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Reduction and Nucleophilic Addition of Carbonyl Compounds

A concise revision guide to the reduction of aldehydes and ketones with NaBH₄ and to nucleophilic addition of hydrogen cyanide (KCN followed by dilute acid): reagents, equations, the mechanism with curly arrows, and why the hydroxynitrile can form as a racemic mixture.

Paper 2 AQA
3.3.8 Aldehydes and ketones
7405/2
AQA specification10 spec points in this lesson
  • 3.3.8iii-a
  • 3.3.8iii-b
  • 3.3.8iv-a
  • 3.3.8iv-b
  • 3.3.8v
  • 3.3.8vi
  • 3.3.8vii
  • 3.3.8viii
  • 3.3.8ix
  • 3.3.8x
What these spec points say
  • 3.3.8iii-a know that aldehydes can be reduced to primary alcohols using NaBH4 in aqueous solution as examples of nucleophilic addition
  • 3.3.8iii-b know that ketones can be reduced to secondary alcohols using NaBH4 in aqueous solution as examples of nucleophilic addition
  • 3.3.8iv-a write overall equations for reduction reactions of aldehydes using [H] as the reductant
  • 3.3.8iv-b write overall equations for reduction reactions of ketones using [H] as the reductant
  • 3.3.8v outline the nucleophilic addition mechanism for reduction reactions with NaBH4 (the nucleophile shown as H-)
  • 3.3.8vi know the nucleophilic addition reactions of carbonyl compounds with KCN followed by dilute acid to produce hydroxynitriles
  • 3.3.8vii write overall equations for the formation of hydroxynitriles using HCN
  • 3.3.8viii outline the nucleophilic addition mechanism for the reaction with KCN followed by dilute acid
  • 3.3.8ix explain why nucleophilic addition reactions of KCN followed by dilute acid can produce a mixture of enantiomers
  • 3.3.8x know the hazards of using KCN

Nucleophilic Addition to a Planar Carbonyl

A cyanide ion can attack the flat C=O group of ethanal from above or from below with equal chance; an H⁺ ion then protonates the oxygen, so the product is an equal mixture of two mirror-image molecules.

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Carbon Hydrogen Oxygen Nitrogen Electron

© Dr. Mohammed Al-Fatah – onlinelearningsystem.net

1

Before You Start

This page uses the polar carbonyl group from Aldehydes and Ketones: Structure, Naming and Properties and the idea of a racemic mixture (racemate) from Optical Activity and Racemic Mixtures.

  • The carbon atom of C=O is δ+, so it attracts nucleophiles: species that donate a lone pair to form a new covalent bond.
  • The C=O group is planar, so a nucleophile can approach from either side.
  • Reduction is the reverse of the oxidation met on the previous page.
2

Reduction of Aldehydes and Ketones

Reducing agents turn carbonyl compounds back into alcohols. In equations the reducing agent is written as [H].

Carbonyl compoundProductExample
aldehydeprimary alcoholCH₃CHO + 2[H] → CH₃CH₂OH
ketonesecondary alcoholCH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃

Reagents and conditions

Reducing agentConditionsNote
NaBH₄, sodium tetrahydridoborate(III)in aqueous solutionmilder and safer: it can be used in water
  • The reagent acts as a source of hydride ions, H⁻.
  • Two hydrogen atoms are added across the C=O bond: one to the carbon and one to the oxygen.
  • The hydride ion is a nucleophile. It is repelled by the electron-rich C=C bond, so an alkene group in the molecule is not reduced: CH₂=CHCHO gives CH₂=CHCH₂OH.
  • Butanone gives butan-2-ol, which has a chiral centre. Attack on the planar C=O from either side gives a racemic mixture.
Poster of a two-way map between alcohols and carbonyl compounds: oxidation arrows from primary alcohol to aldehyde and from secondary alcohol to ketone, reduction arrows back again, the equations for ethanal and propanone written with two reducing hydrogens, and a note that the C=C bond is not reduced.

Oxidation and reduction as opposite journeys: primary alcohol and aldehyde, secondary alcohol and ketone, with the two reduction equations.

Exam focus: Balance reduction equations with 2[H]. An aldehyde always gives a primary alcohol and a ketone always gives a secondary alcohol, with the –OH on the carbon that carried the C=O.

3

The Mechanism of Reduction

Reduction with NaBH₄ is a nucleophilic addition. The nucleophile is the hydride ion, :H⁻, which has a lone pair of electrons.

  1. The lone pair on :H⁻ is attracted to the δ+ carbon of the C=O group and forms a new C–H bond.
  2. At the same time the π pair of the C=O bond moves onto the oxygen, which becomes O⁻.
  3. A lone pair on the O⁻ of the intermediate takes an H⁺ from water or from the dilute acid added, forming the –OH group.
ChemDraw mechanism of the reduction of propanone by a hydride ion: the lone pair on H⁻ attacks the δ+ carbonyl carbon as the C=O π pair moves to oxygen, then a lone pair on the O⁻ takes a proton from a water molecule, giving propan-2-ol and a hydroxide ion.

Nucleophilic addition of a hydride ion to propanone: attack on the δ+ carbon, the π pair moving to oxygen, then the O⁻ taking a proton to give propan-2-ol.

Exam focus: Draw the first curly arrow from the lone pair on :H⁻ to the carbonyl carbon and the second from the C=O bond to the oxygen. Show the lone pair and the negative charge on the intermediate.

Check your understanding

Check: Reducing Carbonyl Compounds

Predict the alcohols formed and balance equations with [H], using carbonyl compounds not used on this page.

4

Addition of Hydrogen Cyanide

Hydrogen cyanide adds across the C=O bond of aldehydes and ketones to form a hydroxynitrile: a molecule with an –OH group and a –CN group on the same carbon.

The reagent is KCN followed by dilute acid.

Carbonyl compoundEquationProduct
ethanalCH₃CHO + HCN → CH₃CH(OH)CN2-hydroxypropanenitrile
propanoneCH₃COCH₃ + HCN → (CH₃)₂C(OH)CN2-hydroxy-2-methylpropanenitrile
propanalCH₃CH₂CHO + HCN → CH₃CH₂CH(OH)CN2-hydroxybutanenitrile
  • In the name, the carbon of the –CN group is carbon 1, so the –OH is on carbon 2.
  • The product has one more carbon atom than the carbonyl compound: a new C–C bond has formed.
  • The reaction is an addition: two molecules join and nothing is lost.

Why a cyanide salt is used

  • HCN is a very weak acid. On its own it gives very few CN⁻ ions, so the reaction would be extremely slow.
  • KCN is fully ionised and supplies a high concentration of CN⁻ ions, the nucleophile that starts the reaction.
  • The dilute acid added afterwards supplies the H⁺ ions needed for the second step.
  • HCN is a highly toxic gas that is hard to handle safely. Cyanide salts are also toxic, so the reaction is done in a fume cupboard.
Poster summarising the addition of hydrogen cyanide to carbonyl compounds: the two steps described in words, the equations for ethanal and propanone giving hydroxynitriles, the carbon chain growing by one carbon atom, and a safety panel on cyanide.

Adding HCN across the C=O bond: the two steps in words, the equations for ethanal and propanone, the extra carbon atom and the safety precautions.

5

The Mechanism: Nucleophilic Addition

The nucleophile is the cyanide ion, :CN⁻. Its lone pair and its negative charge are on the carbon atom.

Step 1

  • The lone pair on the carbon of :CN⁻ attacks the δ+ carbon of the C=O group, forming a C–C bond.
  • The π electron pair of the C=O bond moves onto the oxygen atom.
  • The intermediate has a negatively charged oxygen, O⁻, carrying three lone pairs.

Step 2

  • A lone pair on the O⁻ forms a bond to an H⁺ taken from the dilute acid.
  • The –OH group of the hydroxynitrile is complete.
Nucleophilic addition of cyanide to ethanal: the cyanide ion attacks the δ+ carbonyl carbon (step 1), then the O⁻ lone pair takes H⁺ from the dilute acid (step 2) to give 2-hydroxypropanenitrile.

Nucleophilic addition of cyanide to ethanal: cyanide attacks the δ+ carbon, the π pair moves to oxygen, then the O⁻ takes H⁺ from the dilute acid.

Curly arrowStarts atEnds at
1the lone pair on the C of :CN⁻the δ+ carbon of C=O
2the C=O double bondthe oxygen atom
3a lone pair on the O⁻the H⁺ ion

Exam focus: Start the first arrow at the lone pair on the carbon of :CN⁻, not on the nitrogen and not on the charge. Show δ+ and δ− on C=O and the lone pair and charge on the intermediate oxygen.

Check your understanding

Check: Hydroxynitriles and the Mechanism

Name the products of adding HCN and follow each curly arrow of nucleophilic addition, with carbonyl compounds not used on this page.

6

Why the Product Can Be a Racemic Mixture

The carbonyl carbon and the three atoms bonded to it are planar. The cyanide ion can attack from above the plane or from below it.

  • The two directions of attack are equally likely.
  • If the product carbon carries four different groups, attack from above gives one enantiomer and attack from below gives the other.
  • Equal amounts of the two enantiomers form: a racemic mixture (racemate), which is optically inactive.

Is the product chiral?

Carbonyl compoundGroups on the product carbonProduct
methanal, HCHOH, H, OH, CNnot chiral
ethanal, CH₃CHOH, CH₃, OH, CNchiral: racemic mixture (racemate)
propanone, CH₃COCH₃CH₃, CH₃, OH, CNnot chiral
butanone, CH₃COCH₂CH₃CH₃, C₂H₅, OH, CNchiral: racemic mixture (racemate)
pentan-3-oneC₂H₅, C₂H₅, OH, CNnot chiral
  • Aldehydes other than methanal give a chiral hydroxynitrile.
  • Unsymmetrical ketones give a chiral hydroxynitrile; symmetrical ketones do not.
  • The same reasoning applies to reduction: butanone gives racemic butan-2-ol.

Exam focus: Explain a racemic mixture (racemate) in three steps: the C=O group is planar; the nucleophile attacks from either side with equal probability; equal amounts of the two enantiomers form.

Check your understanding

Quick Check: Is the Alcohol Racemic?

Each carbonyl compound is reduced with a hydride reducing agent. Decide what forms.

7

Why Hydroxynitriles Matter

Adding HCN is one of the few reactions that makes a new carbon–carbon bond, so it is used to lengthen a carbon chain in a synthesis.

  • The –CN group can then be changed into other groups.
  • Hydrolysis with dilute acid turns –CN into –COOH: CH₃CH(OH)CN gives 2-hydroxypropanoic acid, CH₃CH(OH)COOH (see Carboxylic Acids: Structure, Properties and Preparation).
  • The two-carbon aldehyde ethanal has become a three-carbon hydroxy acid.
ReactionReagentNucleophileProduct
reductiona hydride reducing agent, [H]:H⁻alcohol
addition of HCNKCN followed by dilute acid:CN⁻hydroxynitrile
8

Common Exam Points

Say

  • “The mechanism is nucleophilic addition.”
  • “The lone pair on the carbon of :CN⁻ attacks the δ+ carbon.”
  • “An aldehyde is reduced to a primary alcohol; a ketone to a secondary alcohol.”
  • “The O⁻ of the intermediate gains H⁺ to form the –OH group.”

Do not say

  • “Electrophilic addition” (that is the mechanism for alkenes).
  • “HCN attacks the carbonyl group” (the attacking species is the CN⁻ ion).
  • “The product is hydroxypropanenitrile” without the number: give 2-hydroxypropanenitrile.
  • “NaBH₄ reduces the C=C bond as well” (it does not).

Watch for

  • Curly arrows that start from the negative charge or from the nitrogen of CN⁻.
  • Missing lone pair or charge on the oxygen of the intermediate.
  • The extra carbon: count the –CN carbon when naming the hydroxynitrile.
  • Products with no chiral centre, from methanal or a symmetrical ketone: no enantiomers form.
Check your understanding

Check: Mechanism, Chain Length and Optical Activity

Bring together reduction, HCN addition and the formation of racemic mixtures in examples not used on this page.

FAQs

Use these quick answers to check reduction and the nucleophilic addition of hydrogen cyanide.

What do aldehydes and ketones give when they are reduced?

An aldehyde is reduced to a primary alcohol and a ketone to a secondary alcohol. The reducing agent is written as [H] in equations: CH₃CHO + 2[H] → CH₃CH₂OH and CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃.

Does NaBH₄ reduce a C=C double bond as well?

No. NaBH₄ supplies hydride ions, H⁻, which are nucleophiles attracted to the δ+ carbon of the polar C=O bond. A C=C bond is electron-rich and non-polar, so it repels H⁻ and is left unchanged.

Why is the reaction with HCN called nucleophilic addition?

The cyanide ion, :CN⁻, uses its lone pair to attack the δ+ carbonyl carbon, and the π electrons of C=O move onto oxygen. The O⁻ then gains H⁺ from the dilute acid. Overall HCN adds across the C=O bond.

Why is a cyanide salt used rather than HCN on its own?

The reagent is KCN followed by dilute acid. HCN is a very weak acid, so alone it gives very few CN⁻ ions and reacts very slowly, and it is a highly toxic gas. The salt supplies plenty of the CN⁻ nucleophile.

Why is the hydroxynitrile made from ethanal not optically active?

2-Hydroxypropanenitrile, CH₃CH(OH)CN, has a chiral centre, but the planar carbonyl group is attacked by CN⁻ from either side with equal probability. Equal amounts of the two enantiomers form, so the product is a racemic mixture.

Dr. Mohammed Al-Fatah

Written by:
Dr. Mohammed Al-Fatah

Chemistry specialist revision notes for A Level Chemistry.

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Related 3.3.8 Aldehydes and Ketones Pages

Use these pages to connect the ideas across 3.3.8 Aldehydes and Ketones and the rest of the course.

Aldehydes and Ketones: Structure, Naming and Properties Oxidation and Tests for Carbonyl Compounds 3.3.8 Aldehydes and Ketones Overview 3.3.1 Introduction to organic chemistry 3.3.7 Optical Isomerism 3.3.9 Carboxylic Acids and Derivatives 3.3.15 NMR Spectroscopy 3.3.16 Chromatography

Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.