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3.8.3Organic Chemistry

Nucleophilic Addition

5 min read 6 parts Lesson 31 of 55

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Section 1 of 6Why Carbonyls Are Attacked

3.8.3 Nucleophilic Addition

Nucleophilic addition is the typical reaction of aldehydes and ketones.

Polar C=O
  • Oxygen is more electronegative than carbon, so the carbonyl carbon is δ+ and the oxygen is δ−.
Attacking species
  • A nucleophile such as CN⁻ donates a lone pair to the δ+ carbon, which is why this carbon is the site of attack.
Addition
  • The π bond breaks and the nucleophile and a hydrogen add across the C=O bond, so only one product forms.
Alkenes compared
  • Alkenes undergo electrophilic addition because their double bond is electron-rich, whereas carbonyl carbon is electron-poor.

Section 2 of 6Mechanism with HCN

Hydrogen cyanide adds to aldehydes and ketones to form hydroxynitriles.

1
Cyanide ions are produced in solution, usually from KCN with dilute acid, giving a mixture of CN⁻ and HCN.
2
The lone pair on the carbon of CN⁻ attacks the δ+ carbonyl carbon and the C=O π electrons move onto oxygen.
3
This gives a negatively charged intermediate in which carbon now bears the CN group.
4
The negative oxygen quickly accepts H⁺ from HCN or from dilute acid, forming the O–H group.
5
Overall, CH₃CHO + HCN → CH₃CH(OH)CN, named 2-hydroxypropanenitrile.

Section 3 of 6Optical Isomerism in the Products

Optical Isomerism in the Products
Planar carbonyl
  • The trigonal planar carbonyl group can be attacked by CN⁻ from above or below the plane.
Enantiomers
  • For ethanal the new carbon has four different groups (H, OH, CH₃, CN), so attack from each face gives a different enantiomer.
Racemate
  • Both faces are equally likely, so the product is a 50:50 racemic mixture that does not rotate plane-polarised light.
Ketones
  • Propanone gives (CH₃)₂C(OH)CN, 2-hydroxy-2-methylpropanenitrile, which has two identical CH₃ groups and so is not chiral.

Section 4 of 6Practical and Safety Points

Practical and Safety Points
Toxicity
  • HCN and cyanide salts are extremely poisonous, so the reaction is done in a fume cupboard with great care.
Safer source
  • Using KCN with acid makes HCN in the flask so that the gas is not handled directly.
Use in synthesis
  • Hydroxynitriles are useful because the nitrile can be converted to an acid, which lengthens a carbon chain by one carbon.
Test of racemate
  • An optically inactive product from a reaction that makes a chiral centre indicates that a racemic mixture has formed.

Section 5 of 6Worked Example

Question Explain why the product of the reaction between propanal and hydrogen cyanide is a racemic mixture.

  1. Propanal is CH₃CH₂CHO and its carbonyl carbon is attached to H, C₂H₅ and =O, so it is planar.
  2. CN⁻ can attack from either side of this plane with equal probability.
  3. The product CH₃CH₂CH(OH)CN has a carbon bonded to H, OH, C₂H₅ and CN, which are four different groups, so it is chiral.
  4. Attack from each side gives a different enantiomer, so equal amounts form and the mixture is optically inactive.

Answer Equal attack from both faces gives equal amounts of two enantiomers, so a racemic mixture forms.

Section 6 of 6Exam Checklist & Tips

3.8.3 – Key Points for the Exam

  • Nucleophile A species that attacks a δ+ carbon by donating a lone pair.
  • Nucleophilic addition Addition across a double bond that begins with attack by a nucleophile.
  • Hydroxynitrile A compound containing both –OH and –CN groups.
  • Racemic mixture An equal mixture of two enantiomers with no optical activity.

Common Mistakes

  • Describing the attack on carbonyls as electrophilic; it is nucleophilic because carbon is δ+.
  • Putting the lone pair of CN⁻ on nitrogen in the mechanism; the bond forms from the carbon.
  • Saying every product of HCN addition is chiral, when propanone gives an achiral product.
  • Forgetting the proton step, so the final O–H group is missing from the product.
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