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Chapter 23
Introduction to General, Organic, and Biochemistry, 10e John Wiley & Sons, Inc
Morris Hein, Scott Pattison, and Susan Arena
Aldehydes and Ketones
Ketones are common solvents for quick-drying paints.
23.1 Structures of Aldehydes and Ketones
23.2 Naming Aldehydes and Ketones
23.3 Bonding and Physical Properties
23.4 Chemical Properties of Aldehydes and Ketones
23.5 Common Aldehydes and Ketones
23.6 Condensation Polymers
Chapter 23 Summary
Course Outline
2
3
Aldehydes (i.e. RCHO) and ketones (i.e. R2CO) are compounds with a chemical structure that contains the carbonyl (C=O) functional group as shown in these structures (R– is an alkyl group and Ar– is an aromatic group).
Structures of Aldehydes and Ketones
4
The aldehyde group is often written as CHO. For example,
CH3C
O
HCH3CHO is equivalent to
The ketone group is often written as CO. For example,
CH3CCH3
O
CH3COCH3 is equivalent to
Structures of Aldehydes and Ketones
5
IUPAC Rules for Naming Aldehydes
1. Name the longest continuous carbon chain containing the CHO group.
2. The CHO carbon is the #1 carbon atom. Notice that the aldehyde group is not given a number in the name.
This is the CHO carbon and is numbered as carbon one
Naming Aldehydes and Ketones
6
3. Drop an –e from the corresponding alkane parent name and add the suffix –al.
4. Number and name groups attached to the longest carbon chain.
Naming Aldehydes and Ketones
7
CH3 C H
O
H C CH2CH2CHCH2CH3
O
CH3
Ethane is the longest chain contains two carbon atoms soethan + al = ethanal.
2 3 4 5 61
The longest chain contains six carbon atoms so this would behexan + al = hexanal. The methyl group is attached to C4 so the name is 4-methyhexanal.
Naming Aldehydes and Ketones
8
The IUPAC names and commonly used names of some aldehydes are on Table 23.1.
Naming Aldehydes and Ketones
9
H
O
benzaldehyde
The simplest aromatic aldehyde is called benzaldehyde.
Naming Aldehydes and Ketones
10 butanedial
Naming Aldehydes and Ketones
Dialdehydes are compounds that contain two aldehyde groups. They are named by adding the suffix –dial to the corresponding hydrocarbon name.
For example a molecule with four carbons and two
aldehyde groups is called butanedial.
11
IUPAC Rules for Naming Ketones
1. Name the longest continuous carbon chain containing the C=O group.
2. Drop the –e from the corresponding alkane parent name and add the suffix –one.
Naming Aldehydes and Ketones
12
3. Carbon chains with four or more carbon atoms are numbered so the carbonyl (C=O) carbon is given the lowest possible number.
4. Attached groups are named and numbered as is done when naming aldehydes.
Naming Aldehydes and Ketones
13
Naming Aldehydes and Ketones
An alternative non-IUPAC method that is often used to name simple ketones lists the names of the alkyl or aromatic groups attached to the carbonyl carbon together with the word ketone. Thus, butanone is also called methyl ethyl ketone.
14
Naming Aldehydes and Ketones
Aromatic ketones are named in a fashion similar to that of aliphatic ketones and often have special names as well.
17
Your Turn!
Give names for the following compounds.
CH3CHCH2CCH2CHCH2CH3
O
CH3
Br
CH3CHCH2CHC
Cl
Cl
O
H
18
Your Turn!
Give names for the following compounds.
CH3CHCH2CCH2CHCH2CH3
O
CH3
Br
CH3CHCH2CHC
Cl
Cl
O
H
2-methyl-6-bromo-4-octanone 2,4-dichloropentanal
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Bonding and Physical Properties
All aldehydes and ketones have carbonyl groups. The carbonyl carbon in a carbonyl group is sp2-hybridized with one pi (π) bond and three sigma (σ) bonds.
The pi (π) bond of the carbonyl group is reactive and
undergoes addition reactions. The carbonyl group is polarized which causes aldehydes
and ketones to be reactive.
22
Bonding and Physical Properties
Unlike alcohols, aldehydes and ketones cannot hydrogen-bond to themselves, because no hydrogen atom is attached to the oxygen atom of the carbonyl group.
23
Bonding and Physical Properties
Therefore aldehydes and ketones have lower boiling points than alcohols of comparable molar mass as seen on Table 23.2 on the next slide . . .
25
Bonding and Physical Properties
Low-molar-mass aldehydes and ketones are soluble in water.
The lower-molar-mass aldehydes have a penetrating,
disagreeable odor. As the molar mass increases, the odor of both aldehydes and ketones—especially the aromatic ones—becomes more fragrant.
Some aldehydes and ketons are used in flavorings and
perfumes . . .
28
Your Turn!
Rank these molecules based on boiling point from lowest to highest.
1. 3-hexanol (molar mass = 102 g/mol) 2. 3-hexanone (molar mass = 100 g/mol) 3. heptane (molar mass = 100 g/mol)
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Your Turn!
Rank these molecules based on boiling point from lowest to highest.
1. 3-hexanol (molar mass = 102 g/mol) 2. 3-hexanone (molar mass = 100 g/mol) 3. heptane (molar mass = 100 g/mol)
heptane 3-hexanone 3-hexanolLowest boilingpoint
Highest boilingpoint
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The carbonyl functional group is the reactive site for aldehydes and ketones. These compounds undergo three broad classes of reactions.
Chemical Properties of Aldehydes and Ketones
Oxidation Reduction Addition
Aldehydes Only Both Both
Reaction Type Aldehydes or Ketones
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Chemical Properties of Aldehydes and Ketones
Oxidation Aldehydes (RCHO) are oxidized with dichromate to
carboxylic acids (RCOOH). Here acetaldehyde oxidizes to acetic acid.
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Three well-known identification tests for aldehydes are based on the fact that aldehydes are much easier to oxidize than ketones. These tests are Tollens, Fehling, and Benedict tests . . .
Chemical Properties of Aldehydes and Ketones
33
The Tollens test gives the characteristicsilver-mirror f rom the deposition of silveron a glass surface.
Chemical Properties of Aldehydes and Ketones
Aldehydes can be identified using the Tollens test (i.e. the silver-mirror test).
34
The aldehyde group (RCHO) is oxidized to a carboxylic acid by Cu2+ ions in both the Fehling and Benedict tests.
The tests are very similar except the Fehling test
uses tartaric acid to complex Cu2+ while the Benedict test uses citric acid.
Chemical Properties of Aldehydes and Ketones
35
The positive test results for these tests are shown here.
Tollens Fehling Benedict
Silver mirror Blue → Brick-red Blue → Brick-red
Test Positive Test Result for Aldehydes
Chemical Properties of Aldehydes and Ketones
36
All three tests are used to distinguish aldehydes from ketones.
These tests give a positive test result for aldehydes and
a negative test result for ketones.
Tollens test
Fehling/Benedicttests
Chemical Properties of Aldehydes and Ketones
37
Write the structures of the products, or indicate no reaction, for the following.
Your Turn!
1. Propanone in the Tollens test
2. Pentanal in the Fehling test
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Write the structures of the products, or indicate no reaction, for the following.
Your Turn!
1. Propanone in the Tollens test
2. Pentanal in the Fehling test
CH3CCH3
O
+ 2Ag+NH3
H2O No reaction
CH3CH2CH2CH2C
O
H + 2Cu2+NaOH
H2O CH3CH2CH2CH2C
O
O- Na+ + Cu2O(s)
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Reduction Aldehydes and ketones are easily reduced to
alcohols using LiAlH4, NaBH4 , or H2/Ni . Aldehydes yield primary alcohols and ketones yield secondary alcohols.
Chemical Properties of Aldehydes and Ketones
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Chemical Properties of Aldehydes and Ketones
Oxidation and reduction of aldehydes and ketones is important in biochemistry.
Pyruvic acid is reduced to lactic acid in muscle cells and
then lactic acid is oxidized back to pyruvic acid in the liver.
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This reaction sequence is important because the body uses this sequence as an additional pathway to regenerate glucose.
Chemical Properties of Aldehydes and Ketones
42
RCHO + ROH R2CO + ROH RCHO + 2ROH R2CO + 2ROH RCHO or R2CO + HCN 2RCHO 2R2CO
Hemiacetal Hemiketal Acetal Ketal Cyanohydrin Aldol β- hydroxy ketone
Reactants Product
Addition Addition reactions of aldehydes (RCHO) and ketones
(R2CO).
Chemical Properties of Aldehydes and Ketones
43
Hemiacetals, Hemiketals, Acetals and Ketals Compounds derived from aldehydes and ketones that
contain an alkoxy (ether) and a hydroxy group on the same carbon atom are known as hemiacetals and hemiketals.
Chemical Properties of Aldehydes and Ketones
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Compounds that have two alkoxy (ether) groups on the same carbon atom are known as acetals and ketals.
Chemical Properties of Aldehydes and Ketones
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Addition of Alcohols Aldehydes react with alcohols and a trace of acid to yield
hemiacetals.
Chemical Properties of Aldehydes and Ketones
46
In the presence of excess alcohol and a strong acid such as dry HCl, aldehydes or hemiacetals react with a second molecule of the alcohol to yield an acetal.
Chemical Properties of Aldehydes and Ketones
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Cyclic hemiacetals or hemiketals can form when the alcohol and the carbonyl group exist within the same molecule.
Chemical Properties of Aldehydes and Ketones
48
Write the structures and IUPAC names for the hemiacetal and acetal formed from the reaction of butanal and ethanol.
Your Turn!
49
Write the structures and IUPAC names for the hemiacetal and acetal formed from the reaction of butanal and ethanol.
Formation of the hemiacetal
Your Turn!
CH3CH2CH2C
O
H CH3CH2 OH+H+
CH3CH2CH2C
OH
H
OCH2CH3
1-ethoxy-1-butanol
50
Write the structures and IUPAC names for the hemiacetal and acetal formed from the reaction of butanal and ethanol.
Formation of the acetal
Your Turn!
CH3CH2CH2C
OH
H
OCH2CH3
1-ethoxy-1-butanol
dry HClCH3CH2 OH+ CH3CH2CH2C
OCH2CH3
H
OCH2CH3
1,1-diethoxybutane
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Addition of HCN Addition of HCN to aldehydes and ketones under basic
conditions yields cyanohydrins resulting in an increase in the carbon chain by one carbon atom.
A cyanohydrin is a compound that has a cyano group (–CN)
and a hydroxyl group (–OH) on the same carbon . . .
Chemical Properties of Aldehydes and Ketones
52
This is the reaction of acetaldehyde and acetone with HCN.
Chemical Properties of Aldehydes and Ketones
53
Hydrolysis of cyanohydrins yields an α-hydroxy carboxylic acid. Cyanohydrins are important synthetic intermediates for α-hydroxy acids as well as amino acids.
Chemical Properties of Aldehydes and Ketones
54
Aldol Condensations (Self-Addition) In a carbonyl compound, the carbon atoms are labeled
using the Greek alphabet: alpha (α), beta (β), gamma (γ), delta (δ), and so on, according to their positions with respect to the carbonyl group.
Chemical Properties of Aldehydes and Ketones
55
In an aldol condensation an aldehyde or ketone that contains α-hydrogens may add to itself or to another α-hydrogen-containing aldehyde or ketone. The product of this reaction contains both a carbonyl group and an alcohol group within the same molecule.
Chemical Properties of Aldehydes and Ketones
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The transfer of the α-hydrogen occurs in the first step and the addition of the anion to the cation occurs in the second step.
Chemical Properties of Aldehydes and Ketones
57
Ketones with α-hydrogen atoms also undergo aldol reactions.
Chemical Properties of Aldehydes and Ketones
59
Write the equation for the aldol condensation of pentanal. First step
Your Turn!
CH3CH2CH2CH2C
O
H
CH3CH2CHCH2C
O
H
H
dilute
NaOH CH3CH2CH2CH2C
OH
H
+ CHCH2C
CH2CH3
O
H-+
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Write the equation for the aldol condensation of pentanal. Second step
Your Turn!
CH3CH2CH2CH2C
OH
H
+
CHCH2C
CH2CH3
O
H-
CH3CH2CH2CH2CHCHCH2C
OH
CH2CH3
O
H
61
The conversion of 2-pentanone to 2-pentanol is a(n): • Hydration • Dehydration • Hydrolysis • Oxidation • Reduction
Your Turn!
62
Your Turn!
The conversion of 2-pentanone to 2-pentanol is a(n): • Hydration • Dehydration • Hydrolysis • Oxidation • Reduction 2-Pentanone is a ketone and 2-pentanol is an alcohol. The
conversion of a ketone to an alcohol is a reduction.
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Chemical Properties of Aldehydes and Ketones
Aldol condensations are common in biochemistry. For example, the strength of collagen depends on aldol condensation reactions. The cross-linking bonds form a strong network.
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Common Aldehydes and Ketones
Formaldehyde (methanal) is made from methanol by reaction with oxygen in the presence of a silver or copper catalyst.
Formaldehyde is marketed as a 37% aqueous solution called formalin. The largest use of formaldehyde is in the manufacture of polymers. About 1.33 x 109 kg of formaldehyde are manufactured annually in the United States.
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Common Aldehydes and Ketones
Acetaldehyde (ethanal) has a general narcotic action and, in large doses, may cause respiratory paralysis.
Its principal use is as an intermediate in the manufacture
of other chemicals, such as acetic acid and 1-butanol. Acetic acid is made by air oxidation of acetaldehyde.
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Common Aldehydes and Ketones
Acetone (propanone) is widely used as an organic solvent in the manufacture of drugs, chemicals, and explosives; for the removal of paints, varnishes, and fingernail polish; and as a solvent in the plastics industry. It is manufactured by the oxidation of 2-propanol.
67
Common Aldehydes and Ketones
Methyl ethyl ketone (MEK) is also widely used as a solvent, especially for lacquers. MEK is made by oxidation of 2-butanol.
68
Condensation Polymers
Condensation polymers are substances that are produced when a small molecule such as water is eliminated during polymerization.
Polyesters, polyamides, polyurethanes, and phenolics represent four important classes of condensation polymers.
69
Condensation Polymers
A phenol–formaldehyde condensation polymer (Bakelite) was first marketed a century ago. Polymers of this type are still widely used, especially in electrical equipment, because of their insulating and fire-resistant properties.
Polymers made from phenol are known as phenolics.
70
Condensation Polymers
Each phenol molecule can react with formaldehyde to lose an H atom from the para position and from each ortho position.
Each formaldehyde molecule reacts with two phenol
molecules and eliminates water.
71
Condensation Polymers
Similar reactions can occur at the other two reactive sites on each phenol molecule, leading to the formation of the polymer. This polymer is thermosetting because it has an extensively cross-linked network structure.
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Chapter 23 Summary
• Both aldehydes and ketones contain the carbonyl group. Aldehydes have at least one hydrogen atom bonded to the carbonyl carbon. Ketones have only alkyl or aryl groups bonded to the carbonyl carbon.
• The IUPAC naming of aldehydes and ketones follows a similar process to that used for other organic molecules.
• The carbonyl group in aldehydes and ketones is very polar, with the oxygen pulling electrons from the carbon.
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Chapter 23 Summary
• Aldehydes and ketones do not hydrogen-bond to themselves. They have lower boiling points than alcohols of comparable molar mass.
• Aldehydes are easily oxidized to carboxylic acids. Ketones are unreactive under similar conditions.
• Both aldehydes and ketones can be reduced. Aldehydes yield primary alcohols, while ketones yield secondary alcohols when reduced.
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Chapter 23 Summary
• A hemiacetal is formed when an alcohol adds across an aldehyde carbonyl double bond. If an alcohol adds to a ketone, a hemiketal is produced.
• With an excess of alcohol, two alcohols can react with a carbonyl to form an acetal (from an aldehyde) and a ketal (from a ketone).
• Cyanohydrins are formed by the addition of HCN to a carbonyl group.
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Chapter 23 Summary
• In an aldol condensation, two carbonyl-containing molecules (either aldehydes or ketones) connect together.
• Common aldehydes are formaldehyde and acetaldehyde. Formaldehyde (methanal) is used primarily in the manufacture of polymers. Acetaldehyde (ethanal) is used to produce acetic acid and 1-butanol.
• Common ketones are acetone and methyl ethyl ketone (MEK).