Introduction to Ketones and Aldehydes

Contributed by:

Jonathan James Mon, Jan 17, 2022 06:42 PM UTC

The highlights are:
1. Carbonyl compounds
2. IUPAC names for ketones
3. Naming aldehydes
4. Boiling points
5. Solubility
6. IR Spectroscopy
7. Nucleophilic Addition

1. Organic Chemistry, 5th Edition
L. G. Wade, Jr.
Chapter 18
Ketones and Aldehydes
Jo Blackburn
Richland College, Dallas, TX
Dallas County Community College District
2003,Prentice Hall

2. Carbonyl Compounds
=>
Chapter 18 2

3. Carbonyl Structure
• Carbon is sp2 hybridized.
• C=O bond is shorter, stronger, and
more polar than C=C bond in alkenes.
=>
Chapter 18 3

4. IUPAC Names
for Ketones
• Replace -e with -one. Indicate the
position of the carbonyl with a number.
• Number the chain so that carbonyl
carbon has the lowest number.
• For cyclic ketones the carbonyl carbon
is assigned the number 1.
=>
Chapter 18 4

5. Examples
O O
CH3 C CH CH3
CH3
Br
3-bromocyclohexanone
O
CH3 C CH CH2OH
CH3
4-hydroxy-3-methyl-2-butanone
=>
Chapter 18 5

6. Naming Aldehydes
• IUPAC: Replace -e with -al.
• The aldehyde carbon is number 1.
• If -CHO is attached to a ring, use the
suffix -carbaldehyde.
=>
Chapter 18 6

7. Examples
CH3 O
CH3 CH2 CH CH2 C H
3-methylpentanal
CHO
2-cyclopentenecarbaldehyde
=>
Chapter 18 7

8. Name as Substituent
• On a molecule with a higher priority
functional group, C=O is oxo- and -CHO
is formyl.
• Aldehyde priority is higher than ketone.
COOH
O CH3 O
CH3 C CH CH2 C H
CHO
3-methyl-4-oxopentanal 3-formylbenzoic acid
=>
Chapter 18 8

9. Common Names
for Ketones
• Named as alkyl attachments to -C=O.
• Use Greek letters instead of numbers.
O O
CH3 C CH CH3 CH3CH C CH CH3
CH3 Br CH3
methyl isopropyl ketone bromoethyl isopropyl ketone
=>
Chapter 18 9

10. Historical Common
Names O
C
O CH3
CH3 C CH3
acetone acetophenone
O
C
benzophenone
=>
Chapter 18 10

11. Aldehyde Common
Names
• Use the common name of the acid.
• Drop -ic acid and add -aldehyde.
1 C: formic acid, formaldehyde
2 C’s: acetic acid, acetaldehyde
3 C’s: propionic acid, propionaldehyde
4 C’s: butyric acid, butyraldehyde.
=>
Chapter 18 11

12. Boiling Points
• More polar, so higher boiling point than
comparable alkane or ether.
• Cannot H-bond to each other, so lower
boiling point than comparable alcohol.
=>
Chapter 18 12

13. Solubility
• Good solvent for alcohols.
• Lone pair of electrons on oxygen of
carbonyl can accept a hydrogen bond
from O-H or N-H.
• Acetone and acetaldehyde are miscible
in water.
=>
Chapter 18 13

14. Formaldehyde
• Gas at room temperature.
• Formalin is a 40% aqueous solution.
H H
C O HO
O O heat H2O OH
H H C H H C
C C H H
O formaldehyde,
H H formalin
b.p. -21C
trioxane, m.p. 62C
=>
Chapter 18 14

15. IR Spectroscopy
• Very strong C=O stretch around 1710 cm-1.
• Conjugation lowers frequency.
• Ring strain raises frequency.
• Additional C-H stretch for aldehyde: two
absorptions at 2710 cm-1 and 2810 cm-1.
=>
Chapter 18 15

16. H NMR Spectroscopy
=>
Chapter 18 16

17. C NMR Spectroscopy
Chapter 18 17 =>

18. MS for 2-Butanone
=>
Chapter 18 18

19. MS for Butyraldehyde
=>
Chapter 18 19

20. McLafferty
Rearrangement
• Loss of alkene (even mass number)
• Must have -hydrogen
=>
Chapter 18 20

21. UV Spectra,   *
• C=O conjugated with another double bond.
• Large molar absorptivities (> 5000)
=>
Chapter 18 21

22. UV Spectra, n  *
• Small molar absorptivity.
• “Forbidden” transition occurs less frequently.
=>
Chapter 18 22

23. Industrial Importance
• Acetone and methyl ethyl ketone are
important solvents.
• Formaldehyde used in polymers like
Bakelite.
• Flavorings and additives like vanilla,
cinnamon, artificial butter.
=>
Chapter 18 23

24. Synthesis Review
• Oxidation
2 alcohol + Na2Cr2O7  ketone
1 alcohol + PCC  aldehyde
• Ozonolysis of alkenes.
H R' H R'
1) O3
C C C O + O C
2) (CH3)2S
R R'' R R''
=>
Chapter 18 24

25. Synthesis Review (2)
• Friedel-Crafts acylation
Acid chloride/AlCl3 + benzene  ketone
CO + HCl + AlCl3/CuCl + benzene 
benzaldehyde (Gatterman-Koch)
• Hydration of terminal alkyne
Use HgSO4, H2SO4, H2O for methyl ketone
Use Sia2BH followed by H2O2 in NaOH for
aldehyde.
=>
Chapter 18 25

26. Synthesis Using
1,3-Dithiane
• Remove H+ with n-butyllithium.
BuLi
S S S S
_
H H H
• Alkylate with primary alkyl halide,
then hydrolyze.
+
O
CH3CH2Br H , HgCl2
C
S S S S H2O H CH2CH3
_
H CH2CH3 =>
H
Chapter 18 26

27. Ketones from
1,3-Dithiane
• After the first alkylation, remove the
second H+, react with another primary
alkyl halide, then hydrolyze.
+ O
BuLi CH3Br H , HgCl2
S S S S C
S S _ H2O
CH3 CH2CH3
CH3 CH2CH3
H CH2CH3 CH2CH3
=>
Chapter 18 27

28. Ketones from
Carboxylates
• Organolithium compounds attack the
carbonyl and form a diion.
• Neutralization with aqueous acid
produces an unstable hydrate that loses
water to form a ketone.
_ +
O O Li OH O
_ +
C _ C O Li C
O Li + C OH _
H3O
+ H2O CH3
CH3 CH3
CH3Li
=>
Chapter 18 28

29. Ketones from Nitriles
• A Grignard or organolithium reagent
attacks the nitrile carbon.
• The imine salt is then hydrolyzed to
form a ketone.
N MgBr O
C N C
CH2CH3 + C
CH3CH2MgBr + H3O CH2CH3
ether
=>
Chapter 18 29

30. Aldehydes from
Acid Chlorides
Use a mild reducing agent to prevent
reduction to primary alcohol.
O O
LiAlH(O-t-Bu)3
CH3CH2CH2C Cl CH3CH2CH2C H
=>
Chapter 18 30

31. Ketones from
Acid Chlorides
Use lithium dialkylcuprate (R2CuLi),
formed by the reaction of 2 moles of
R-Li with cuprous iodide.
CuI
2 CH3CH2CH2Li (CH3CH2CH2)2CuLi
O O
(CH3CH2CH2)2CuLi + CH3CH2C Cl CH3CH2C CH2CH2CH3
=>
Chapter 18 31

32. Nucleophilic Addition
• A strong nucleophile attacks the
carbonyl carbon, forming an alkoxide
ion that is then protonated.
• A weak nucleophile will attack a
carbonyl if it has been protonated,
thus increasing its reactivity.
• Aldehydes are more reactive than
ketones.
Chapter 18 32
=>

33. Wittig Reaction
• Nucleophilic addition of phosphorus ylides.
• Product is alkene. C=O becomes C=C.
Chapter 18 33
=>

34. Phosphorus Ylides
• Prepared from triphenylphosphine and an
unhindered alkyl halide.
• Butyllithium then abstracts a hydrogen
from the carbon attached to phosphorus.
+ _
Ph3P + CH3CH2Br Ph3P CH2CH3 Br
_
+ +
BuLi
Ph3P CH2CH3 Ph3P CHCH3
ylide =>
Chapter 18 34

35. Mechanism for Wittig
• The negative C on ylide attacks the
positive C of carbonyl to form a betaine.
• Oxygen combines with phosphine to
form the phosphine oxide. +
_ Ph3P O
+ H3C
Ph3P CHCH3 C O H C C CH3
Ph CH3 Ph
+ _ Ph3P O
Ph3P O Ph3P O
H CH3
H C C CH3 H C C CH3 C C
H3C Ph
CH3 Ph CH3 Ph
Chapter 18
=>
35

36. Addition of Water
• In acid, water is the nucleophile.
• In base, hydroxide is the nucleophile.
• Aldehydes are more electrophilic since
they have fewer e--donating alkyl groups.
O OH
HO
C + H2O C
H H H H K = 2000
O OH
HO
C + H2O C
CH3 CH3 CH3 CH3 K = 0.002
=>
Chapter 18 36

37. Addition of HCN
• HCN is highly toxic.
• Use NaCN or KCN in base to add
cyanide, then protonate to add H.
• Reactivity formaldehyde > aldehydes >
ketones >> bulky ketones.
O
CN
C HO
CH3CH2 CH3 + HCN C
CH3CH2 CH3
=>
Chapter 18 37

38. Formation of Imines
• Nucleophilic addition of ammonia or
primary amine, followed by elimination
of water molecule.
• C=O becomes C=N-R
CH3 CH3
H3C R R
_
RNH2 C O H2N C O N C OH
Ph + Ph H Ph
CH3 CH3
R R
N C OH N C
H Ph Ph =>38
Chapter 18

39. pH Dependence
• Loss of water is acid catalyzed, but acid
destroys nucleophiles.
• NH3 + H+  NH4+ (not nucleophilic)
• Optimum pH is around 4.5
=>
Chapter 18 39

40. Other Condensations
Chapter 18 40 =>

41. Addition of Alcohol
=>
Chapter 18 41

42. Mechanism
• Must be acid-catalyzed.
• Adding H+ to carbonyl makes it more
reactive with weak nucleophile, ROH.
• Hemiacetal forms first, then acid-
catalyzed loss of water, then addition of
second molecule of ROH forms acetal.
• All steps are reversible.
=>
Chapter 18 42

43. Mechanism for
Hemiacetal
O + OH OH
H+ +
H
OH HO OCH3
HO OCH3
+
+ HOCH3
HOCH3 +
+ H2OCH3
Chapter 18 43 =>

44. Hemiacetal to Acetal
H
+ OCH3
HO OCH3 HO OCH3
+
H+ + HOH
HOCH3
H
OCH3 +
CH3O OCH3 CH3O OCH3
+
=>
Chapter 18 44

45. Cyclic Acetals
• Addition of a diol produces a cyclic acetal.
• Sugars commonly exist as acetals or
hemiacetals.
CH2 CH2
O O
O
CH2 CH2
+
HO OH
=>
Chapter 18 45

46. Acetals as
Protecting Groups
• Hydrolyze easily in acid, stable in base.
• Aldehydes more reactive than ketones.
O
O CH2 CH2
HO OH
+ O
H H C
C
O
O
=>
Chapter 18 46

47. Selective Reaction
of Ketone
• React with strong nucleophile (base)
• Remove protective group.
+ _
O MgBr O CH HO CH3
3
+
CH3MgBr H3O
O H
O C C
C
O O
O
=>
Chapter 18 47

48. Oxidation of Aldehydes
Easily oxidized to carboxylic acids.
=>
Chapter 18 48

49. Tollens Test
• Add ammonia solution to AgNO3
solution until precipitate dissolves.
• Aldehyde reaction forms a silver mirror.
O O
+
_ H2O _
R C H + 2 Ag(NH3)2 + 3 OH 2 Ag + R C O + 4
O
+
_ H2O _
NH3)2 + 3 OH 2 Ag + R C O + 4 NH3 + 2 H2O
=>
Chapter 18 49

50. Reduction Reagents
• Sodium borohydride, NaBH4, reduces
C=O, but not C=C.
• Lithium aluminum hydride, LiAlH4, much
stronger, difficult to handle.
• Hydrogen gas with catalyst also
reduces the C=C bond.
=>
Chapter 18 50

51. Catalytic Hydrogenation
• Widely used in industry.
• Raney nickel, finely divided Ni powder
saturated with hydrogen gas.
• Pt and Rh also used as catalysts.
O OH
Raney Ni
H
=>
Chapter 18 51

52. Deoxygenation
• Reduction of C=O to CH2
• Two methods:
Clemmensen reduction if molecule is
stable in hot acid.
Wolff-Kishner reduction if molecule is
stable in very strong base.
=>
Chapter 18 52

53. Clemmensen Reduction
O
C CH2CH2CH3
CH2CH3 Zn(Hg)
HCl, H2O
O
Zn(Hg)
CH2 C CH2 CH3
H HCl, H2O
=>
Chapter 18 53

54. Wolff-Kisher Reduction
• Form hydrazone, then heat with strong
base like KOH or potassium t-butoxide.
• Use a high-boiling solvent: ethylene
glycol, diethylene glycol, or DMSO.
CH2 C H CH2 C H KOH CH2 CH3
H2N NH2
heat
O NNH2
=>
Chapter 18 54

55. End of Chapter 18
Chapter 18 55

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