The actual negative charge on the cyanide ion is on the carbon atom and not on the nitrogen atom

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Philippa Platt

Author: Philippa Platt

Expertise: Chemistry Content Creator

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener