Carbonyl Compounds

1
Chapter 34
Carbonyl Compounds
34.1 Introduction 34.2 Nomenclature of Carbonyl
Compounds 34.3 Physical Properties of Carbonyl
Compounds 34.4 Preparation of Carbonyl
Compounds 34.5 Reactions of Carbonyl
Compounds 34.6 Uses of Carbonyl Compounds
2
34.1 Introduction (SB p.2)
General formula of aldehydes
Examples
3
34.1 Introduction (SB p.2)
General formula of ketones Examples
4
34.1 Introduction (SB p.3)

• Carbonyl carbon is sp2-hybridized
• The ? bond is formed by the head-on overlap of an
  sp2 hybrid orbital of C and one p prbital of O
• The ? bond is formed by the side-way overlap of p
  orbitals of C and O

5
34.1 Introduction (SB p.3)

• The three atoms that are bonded to the carbonyl
  carbon forms a trigonal planar structure
• The bond angles between three attached atoms are
  ?120?

6
34.1 Introduction (SB p.3)

• Oxygen is more electronegative
• The carbonyl oxygen bears a partial negative
  charge and the carbonyl carbon bears partial
  positive charge

7
34.2 Nomenclature of Carbonyl Compounds (SB p.3)
Aldehydes
Aldehydes are named by replacing the final -e
of the name of the corresponding alkane with
-al Examples
8
34.2 Nomenclature of Carbonyl Compounds (SB p.4)
Ketones

• Ketones are named by replacing the final -e of
  the name of the corresponding alkane with -one.
  
• The parent chain is then numbered in the way that
  gives the carbonyl carbon atom the lowest
  possible number, and this number is used to
  indicate its position.
• Examples

9
34.2 Nomenclature of Carbonyl Compounds (SB p.4)
Check Point 34-1 (a) Draw the structural
formulae of all carbonyl compounds having the
molecular formula C4H8O. Give their IUPAC names.
Answer
10
34.2 Nomenclature of Carbonyl Compounds (SB p.4)
Check Point 34-1 (b) Draw the structural
formulae of all straight-chain carbonyl
compounds having the molecular formula C5H10O.
Answer
11
34.2 Nomenclature of Carbonyl Compounds (SB p.4)
Check Point 34-1 (c) Explain why there is no
such a compound called ethanone.
Answer
12
34.3 Physical Properties of Carbonyl Compounds
(SB p.5)

• Simple aldehydes and ketones are gases or liquids
  at room temperature
• Aliphatic aldehydes have unpleasant and pungent
  smell
• Ketones and benzaldehyde have a pleasant and
  sweet odour

Carbonyl compound Formula Boiling point (C) Melting point (C) Density at 20C (g cm3)
Aldehydes Methanal Ethanal Propanal Butanal Methylpropanal Benzaldehyde HCHO CH3CHO CH3CH2CHO CH3(CH2)2CHO (CH3)2CHCHO C6H5CHO 21 20.8 48.8 75.7 64.2 179 92 124 81 99 65.9 26 0.783 0.807 0.817 0.790 1.046
13
34.3 Physical Properties of Carbonyl Compounds
(SB p.5)
Carbonyl compound Formula Boiling point (C) Melting point (C) Density at 20C (g cm3)
Ketones Propanone Butanone Pentan-3-one Pentan-2-one 3-Methylbutan-2-one Hexan-2-one Phenylethanone CH3COCH3 CH3COCH2CH3 CH3CH2COCH2CH3 CH3CO(CH2)2CH3 CH3COCH(CH3)2 CH3CO(CH2)3CH3 C6H5COCH3 56.2 79.6 102 102 95 127 202 95.4 86.9 39.9 77.8 92 56.9 19.6 0.791 0.806 0.814 0.811 0.803 0.812 1.028
14
34.3 Physical Properties of Carbonyl Compounds
(SB p.5)
Boiling Point and Melting Point

• Carbonyl compounds have higher b.p. and m.p. than
  hydrocarbons of similar relative molecular
  masses? the presence of dipole-dipole
  interactions
• Carbonyl compounds have lower b.p. and m.p. than
  the corresponding alcohols? dipole-dipole
  interactions are weaker than intermolecular
  hydrogen bonds

15
34.3 Physical Properties of Carbonyl Compounds
(SB p.6)
Density

• The densities of aliphatic carbonyl compounds are
  lower than that of water at 20C
• Aromatic carbonyl compounds are slightly denser
  than water 20C
• Densities increase with increasing relative
  molecular masses

16
34.3 Physical Properties of Carbonyl Compounds
(SB p.6)
Solubility

• Aldehydes and ketones of low molecular masses
  show appreciable solubilities in water? carbonyl
  oxygen can form strong hydrogen bonds with water
  molecules

• e.g. propanone and ethanal are soluble in water
  in all proportions

17
34.3 Physical Properties of Carbonyl Compounds
(SB p.7)
Example 34-1 (a) In each pair of compounds below,
select the one you would expect to have a higher
boiling point. (i) A CH3CH2CHO B
CH3CH2CH2OH (ii) C D (iii) E
CH3CH2CH2CHO F CH3CH2CH2CH3 (iv) G H
Solution (a) (i) B (ii) D (iii) E (iv) H
Answer
18
34.3 Physical Properties of Carbonyl Compounds
(SB p.7)
Example 34-1 (b) Propanone, CH3COCH3, is
completely soluble in water, but octan-4-one,
CH3CH2CH2COCH2CH2CH2CH3, is almost insoluble in
soluble in water. Explain their difference in
solubility.
Answer
Solution (b) This is because the solubility in
water decreases as the hydrophobic hydrocarbon
portion lengthens
19
34.4 Preparation of Carbonyl Compounds (SB p.7)
Dehydrogenation of Alcohols
Industrially, lower members of aldehydes and
ketones are prepared by passing alcohol vapour
over hot silver catalyst
20
34.4 Preparation of Carbonyl Compounds (SB p.8)
Oxidation of Alcohols

• Example of oxidizing agents acidified K2Cr2O7
• Aldehydes are prepared by oxidation of 1
  alcohols

• Ketones are prepared by oxidation of 2 alcohols

21
34.4 Preparation of Carbonyl Compounds (SB p.8)
Oxidative Cleavage of Alkenes (Ozonolysis)

• Ozone reacts with alkenes vigorously to from
  ozonides
• Ozonides are reduced by Zn and H2O to give
  aldehydes and/or ketones

22
34.4 Preparation of Carbonyl Compounds (SB p.8)
Decarboxylation of Acid Salts
Aldehydes can be prepared by heating a mixture of
calcium methanoate and calcium carboxylate e.g.
23
34.4 Preparation of Carbonyl Compounds (SB p.9)
Ketones can be prepared by heating calcium
carboxylate e.g.
24
34.4 Preparation of Carbonyl Compounds (SB p.9)
Reduction of Acyl Chlorides

• Aldehydes can be prepared by reducing acyl
  chlorides by treatment with H2 in the presence of
  Pd / BaSO4 catalyst and S
• The purpose of adding sulphur is to poison the
  catalyst, so that the reduction does not proceed
  to produce alcohols

25
34.5 Reactions of Carbonyl Compounds (SB p.9)
Nucleophilic Addition Reactions

• Carbonyl group is susceptible to nucleophilic
  attack? carbonyl carbon bears a partial positive
  charge
• Nucleophiles use its lone pair electrons to form
  a bond with carbonyl carbon
• One pair of bonding electrons of the
  carbon-oxygen bond shift out to the carbonyl
  oxygen

26
34.5 Reactions of Carbonyl Compounds (SB p.10)

• The electron-rich oxygen transfers its electron
  pair to a proton? addition of Nu H to the
  carbonyl group

• The carbonyl carbon changes from a trigonal
  planar geometry (i.e. sp2-hybridized) to a
  tetrahedral geometry (i.e. sp3-hybridized)

27
34.5 Reactions of Carbonyl Compounds (SB p.10)

• Aldehydes are more reactive than ketones ?
  inductive effect and steric effect

1. The inductive effectThe carbonyl carbon in
ketones is less electron-deficient because two
alkyl groups release electrons whereas only one
present in aldehydes
28
34.5 Reactions of Carbonyl Compounds (SB p.10)

• The steric effect
• Aldehyde molecules are relatively open to the
  attack of nucleophiles? one group being attached
  to the carbonyl carbon is a small hydrogen atom

• In ketones, the two alkyl or aryl substituents
  cause a greater steric hindrance to the
  nucleophiles

29
34.5 Reactions of Carbonyl Compounds (SB p.11)

• Due to the above 2 factors, the general order of
  reactivity of carbonyl compounds

• The delocalization of ? electrons from the
  benzene ring reduce the electron deficiency of
  the carbonyl carbon atom and makes aromatic
  carbonyl compounds even less reactive than
  aliphatic ketones

30
34.5 Reactions of Carbonyl Compounds (SB p.11)
Addition of Hydrogen Cyanide
Addition of hydrogen cyanide to the carbonyl
group to form 2-hydroxyalkanenitriles (also known
as cyanohydrins)
31
34.5 Reactions of Carbonyl Compounds (SB p.11)
Examples
32
34.5 Reactions of Carbonyl Compounds (SB p.11)
Mechanism for the nucleophilic addition of
hydrogen cyanide to the carbonyl group
33
34.5 Reactions of Carbonyl Compounds (SB p.11)

• HCN is a poor nucleophile while CN is much
  stronger? the reaction can be catalyzed by a
  base stronger than CN, as the base can increase
  the concentration of CN
• HCN OH ?? CN H2O

• As HCN is very toxic and volatile, it is safer to
  generate it in the reaction mixture
• Mixing KCN or NaCN with dilute H2SO4 at 10 20C
  gives HCN 2KCN H2SO4 ?? 2HCN K2SO4 2NaCN
  H2SO4 ?? 2HCN Na2SO4

34
34.5 Reactions of Carbonyl Compounds (SB p.12)

• 2-Hydroxyalkanenitriles are useful intermediates
  in organic synthesis

• On acid hydrolysis, 2-hydroxyalkanenitriles are
  converted to 2-hydroxycarboxylic acids or
  2-alkenoic acids
• e.g.

35
34.5 Reactions of Carbonyl Compounds (SB p.12)

• With the use of reducing agents (e.g. LiAlH4),
  2-hydroxyalkanenitriles are
  reduced to amines
• e.g.

36
34.5 Reactions of Carbonyl Compounds (SB p.13)
Addition of Sodium Hydrogensulphate(IV)
Carbonyl compounds react reversibly with excess
40 aqueous hydrogensulphate(IV) solutions at
room temperature
37
34.5 Reactions of Carbonyl Compounds (SB p.13)
Examples
38
34.5 Reactions of Carbonyl Compounds (SB p.13)
The reaction mechanism

• The reaction is initiated by the attack of
  nucleophile, HSO3

39
34.5 Reactions of Carbonyl Compounds (SB p.13)

• This reaction is very sensitive to steric
  hindrance and is limited to aliphatic aldehydes
  and sterically unhindered ketones
• This reaction can be used for the separation and
  purification of the aldehydes and ketones, as
  they can be regenerated by treating the
  bisulphite addition product with aqueous alkalis
  or dilute acids.

40
34.5 Reactions of Carbonyl Compounds (SB p.14)
Example 34-2 Outline how you are going to
separate a mixture of butanone (b.p. 79.6C) and
1-chlorobutane (b.p. 78.5C) in diethyl ether.
Answer
41
34.5 Reactions of Carbonyl Compounds (SB p.14)
Solution The mixture of butanone and
1-chlorobutane cannot be separated by
distillation as their boiling points are too
close. However, they can be separated through the
nucleophilic addition reaction of sodium
hydrogensulphate(IV). With the addition of sodium
hydrogensulphate(IV) to the mixture, only
butanone reacts to give the bisulphite addition
product which is soluble in water. Then the
organic later (containing 1-chlorobutane) and the
aqueous layer (containing the bisulphite addition
product of butanone) are separated using a
separating funnel. 1-Chlorobutane can be obtained
by distilling off the ether. On the other hand,
with the addition of a dilute acid, the
bisulphite addition product is converted to the
carbonyl compound (i.e. butanone) which dissolves
in diethyl ether. The organic layer (containing
butanone) is separated from the aqueous layer by
means of a separating funnel. Butanone is
obtained after distilling off the ether.
42
34.5 Reactions of Carbonyl Compounds (SB p.14)
Addition Elimination (Condensation) Reactions

• Addition elimination reactions involve the
  first addition of two molecules to form an
  unstable intermediate followed by the spontaneous
  elimination of the elements of water
• e.g. reaction of aldehydes or ketones with the
  derivatives of ammonia

43
34.5 Reactions of Carbonyl Compounds (SB p.14)
Reaction with Hydroxylamine

• Carbonyl compounds react with hydroxylamine
  (NH2OH) to form oximes

44
34.5 Reactions of Carbonyl Compounds (SB p.15)
Examples
45
34.5 Reactions of Carbonyl Compounds (SB p.15)
Reaction with 2,4-dinitrophenylhydrazine

• Carbonyl compounds react with 2,4-dinitrophenylhyd
  razine to form 2,4-dinitrophenylhydrazones

46
34.5 Reactions of Carbonyl Compounds (SB p.15)
Examples
47
34.5 Reactions of Carbonyl Compounds (SB p.16)

• The oximes and 2,4-dinitrophenylhydrazones are
  used to identify unknown aldehydes and ketones

• They are insoluble solids and have sharp
  characteristic melting points
• The products are purified by recrystallization
  from ethanol and then filtered and washed under
  suction
• Their melting points are determined and compared
  with that in data book to identify the original
  aldehyde or ketone

48
34.5 Reactions of Carbonyl Compounds (SB p.16)
Carbonyl compound Formula Melting point of 2,4-dinitrophenylhydrazone (C)
Aldehydes Methanal Ethanal Propanal Butanal Benzaldehyde HCHO CH3CHO CH3CH2CHO CH3CH2CH2CHO C6H5CHO 167 146, 164 (2 forms) 156 123 237
Ketones Propanone Butanone Pentan-2-one Pentan-3-one Hexan-2-one Phenylethanone CH3COCH3 CH3CH2COCH3 CH3CH2CH2COCH3 CH3CH2COCH2CH3 CH3CH2CH2CH2COCH3 C6H5COCH3 128 115 141 156 107 250
49
34.5 Reactions of Carbonyl Compounds (SB p.17)
Check Point 34-2 (a) Compare the addition
reactions of carbonyl compounds and alkenes.
Answer
50
34.5 Reactions of Carbonyl Compounds (SB p.17)
Check Point 34-2 (b) Describe briefly how you
can distinguish between two carbonyl compounds
having similar boiling points.
Answer
51
34.5 Reactions of Carbonyl Compounds (SB p.17)
Oxidations

• Aldehydes can be oxidized to carboxylic acids by
  strong oxidizing agents such as KMnO4 and
  K2Cr2O7, and also by mild oxidizing agents such
  as Tollens reagent and Fehlings reagent

52
34.5 Reactions of Carbonyl Compounds (SB p.18)
Reaction with Potassium Manganate(VII) and
Potassium Dichromate(VI)

• Aldehydes are oxidized readily by common
  oxidizing agents such as KMnO4/H and K2Cr2O7/H

53
34.5 Reactions of Carbonyl Compounds (SB p.18)

• Generally, ketones do not undergo oxidation as
  their oxidation involves the cleavage of the
  strong carbon-carbon bond

• More severe conditions are required to bring
  about the oxidation

54
34.5 Reactions of Carbonyl Compounds (SB p.18)
Reaction with Tollens Reagent (Silver Mirror
Test)

• Tollens reagent contains Ag(NH3)2
• Ag(NH3)2 oxidizes aldehydes to carboxylic acids
  while it is reduced to metallic silver which
  deposits on the wall of the reaction vessel as
  silver mirror

55
34.5 Reactions of Carbonyl Compounds (SB p.19)

• Aldehydes are mixed with Tollens reagent in a
  clean test tube and placed in water bath kept at
  60C

• If the wall of the reaction vessel is not clean
  enough, a silver mirror cannot be formed and a
  black precipitate is deposited instead

• All ketones give a negative result of the silver
  mirror test
• This reaction can be used to distinguish
  aldehydes from ketones

56
34.5 Reactions of Carbonyl Compounds (SB p.18)
Reaction with Fehlings Solution (Fehlings Test)

• Fehlings solution is an alkaline solution of
  copper(II) tartrate. It is a blue solution
• Aliphatic aldehydes reduce the Cu2 ion in
  Fehlings solution to form a brick-red
  precipitate of Cu2O

57
34.5 Reactions of Carbonyl Compounds (SB p.18)
Fehlings solution

• Ketones and aromatic aldehydes give a negative
  result of Fehlings test
• This reaction can be used to distinguish
  aliphatic aldehydes from ketones and aromatic
  aldehydes

58
34.5 Reactions of Carbonyl Compounds (SB p.20)
Reductions

• Aldehydes and ketones undergo reduction reactions
  forming 1 and 2 alcohols respectively

59
34.5 Reactions of Carbonyl Compounds (SB p.20)
Reaction with Lithium Tetrahydridoaluminate

• Lithium tetrahydridoaluminate (also called
  lithium aluminium hydride, LiAlH4) is a powerful
  reducing agent
• It reduces aldehydes to 1 alcohols and ketones
  to 2 alcohols

60
34.5 Reactions of Carbonyl Compounds (SB p.20)

• LiAlH4 is able to reduce carboxylic acid and
  esters to give alcohols
• LiAlH4 reacts violently with water, therefore the
  reaction must be carried out in anhydrous
  solutions, usually in dry ether

61
34.5 Reactions of Carbonyl Compounds (SB p.21)
Reaction with Sodium Tetrahydridoborate
In practice, the reduction of aldehydes and
ketones to alcohols is carried out by sodium
tetrahydridoborate (also called sodium
borohydride, NaBH4)
62
34.5 Reactions of Carbonyl Compounds (SB p.21)

• NaBH4 is a less powerful reducing agent than
  LiAlH4
• NaBH4 reduces only aldehydes and ketones
• The reduction by NaBH4 can be carried out in
  water or alcohol solutions

63
34.5 Reactions of Carbonyl Compounds (SB p.21)
Triiodomethane Formation (Iodoform Reaction)
64
34.5 Reactions of Carbonyl Compounds (SB p.21)
65
34.5 Reactions of Carbonyl Compounds (SB p.22)
Check Point 34-3 Draw the structural formulae of
the major organic products A to H in the
following reactions (a) KCN/H2SO4 conc.
HCl CH3CH2CHO ??????? A ???? B 20C (b) 2,4
-dinitrophenylhydrazine CH3CH2CHO ?????????
C (c) 1. LiAlH4 / dry ether CH3CH2CHO ???????
D 2. H3O (d) NaBH4 CH3CH CHCH2CHO
????? E H2O
Answer
66
34.5 Reactions of Carbonyl Compounds (SB p.22)
Check Point 34-3 Draw the structural formulae of
the major organic products A to H in the
following reactions (e) (f)
Answer
67
34.6 Uses of Carbonyl Compounds (SB p.22)
As Raw Materials for Making Plastics
Urea-methanal
68
34.6 Uses of Carbonyl Compounds (SB p.23)
In the presence of excess methanal, cross
linkages will be formed
69
34.6 Uses of Carbonyl Compounds (SB p.24)
Urea-methanal

• thermosetting polymer (cannot be softened and
  insoluble in any solvents)
• excellent electrical insulator
• resistant to chemical attack
• used for moulding electrical sockets

70
34.6 Uses of Carbonyl Compounds (SB p.24)
Perspex
Propanone is converted to methyl
2-methylpropenoate, which is the monomer for the
production of perspex

• Perspex is a dense, transparent solid
• Used to make safety goggles, advertising signs
  and carside light protectors

71
34.6 Uses of Carbonyl Compounds (SB p.24)
As Solvents

• Propanone
• Liquid with a boiling point of 56.2C
• Can dissolve a variety of organic compounds
• Important solvent used in industry and in the
  laboratory

72
The END