Transcript Chemical Properties of Aldehydes and Ketones

Chapter 23
Aldehydes and Ketones
Ketones are
common
solvents
for quickdrying
paints.
Introduction to General, Organic, and Biochemistry, 10e
John Wiley & Sons, Inc
Morris Hein, Scott Pattison, and Susan Arena
Course Outline
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
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Structures of Aldehydes and Ketones
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).
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Structures of Aldehydes and Ketones
The aldehyde group is often written as CHO. For
example,
O
CH3CHO is equivalent to
CH3C
H
The ketone group is often written as CO. For example,
O
CH3COCH3 is equivalent to
CH3CCH3
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Naming Aldehydes and Ketones
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.
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Naming Aldehydes and Ketones
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.
6
Naming Aldehydes and Ketones
O
CH3 C
H
Ethane is the longest chain contains two carbon atoms so
ethan + al = ethanal.
O
H
1
C
2 3 4 5 6
CH2CH2CHCH2CH3
CH3
The longest chain contains six carbon atoms so this would be
hexan + al = hexanal. The methyl group is attached to C4 so the
name is 4-methyhexanal.
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Naming Aldehydes and Ketones
The IUPAC names and commonly used names of some
aldehydes are on Table 23.1.
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Naming Aldehydes and Ketones
The simplest aromatic aldehyde is called benzaldehyde.
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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.
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Naming Aldehydes and Ketones
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.
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Naming Aldehydes and Ketones
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.
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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.
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Naming Aldehydes and Ketones
Aromatic ketones are named in a fashion similar to that of
aliphatic ketones and often have special names as well.
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Your Turn!
Write the structure of butyl ethyl ketone.
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Your Turn!
Write the structure of butyl ethyl ketone.
O
CH3CH2CH2CH2
C
CH2CH3
16
Your Turn!
Give names for the following compounds.
O
Br
CH 3CHCH 2CCH 2CHCH 2CH 3
CH 3
Cl
O
CH 3CHCH 2CHC
H
Cl
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Your Turn!
Give names for the following compounds.
O
Br
CH 3CHCH 2CCH 2CHCH 2CH 3
CH 3
2-methyl-6-bromo-4-octanone
Cl
O
CH 3CHCH 2CHC
H
Cl
2,4-dichloropentanal
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Your Turn!
Write the structure of cyclohexanone.
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Your Turn!
Write the structure of cyclohexanone.
O
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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.
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Bonding and Physical Properties
Unlike alcohols, aldehydes and ketones cannot hydrogenbond to themselves, because no hydrogen atom is
attached to the oxygen atom of the carbonyl group.
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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 . . .
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Bonding and Physical Properties
24
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 . . .
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Bonding and Physical Properties
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Bonding and Physical Properties
Other naturally-occurring aldehydes and ketones are
shown below.
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Your Turn!
Rank these molecules based on boiling point from lowest
to highest.
1.
2.
3.
3-hexanol (molar mass = 102 g/mol)
3-hexanone (molar mass = 100 g/mol)
heptane (molar mass = 100 g/mol)
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Your Turn!
Rank these molecules based on boiling point from lowest
to highest.
1.
2.
3.
3-hexanol (molar mass = 102 g/mol)
3-hexanone (molar mass = 100 g/mol)
heptane (molar mass = 100 g/mol)
heptane
Lowest boiling
point
3-hexanone
3-hexanol
Highest boiling
point
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Chemical Properties of Aldehydes
and Ketones
The carbonyl functional group is the reactive site for
aldehydes and ketones. These compounds undergo three
broad classes of reactions.
Reaction Type
Aldehydes or Ketones
Oxidation
Aldehydes Only
Reduction
Both
Addition
Both
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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.
31
Chemical Properties of Aldehydes
and Ketones
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 . . .
32
Chemical Properties of Aldehydes
and Ketones
Aldehydes can be identified using the Tollens test (i.e. the
silver-mirror test).
33
Chemical Properties of Aldehydes
and Ketones
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.
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Chemical Properties of Aldehydes
and Ketones
The positive test results for these tests are
shown here.
Test
Positive Test Result for Aldehydes
Tollens
Silver mirror
Fehling
Blue → Brick-red
Benedict
Blue → Brick-red
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Chemical Properties of Aldehydes
and Ketones
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.
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Your Turn!
Write the structures of the products, or indicate no
reaction, for the following.
1. Propanone in the Tollens test
2. Pentanal in the Fehling test
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Your Turn!
Write the structures of the products, or indicate no
reaction, for the following.
1. Propanone in the Tollens test
O
NH3
CH 3CCH 3 +
2Ag+
H2O
No reaction
2. Pentanal in the Fehling test
O
O
NaOH
CH3CH2CH2CH2C
H + 2Cu
2+
H2O
CH3CH2CH2CH2C
O- Na+ + Cu2O(s)
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Chemical Properties of Aldehydes
and Ketones
Reduction
Aldehydes and ketones are easily reduced to
alcohols using LiAlH4, NaBH4 , or H2/Ni .
Aldehydes yield primary alcohols and ketones
yield secondary alcohols.
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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.
40
Chemical Properties of Aldehydes
and Ketones
This reaction sequence is important because the
body uses this sequence as an additional
pathway to regenerate glucose.
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Chemical Properties of Aldehydes
and Ketones
Addition
Addition reactions of aldehydes (RCHO) and ketones
(R2CO). Reactants
Product
RCHO +
R2CO +
RCHO +
R2CO +
RCHO or
2RCHO
2R2CO
ROH
ROH
2ROH
2ROH
R2CO + HCN
Hemiacetal
Hemiketal
Acetal
Ketal
Cyanohydrin
Aldol
- hydroxy ketone
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Chemical Properties of Aldehydes
and Ketones
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.
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Chemical Properties of Aldehydes
and Ketones
Compounds that have two alkoxy (ether) groups on the
same carbon atom are known as acetals and ketals.
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Chemical Properties of Aldehydes
and Ketones
Addition of Alcohols
Aldehydes react with alcohols and a trace of acid to yield
hemiacetals.
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Chemical Properties of Aldehydes
and Ketones
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.
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Chemical Properties of Aldehydes
and Ketones
Cyclic hemiacetals or hemiketals can form when the
alcohol and the carbonyl group exist within the same
molecule.
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Your Turn!
Write the structures and IUPAC names for the
hemiacetal and acetal formed from the reaction
of butanal and ethanol.
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Your Turn!
Write the structures and IUPAC names for the
hemiacetal and acetal formed from the reaction
of butanal and ethanol.
Formation of the hemiacetal
OH
O
H+
CH 3CH 2CH 2C
H + CH 3CH 2
OH
CH 3CH 2CH 2C
H
OCH 2CH 3
1-ethoxy-1-butanol
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Your Turn!
Write the structures and IUPAC names for the
hemiacetal and acetal formed from the reaction
of butanal and ethanol.
Formation of the acetal
OH
OCH 2CH 3
dry HCl
CH 3CH 2CH 2C
H
OCH 2CH 3
1-ethoxy-1-butanol
+ CH 3CH 2
OH
CH 3CH 2CH 2C
H
OCH 2CH 3
1,1-diethoxybutane
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Chemical Properties of Aldehydes
and Ketones
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 . . .
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Chemical Properties of Aldehydes
and Ketones
This is the reaction of acetaldehyde and acetone with HCN.
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Chemical Properties of Aldehydes
and Ketones
Hydrolysis of cyanohydrins yields an -hydroxy carboxylic
acid. Cyanohydrins are important synthetic intermediates
for -hydroxy acids as well as amino acids.
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Chemical Properties of Aldehydes
and Ketones
Aldol Condensations (Self-Addition)
In a carbonyl compound, the carbon atoms are labeled
using the Greek alphabet: alpha (), beta (), gamma
(), delta (), and so on, according to their positions with
respect to the carbonyl group.
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Chemical Properties of Aldehydes
and Ketones
In an aldol condensation an aldehyde or ketone that
contains -hydrogens may add to itself or to another hydrogen-containing aldehyde or ketone. The product of
this reaction contains both a carbonyl group and an
alcohol group within the same molecule.
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Chemical Properties of Aldehydes
and Ketones
The transfer of the -hydrogen occurs in the first step
and the addition of the anion to the cation occurs in
the second step.
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Chemical Properties of Aldehydes
and Ketones
Ketones with -hydrogen atoms also undergo aldol
reactions.
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Your Turn!
Write the equation for the aldol condensation of pentanal.
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Your Turn!
Write the equation for the aldol condensation of pentanal.
First step
O
dilute
CH 3CH 2CH 2CH 2C
H
NaOH
OH
O
CH 3CH 2CH 2CH 2C +
- CHCH 2C
+
H
O
CH 3CH 2CHCH 2C
H
H
CH 2CH 3
H
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Your Turn!
Write the equation for the aldol condensation of pentanal.
Second step
OH
OH
CH 3CH 2CH 2CH 2C +
H
O
CH 3CH 2CH 2CH 2CHCHCH 2C
O
- CHCH 2C
H
CH 2CH 3
H
CH 2CH 3
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Your Turn!
The conversion of 2-pentanone to 2-pentanol is a(n):
•
•
•
•
•
Hydration
Dehydration
Hydrolysis
Oxidation
Reduction
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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
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States.
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.
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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.
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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.
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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.
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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.
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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).
75
Chapter 23 Summary
• Condensation polymers form when monomers combine
and eliminate a small-molecule product such as water.
• Common examples of condensation polymers are the
phenol–formaldehyde polymers, also known as
phenolics.
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