Transcript Chapter 18 Ketones and Aldehydes

Chapter 4
Ketones and Aldehydes
4.1 Carbonyl Structure
• Carbon is sp2 hybridized.
• C=O bond is shorter, stronger, and
more polar than C=C bond in alkenes.
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4.2 Carbonyl Compounds
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4.3 Naming Aldehydes
• IUPAC: Replace -e with -al.
• The aldehyde carbon is number 1.
• If -CHO is attached to a ring, use the
suffix -carbaldehyde.
• There are no cycloaldehydes
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Examples
CH3
CH2
CH3
O
CH CH2
C H
CHO
3-methylpentanal
2-cyclopentenealdehyde
benzaldehyde - aromatic
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Small unbranched 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.
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4.4 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.
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Examples
O
O
CH3
C CH CH3
CH3
3-methyl-2-butanone
Br
3-bromocyclohexanone
O
CH3
C CH CH2OH
CH3
4-hydroxy-3-methyl-2-butanone
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Common Names
for Simple Ketones
• Named as alkyl attachments to -C=O.
O
CH3
O
CH3CH2 C CH CH3
C CH CH3
CH3
CH3
methyl isopropyl ketone
ethyl isopropyl ketone
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Historical Common
Names
C
O
CH3
O
CH3
C CH3
acetophenone
acetone
O
C
benzophenone
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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
CH3
O
CH3
O
C
CH CH2
C H
3-methyl-4-oxopentanal
CHO
3-formylbenzoic acid
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4.5 Isomerism in aldehydes and
ketones
• Aldehydes and ketones are
constitutional isomers
• Aldehydes and ketones can have
skeletal and positional isomers if there
are enough carbons.
• Stereoisomers are also possible if there
is a ring or C=C in the molecule
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4.6 Selected Common aldehydes
and ketones
•
•
•
•
•
•
Formaldehyde
Acetone
Vanillin
Benzaldehyde
Cinnamaldehyde
Butanedione
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Formaldehyde
• Gas at room temperature.
• Formalin is a 40% aqueous solution.
H
H
O
H
C
O
O
C
C H
O
H
H
heat
H C H
H2O
formaldehyde,
b.p. -21C
trioxane, m.p. 62C
HO
OH
H C
H
formalin
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Industrial Importance
• Acetone and methyl ethyl ketone are
important solvents.
• Formaldehyde used in polymers like
Bakelite.
• Flavorings and additives like vanilla,
cinnamon, artificial butter.
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•
•
•
•
•
•
Formaldehyde -formalin
Acetone – solvent and metabolic product
Vanillin - vanilla flavoring
Benzaldehyde - almond flavor
Cinnamaldehyde - cinnamon
Butanedione - butter
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4.7 Physical properties
• Boiling points – page 121
• Solubility – water solubility page 123
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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.
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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.
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4.8 Preparation of Aldehydes and
Ketones
• Oxidation
2 alcohol + Na2Cr2O7  ketone
1 alcohol  aldehyde
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4.9 Oxidation and reduction
of Aldehydes and ketones
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Tollens Test
• Add ammonia solution to AgNO3
solution until precipitate dissolves.
• Aldehyde reaction forms a silver mirror.
O
R C H+
+
NH3)2
_
+
3OH
2
+
Ag(NH3)2
_
+
3OH
O
H2O
O
H2O
2 Ag + R C O
_
2 Ag + R C O
+
_
+
4 NH3 +
2 H2O
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4
Oxidation of ketones
Ketones  no reaction
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Reduction –
Catalytic Hydrogenation
• Ketones give secondary alcohols
• Aldehydes give primary alcohols
O
CH3CHO
OH
H2 / Ni
H2 / Ni
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H
CH3CH2OH
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• Reduction: aldehyde + hydrogen --> pri. Alcohol
• Form of addition reaction.
H3C
H2
C
C
H
O + H2
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Reduction: aldehyde + hydrogen
• Break hydrogen bond
H
H
H3C
H2
C
C
H
H
H
O + H2
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Reduction: aldehyde + hydrogen
• Break double bond and use electrons to bond with
hydrogen.
H
H 3C
H2
C
C
H
H
H2 H
O H
C
H3C
C
H
O
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Reduction: aldehyde + hydrogen
• Brief Method
H3C
H2
C
O
+ H
H
C
H
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H2 H
O H
C
H3C
C
H
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4.10 Addition of Alcohol
• Hemiacetal and Acetal Functional Groups
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Hemiacetal and Acetal Functional Groups
• Hemiacetal: alcohol and ether on same carbon
• Acetal: Two ethers on same carbon
H3C
OH
O
H3C
C
H2C
H3C
H3C
H
O
C
H3C
CH3
O
C
CH3
O
CH3
OH
CH3
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Mechanism
• Must be acid-catalyzed.
• Adding H+ to carbonyl makes it more
reactive with weak nucleophile, ROH.
• Hemiacetal forms first, then acidcatalyzed loss of water, then addition of
second molecule of ROH forms acetal.
• All steps are reversible.
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Hemiacetal Synthesis: aldehyde + alcohol
•
•
•
•
Alcohol + aldehyde --> hemiacetal
Ethanal + methanol
Alcohol oxygen becomes an ether
Carbon double bond oxygen becomes an alcohol
H
H3C
C
H
O + H C
3
O
O
H3C
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C
H
H
O
CH3
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Mechanism for
Hemiacetal
O
+ OH
+
H+
H
OH
+
OH
HO
HOCH3
HO
OCH3
+
HOCH3
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OCH3
+
+ H2OCH3
34
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Synthesis of Acetal Functional Group
• Acetal: alcohol plus hemiacetal (ether synthesis)
• Acetal: Two ethers on same carbon
O
H
O
H3C
C
H
H
H
H
O
CH3 +
H3C
H3C
O
O
C
H3C
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CH3
O
H
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Aldehyde and Ketone Reactions
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Mechanism Hemiacetal to Acetal
HO
OCH3
+
HO
H
OCH3
OCH3
+
H+
+ HOH
HOCH3
OCH3
HOCH3
+
CH3O
H
OCH3
CH3O
OCH3
+
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Cyclic Acetals
• Addition of a diol produces a cyclic acetal.
• Sugars commonly exist as acetals or
hemiacetals.
CH2 CH2
O
O
O
+
CH2
HO
CH2
OH
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• SKIP 4.11 AND 4.12
THE END
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