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9476 / 2027
Organic Chemistry

Lesson 13 of 34

Cyanide attacks the carbonyl carbon, then oxygen is protonated

The carbonyl becomes a tetrahedral hydroxynitrile centre and the carbon chain gains one carbon.

A-Level 9476 (2026-2027)

  • I can explain carbonyl reactivity towards nucleophiles.Syllabus

    Syllabus 9476, 11.3(h). Explain carbonyl reactivity towards nucleophiles.

  • I can describe cyanide nucleophilic addition to carbonyl compounds.Syllabus

    Syllabus 9476, 11.3(o). Describe cyanide nucleophilic addition to carbonyl compounds.

  • I can describe HCN addition to aldehydes and ketones.Syllabus

    Syllabus 9476, 11.7(b). Describe HCN addition to aldehydes and ketones.

Cyanide attacks the carbonyl carbon, then oxygen is protonated

The carbonyl becomes a tetrahedral hydroxynitrile centre and the carbon chain gains one carbon.

Use HCN with a small amount of KCN as a catalyst. HCN alone gives very little CN-, and the cyanide ion is the nucleophile. The carbon of CN- gives a lone pair to the δ+ carbon of the carbonyl group. At the same time, the C=O π pair moves onto the oxygen. HCN then gives H+ to the negative oxygen. This forms the hydroxynitrile and makes CN- again.

Nucleophilic addition to ethanal

A lone pair on the carbon end of N triple bond C minus points to the carbonyl carbon. At the same time the carbonyl pi bond points towards oxygen, producing an O-minus alkoxide rather than a carbon with five bonds.

The two arrows occur in the same addition step. Cyanide contributes a carbon atom; it is not attached through nitrogen.
Complete the mechanism and regenerate the catalyst
  1. Form the alkoxide

    CH3CHO + CN- → CH3CH(O-)CN. The negative charge is on oxygen.

  2. Transfer a proton from HCN

    The oxygen lone pair goes to H in H-CN; the H-C bond pair returns to the cyanide carbon.

  3. Write the product

    CH3CH(O-)CN + HCN → CH3CH(OH)CN + CN-. Overall, HCN adds across C=O.

An aldehyde RCHO gives RCH(OH)CN. A ketone R2CO gives R2C(OH)CN. The carbonyl group is flat, so CN- can attack from either face. If this creates a new chiral centre, a racemic mixture forms. Ethanal creates a chiral centre. Propanone does not, because its product has two identical methyl groups.

Check your understanding

When CN- forms a new bond to the carbonyl carbon, why must an arrow also move the C=O pi pair onto oxygen?

  1. Otherwise CN- could not reach carbon; the pi pair has to move first to make room for it.
  2. Otherwise carbon would have five bonds; the pi pair becomes a lone pair on O-.
  3. Otherwise oxygen would have no charge; the arrow is drawn just to show where the minus sits.
Show the answer

Otherwise carbon would have five bonds; the pi pair becomes a lone pair on O-.

Carbon cannot exceed its octet. As CN- bonds, the pi pair moves onto the more electronegative oxygen, giving a tetrahedral alkoxide. Protonation later turns O- into OH.

Exam-style question

Propanal reacts with HCN in the presence of a small amount of KCN at 10-20 °C. Name and describe the mechanism and name the product. [3]

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