Classes Winter09 30BID18 30B-Ch16-Pt2
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Transcript of Classes Winter09 30BID18 30B-Ch16-Pt2
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Chapter 16: Aldehydes and Ketones
Lecture 20
Chem 30B
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Oxidation of Aldehydes
Aldehydes are oxidized to carboxylic acids by avariety of oxidizing agents, including H2CrO4.
They are also oxidized by Ag+.
CHO H2 CrO4 COOH
Hexanal Hexanoic acid
Vanillic acidVanillin
++
CH
HO
CH3 O
O O
CH3 O
HO
COH
Ag2 OTHF, H2 O
NaOHAgHCl
H2 O
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Oxidation of Aldehydes
Aldehydes are oxidized by O2 in a radicalchain reaction.
Liquid aldehydes are so sensitive to air thatthey must be stored under N2.
Benzoic acidBenzaldehyde
+CH
O O
COH2O22
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Oxidation of Ketones
Ketones are not normally oxidized by chromicacid.
They are oxidized by powerful oxidants at
high temperature and high concentrations ofacid or base.
Hexanedioic acid(Adipic acid)
Cyclohexanone(keto form)
Cyclohexanone(enol form)
HNO3
O
HO OH
OO OH
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Reduction
Aldehydes can be reduced to 1 alcohols.
Ketones can be reduced to 2 alcohols.
The C=O group of an aldehyde or ketone can be
reduced to a -CH2- group.
AldehydesCan BeReduced to Ketones
Can BeReduced to
O OOH
RCH
RCH2 OH
RCH3
RCR'RCHR'
RCH2 R'
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Metal Hydride Reduction
The most common laboratory reagents forthe reduction of aldehydes and ketones areNaBH4 and LiAlH4. Both reagents are sources of hydride ion, H:-, a
very powerful nucleophile.
Hydride ionLithium aluminum
hydride (LAH)
Sodium
borohydride
H
H H
H
H-B-H H-Al-HLi +Na+ H:
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NaBH4 Reduction
Reductions with NaBH4 are most commonlycarried out in aqueous methanol, in puremethanol, or in ethanol.
One mole of NaBH4 reduces four moles ofaldehyde or ketone.
4RCH
O
NaBH4
( RCH2O) 4B-
Na+ H2O
4RCH2OH
A tetraalkyl borate
boratesalts
+
+methanol
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NaBH4 Reduction
The key step in metal hydride reduction istransfer of a hydride ion to carbon of the C=Ogroup to form a tetrahedral carbonyl addition
compound.
Na+
H
H
H-B-H
O
R-C-R'
O BH3
H
R-C-R'
Na+
H2O
O-H
H
R-C-R'
This H comes from waterduring hydrolysis
This H comes from thehydride reducing agent
+
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LiAlH4 Reduction
Unlike NaBH4, LiAlH4 reacts violently withwater, methanol, and other protic solvents.
Reductions using it are carried out indiethyl ether or tetrahydrofuran (THF).
4RCR LiAlH4 ( R2 CHO)4Al- Li+ H2OH
+or OH
-
OH
4RCHR+ + aluminumsaltsA tetraalkyl
aluminate
ether
O
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Catalytic Reduction
Catalytic reductions are generally carried out at
from 25 to 100C and 1 to 5 atm H2.
+ 25 oC, 2 atm
Pt
Cyclohexanone Cyclohexanol
O OH
H2
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Catalytic Reduction
A carbon-carbon double bond may also bereduced under these conditions.
1-Butanoltrans- 2-Butenal
(Crotonalde hyde)
2 H2
NiH
O
OH
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Catalytic Reduction
O OH
RCH=CHCR' RCH=CHCHR'1. NaBH4
2. H2O
O
RCH=CHCR' H2Rh
RCH2 CH2CR'
O
+
By careful choice of experimentalconditions, it is often possible toselectively reduce a carbon-carbon doublein the presence of an aldehyde or ketone.
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Clemmensen Reduction
Refluxing an aldehyde or ketone withamalgamated zinc in concentrated HClconverts the carbonyl group to a methylenegroup.
Zn(Hg), HCl
OH O OH
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Wolff-Kishner Reduction
In the original procedure, the aldehyde or ketoneand hydrazine are refluxed with KOH in a high-boiling solvent.
The same reaction can be brought about usinghydrazine and potassium tert-butoxide in DMSO.
+
diethylene glycol
(reflux)
KOH
N2
Hydrazine
+ H2 NNH2
+ H2 O
O
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Racemization
Racemization at an -carbon may becatalyzed by either acid or base.
O
Ph
OH
Ph
O
PhAn achiral
enol(R)-3-Phenyl-2-
butanone(S)-3-Phenyl-2-
butanone
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Deuterium Exchange
Deuterium exchange at an -carbonmay be catalyzed by either acid or
base.
+
Acetone-d 6Acetone
+
O O
CH 3 CCH 3 6 D2 O CD 3 CCD 3 6 HODD
+
or OD
-
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-Halogenation
-Halogenation: aldehydes and ketones withat least one -hydrogen react at an -carbonwith Br2 and Cl2 .
Reaction is catalyzed by both acid and base.
O
Br2 CH3COOH
OBr
HBr
Acetophenone
++
-Bromo-acetophenone
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-Halogenation Acid-catalyzed -halogenation
Step 1: Acid-catalyzed enolization.
Step 2: Nucleophilic attack of the enol on halogen.
Step 3: (not shown) Proton transfer to solvent
completes the reaction.
H
R
R'-C-C-R
O
R'
C C
H-O R
R
slow
C
R
RH-O
C
R'
Br BrR'
C C
O Br
R
R
H
Br:-+fast+
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-Halogenation
Base-promoted
-halogenationStep 1: Formation of an enolate anion.
Step 2: Nucleophilic attack of the enolate
anion on halogen.
+-
-
Resonance-stabilized enolate anion
+slow
O H
R
O
C C
R'R'
C C
O:
R'-C-C-R-:OH H2O
R
R
R
R
+fast
R'
C C
O Br
R
R BrBr Br +
-
R'
C C
O: R
R
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-Halogenation
Acid-catalyzed halogenation: Introduction of a second halogen is slower than the first.
Introduction of the electronegative halogen on the -carbon decreases the basicity of the carbonyl oxygen
toward protonation. Base-promoted -halogenation:
Each successive halogenation is more rapid than theprevious one.
The introduction of the electronegative halogen on the
-carbon increases the acidity of the remaining -hydrogens and, thus, each successive -hydrogen isremoved more rapidly than the previous one.