1. OF ALDEHYDES AND KETONES A guide for A level students
THE CHEMISTRY
OF ALDEHYDES AND KETONES
A guide for A level students

2. ALDEHYDES & KETONES CONTENTS Prior knowledge
Bonding in carbonyl compounds
Structural differences
Nomenclature
Preparation
Identification
Oxidation
Nucleophilic addition
Reduction
2,4-dinitrophenylhydrazine

3. Before you start it would be helpful to…
ALDEHYDES & KETONES
Before you start it would be helpful to…
know the functional groups found in organic chemistry
know the arrangement of bonds around carbon atoms
recall and explain the polarity of covalent bonds

4. CARBONYL COMPOUNDS - BONDING PLANAR WITH BOND ANGLES OF 120°
Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar
PLANAR WITH BOND ANGLES OF 120°

5. CARBONYL COMPOUNDS - BONDING PLANAR WITH BOND ANGLES OF 120°
Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar
the unhybridised 2p orbital of carbon is at 90° to these
P ORBITAL
PLANAR WITH BOND ANGLES OF 120°

6. CARBONYL COMPOUNDS - BONDING PLANAR WITH BOND ANGLES OF 120°
Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar
the unhybridised 2p orbital of carbon is at 90° to these
it overlaps with a 2p orbital of oxygen to form a pi (p) bond
P ORBITAL
PLANAR WITH BOND ANGLES OF 120°

7. CARBONYL COMPOUNDS - BONDING PLANAR WITH BOND ANGLES OF 120°
Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar
the unhybridised 2p orbital of carbon is at 90° to these
it overlaps with a 2p orbital of oxygen to form a pi (p) bond
P ORBITAL
ORBITAL OVERLAP
PLANAR WITH BOND ANGLES OF 120°

8. CARBONYL COMPOUNDS - BONDING PLANAR WITH BOND ANGLES OF 120°
Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar
the unhybridised 2p orbital of carbon is at 90° to these
it overlaps with a 2p orbital of oxygen to form a pi (p) bond
P ORBITAL
ORBITAL OVERLAP
PLANAR WITH BOND ANGLES OF 120°
NEW ORBITAL

9. CARBONYL COMPOUNDS - BONDING PLANAR WITH BOND ANGLES OF 120°
Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar
the unhybridised 2p orbital of carbon is at 90° to these
it overlaps with a 2p orbital of oxygen to form a pi (p) bond
as oxygen is more electronegative than carbon the bond is polar
P ORBITAL
ORBITAL OVERLAP
PLANAR WITH BOND ANGLES OF 120°
NEW ORBITAL

10. CARBONYL COMPOUNDS - STRUCTURE
Structure carbonyl groups consists of
a carbon-oxygen double bond
the bond is polar due to the
difference in electronegativity
Difference ALDEHYDES - at least one H attached to the carbonyl group
C = O H
C = O H
CH3

11. CARBONYL COMPOUNDS - STRUCTURE
Structure carbonyl groups consists of
a carbon-oxygen double bond
the bond is polar due to the
difference in electronegativity
Difference ALDEHYDES - at least one H attached to the carbonyl group
KETONES - two carbons attached to the carbonyl group
C = O H
C = O H
CH3
C = O
CH3
C = O
C2H5
CH3

12. CARBONYL COMPOUNDS - FORMULAE
Molecular C3H6O
Structural C2H5CHO CH3COCH3
Displayed
Skeletal
C = O H
C2H5
C = O
CH3
H C C C H
H O H
H H
H C C C O
H H H
H H O O

13. CARBONYL COMPOUNDS - NOMENCLATURE
Aldehydes C2H5CHO propanal
Ketones CH3COCH3 propanone
CH3CH2COCH3 butanone
CH3COCH2CH2CH3 pentan-2-one
CH3CH2COCH2CH3 pentan-3-one
C6H5COCH3 phenylethanone

14. CARBONYL COMPOUNDS - FORMATION
ALDEHYDES
Oxidation of
primary (1°) alcohols RCH2OH [O] ——> RCHO H2O
beware of further oxidation RCHO [O] ——> RCOOH
Reduction of
carboxylic acids RCOOH [H] ——> RCHO + H2O
KETONES
secondary (2°) alcohols RCHOHR [O] ——> RCOR H2O

15. CARBONYL COMPOUNDS - IDENTIFICATION
Method strong peak around cm-1 in the infra red spectrum
Method formation of an orange precipitate with 2,4-dinitrophenylhydrazine
Although these methods identify a carbonyl group, they cannot tell the difference
between an aldehyde or a ketone. To narrow it down you must do a second test.

16. CARBONYL COMPOUNDS - IDENTIFICATION
Differentiation to distinguish aldehydes from ketones, use a mild oxidising agent
Tollen’s
Reagent ammoniacal silver nitrate
mild oxidising agent which will oxidise aldehydes but not ketones
contains the diammine silver(I) ion - [Ag(NH3)2 ]+
the silver(I) ion is reduced to silver Ag+(aq) + e¯ ——> Ag(s)
the test is known as THE SILVER MIRROR TEST

17. CARBONYL COMPOUNDS - IDENTIFICATION
Differentiation to distinguish aldehydes from ketones, use a mild oxidising agent
Tollen’s
Reagent ammoniacal silver nitrate
mild oxidising agent which will oxidise aldehydes but not ketones
contains the diammine silver(I) ion - [Ag(NH3)2 ]+
the silver(I) ion is reduced to silver Ag+(aq) + e¯ ——> Ag(s)
the test is known as THE SILVER MIRROR TEST
Fehling’s
Solution contains a copper(II) complex ion giving a blue solution
on warming, it will oxidise aliphatic (but not aromatic) aldehydes
the copper(II) is reduced to copper(I)
a red precipitate of copper(I) oxide, Cu2O, is formed
The silver mirror test is the better alternative as it works with all aldehydes
Ketones do not react with Tollen’s Reagent or Fehling’s Solution

18. CARBONYL COMPOUNDS - CHEMICAL PROPERTIES
OXIDATION
• provides a way of differentiating between aldehydes and ketones
• mild oxidising agents are best
• aldehydes are easier to oxidise
• powerful oxidising agents oxidise ketones to a mixture of carboxylic acids
ALDEHYDES easily oxidised to acids
RCHO(l) [O] ——> RCOOH(l)
CH3CHO(l) [O] ——> CH3COOH(l)
KETONES oxidised under vigorous conditions to acids with fewer carbons
C2H5COCH2CH3(l) [O] ——> C2H5COOH(l) CH3COOH(l)

19. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism occurs with both aldehydes and ketones
involves addition to the C=O double bond
unlike alkenes, they are attacked by nucleophiles
attack is at the positive carbon centre due to the
difference in electronegativities
alkenes are non-polar and are attacked by electrophiles
undergoing electrophilic addition
Group
Bond
Polarity
Attacking species
Result
ALKENES
C=C
NON-POLAR
ELECTROPHILES
ADDITION
CARBONYLS
C=O
POLAR
NUCLEOPHILES
ADDITION

20. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Reagent hydrogen cyanide - HCN (in the presence of KCN)
Conditions reflux in alkaline solution
Nucleophile cyanide ion CN¯
Product(s) hydroxynitrile (cyanohydrin)
Equation CH3CHO HCN ——> CH3CH(OH)CN
2-hydroxypropanenitrile
Notes HCN is a weak acid and has difficulty dissociating into ions
HCN H CN¯
the reaction is catalysed by alkali which helps
produce more of the nucleophilic CN¯

21. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism Nucleophilic addition
Step 1 CN¯ acts as a nucleophile and attacks the slightly positive C
One of the C=O bonds breaks; a pair of electrons goes onto the O
STEP 1

22. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism Nucleophilic addition
Step 1 CN¯ acts as a nucleophile and attacks the slightly positive C
One of the C=O bonds breaks; a pair of electrons goes onto the O
Step 2 A pair of electrons is used to form a bond with H+
Overall, there has been addition of HCN
STEP 1
STEP 2

23. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism Nucleophilic addition
Step 1 CN¯ acts as a nucleophile and attacks the slightly positive C
One of the C=O bonds breaks; a pair of electrons goes onto the O
Step 2 A pair of electrons is used to form a bond with H+
Overall, there has been addition of HCN
STEP 1
STEP 2

24. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism Nucleophilic addition
Step 1 CN¯ acts as a nucleophile and attacks the slightly positive C
One of the C=O bonds breaks; a pair of electrons goes onto the O
Step 2 A pair of electrons is used to form a bond with H+
Overall, there has been addition of HCN
STEP 1
STEP 2

25. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
ANIMATED MECHANISM

26. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Watch out for the possibility of optical isomerism in hydroxynitriles
CN¯ attacks from above
CN¯ attacks from below

27. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Watch out for the possibility of optical isomerism in hydroxynitriles
CN¯ attacks
from above
CN¯ attacks
from below

28. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
ANIMATED MECHANISM

29. CARBONYL COMPOUNDS - REDUCTION WITH NaBH4
Reagent sodium tetrahydridoborate(III) (sodium borohydride), NaBH4
Conditions aqueous or alcoholic solution
Mechanism Nucleophilic addition (also reduction as it is addition of H¯)
Nucleophile H¯ (hydride ion)
Product(s) Alcohols Aldehydes are REDUCED to primary (1°) alcohols.
Ketones are REDUCED to secondary (2°) alcohols.
Equation(s) CH3CHO [H] ——> CH3CH2OH
CH3COCH [H] ——> CH3CHOHCH3
Notes The water provides a proton
Question NaBH4 doesn’t reduce C=C bonds. WHY?
CH2 = CHCHO [H] ———> CH2 = CHCH2OH

30. CARBONYL COMPOUNDS - REDUCTION WITH HYDROGEN
Reagent hydrogen
Conditions catalyst - nickel or platinum
Reaction type Hydrogenation, reduction
Product(s) Alcohols Aldehydes are REDUCED to primary (1°) alcohols.
Ketones are REDUCED to secondary (2°) alcohols.
Equation(s) CH3CHO H ——> CH3CH2OH
CH3COCH H2 ——> CH3CHOHCH3
Note Hydrogen also reduces C=C bonds
CH2 = CHCHO H ——> CH3CH2CH2OH

31. CARBONYL COMPOUNDS - REDUCTION
Introduction Functional groups containing multiple bonds can be reduced
C=C is reduced to CH-CH
C=O is reduced to CH-OH
CN is reduced to CH-NH2
Hydrogen H• H2
H+ (electrophile) H¯ (nucleophile)
Reactions Hydrogen reduces C=C and C=O bonds
CH2 = CHCHO [H] ——> CH3CH2CH2OH
Hydride ion H¯ reduces C=O bonds
CH2 = CHCHO [H] ——> CH2=CHCH2OH
Explanation C=O is polar so is attacked by the nucleophilic H¯
C=C is non-polar so is not attacked by the nucleophilic H¯

32. CARBONYL COMPOUNDS - REDUCTION
Example What are the products when Compound X is reduced?
COMPOUND X H2
NaBH4

33. CARBONYL COMPOUNDS - REDUCTION
Example What are the products when Compound X is reduced?
COMPOUND X H2
NaBH4
C=O is polar so is attacked by the nucleophilic H¯
C=C is non-polar so is not attacked by the nucleophilic H¯

34. 2,4-DINITROPHENYLHYDRAZINE C6H3(NO2)2NHNH2
Structure
Use reacts with carbonyl compounds (aldehydes and ketones)
used as a simple test for aldehydes and ketones
makes orange crystalline derivatives - 2,4-dinitrophenylhydrazones
derivatives have sharp, well-defined melting points
also used to characterise (identify) carbonyl compounds.
Identification / characterisation
A simple way of characterising a compound (finding out what it is) is to
measure the melting point of a solid or the boiling point of a liquid.
C6H3(NO2)2NHNH2

35. 2,4-DINITROPHENYLHYDRAZINE C6H3(NO2)2NHNH2
The following structural isomers have similar boiling points because of similar
van der Waals forces and dipole-dipole interactions. They would be impossible
to identify with any precision using boiling point determination.
Boiling point 213°C °C °C
Melting point of
2,4-dnph derivative °C °C °C
By forming the 2,4-dinitrophenylhydrazone derivative and taking its melting point,
it will be easier to identify the unknown original carbonyl compound.
CHO Cl

36. What should you be able to do?
REVISION CHECK
What should you be able to do?
Recall the structure of and bonding in the carbonyl group
Explain the difference in structure between aldehydes and ketones
Recall the different response to oxidation of aldehydes and ketones
Recall and understand the mechanism of nucleophilic addition
Recall the products from the reduction of carbonyl compounds
CAN YOU DO ALL OF THESE? YES NO

37. Try some past paper questions
WELL DONE!
Try some past paper questions