Transcript Chapter 18 Ketones and Aldehydes

Organic Chemistry, 6th Edition
L. G. Wade, Jr.
Chapter 18
Ketones and Aldehydes
Carbonyl Compounds
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Chapter 18
2
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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3
Examples
O
O
CH3
C CH CH3
CH3
3-methyl-2-butanone
3-methylbutan-2-one
Br
3-bromocyclohexanone
O
CH3
C CH CH2OH
CH3
4-hydroxy-3-methyl-2-butanone
4-hydroxy-3-methylbutan-2-one
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Naming Aldehydes
• IUPAC: Replace -e with -al.
• The aldehyde carbon is number 1.
• If -CHO is attached to a ring, use the
suffix -carbaldehyde.
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Examples
CH3
CH2
CH3
O
CH CH2
C H
3-methylpentanal
CHO
2-cyclopentenecarbaldehyde
cyclopent-2-en-1-carbaldehyde
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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
Chapter 18
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7
Historical Common
Names
C
O
CH3
O
CH3
C CH3
acetophenone
acetone
O
C
benzophenone
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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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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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Synthesis Review
• Oxidation
2 alcohol + Na2Cr2O7  ketone
1 alcohol + PCC  aldehyde
• Friedel-Crafts acylation
Acid chloride/AlCl3 + benzene  ketone
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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
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13
Addition of Water
• In acid, water is the nucleophile.
• In base, hydroxide is the nucleophile.
O
H
HO
+ H2O
C
H
O
CH3
C
CH3
C
H
HO
+ H2O
OH
CH3
Chapter 18
H
K = 2000
OH
C
CH3
K = 0.002
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14
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
CH3CH2
C
HO
CH3
+ HCN
Chapter 18
CH3CH2
CN
C
CH3
15
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Formation of Imines
• Nucleophilic addition of ammonia or
primary amine, followed by elimination
of water molecule.
• C=O becomes C=N-R
CH3
H3C
RNH2
C O
Ph
R
CH3
R
_
C
H2N
O
+ Ph
R
Chapter 18
CH3
N C OH
H
Ph
CH3
N C
Ph
N C OH
H
Ph
R
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Other Condensations
Chapter 18
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17
Addition of Alcohol
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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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Mechanism for
Hemiacetal
• Oxygen is protonated.
• Alcohol is the nucleophile.
• H+ is removed.
Chapter 18
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20
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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Acetals as
Protecting Groups
• Hydrolyze easily in acid, stable in base.
• Aldehydes more reactive than ketones.
O
O
CH2
CH2
OH
HO
C
H
+
H
C
O
O
O
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Selective Reaction
of Ketone
• React with strong nucleophile (base).
• Remove protective group.
+
_
MgBr O CH
3
O
HO
CH3MgBr
C
O
O
H3O
C
O
O
CH3
+
C
H
O
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Oxidation of Aldehydes
Easily oxidized to carboxylic acids.
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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 + 2
+
NH3)2
_
+
3 OH
+
Ag(NH3)2
_
+
3 OH
O
H2O
O
H2O
2 Ag + R C O
_
2 Ag + R C O
+
_
+
4 NH3 + 2 H2O
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4
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.
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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
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End of Chapter 18
Chapter 18
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