CIE AS Chemistry (9701) exams from 2022

Revision Notes

3.5.3 Reaction with HCN

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


Carbonyl Compounds Nucleophilic Addition of HCN, downloadable AS & A Level Chemistry revision notes

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

Exam Tip

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

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.

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