1
Created byProfessor William Tam & Dr. Phillis Chang
Chapter 16
Aldehydes & Ketones:Nucleophilic Additionto the Carbonyl Group
Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.
1. Introduction
v Carbonyl compounds
O
R R'ketone
O
R Haldehyde
O
R OR'ester
(R, R' = alkyl, alkenyl, alkynyl or aryl groups)
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v Rules● Aldehyde as parent (suffix)
t Ending with “al”;● Ketone as parent (suffix)
t Ending with “one”● Number the longest carbon chain
containing the carbonyl carbon, starting at the carbonyl carbon
2. Nomenclature of Aldehydes &Ketones
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v ExamplesCl
H
O4-Chloro-2,2-dimethylpentanal
12345
O
Br
12 3 4 5 6
7
6-Bromo-4-ethyl-3-heptanone© 2014 by John Wiley & Sons, Inc. All rights reserved.
2
v group as a prefix: methanoyl or formyl group
O
H
v group as a prefix: ethanoyl or acetyl group (Ac)
O
v groups as a prefix: alkanoyl or acyl groups
O
R
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2-Methanoylbenzoic acid(o-formylbenzoic acid)
CO2H
H
O
4-Ethanoylbenzenesulfonic acid(p-acetylbenzenesulfonic acid)
SO3H
O
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Butanebp -0.5oC(MW = 58)
H
O OOH
Propanalbp 49oC
(MW = 58)
Butanebp 56.1oC(MW = 58)
1-Propanolbp 97.2oC(MW = 60)
3. Physical Properties
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4A. Aldehydes by Oxidation of 1o
Alcohols
4. Synthesis of Aldehydes
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3
OH O
H
PCC
CH2Cl2 (90%)
PCC
CH2Cl2
OH O
H(89%)
v e.g.
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4B. Aldehydes by Ozonolysis ofAlkenes
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O
OH
1. O3, CH2Cl2, -78oC
2. Me2S
+
v e.g.
H3C
1. O3, CH2Cl2, -78oC
2. Me2S
O
H3C
H
+O
H H© 2014 by John Wiley & Sons, Inc. All rights reserved.
4
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4C. Aldehydes by Reduction of AcylChlorides, and Esters
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v LiAlH4 is a very powerful reducing agent and aldehydes are easily reduced● Usually reduced all the way to the
corresponding 1o alcohol● Difficult to stop at the aldehyde
staget Not a good method to
synthesize aldehydes using LiAlH4
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5
v Two aluminum hydride derivatives that are less reactive than LAH:
Lithium tri-tert-butoxyaluminum hydride
AlR
OtBuAlLi+ H OtBuOtBu
-
Diisobutylaluminum hydride(abbreviated i-Bu2AlH or DIBAL-H)
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1. LiAlH(OtBu)3, -78oC
2. H2O
O
R Cl
O
R OR'
R C N
O
R H
1. DIBAL-H, hexane, -78oC
2. H2O
1. DIBAL-H, hexane
2. H2O
Acyl chloride
Ester
Nitrile© 2014 by John Wiley & Sons, Inc. All rights reserved.
© 2014 by John Wiley & Sons, Inc. All rights reserved.
v Aldehydes from acyl chlorides: RCOCl ® RCHO
1.
2.
O
R Cl
O
R OH
O
R H
SOCl2LiAlH(OtBu)3,Et2O, -78oCH2O
v e.g.
1. LiAlH(OtBu)3, Et2O, -78oC
2. H2O
Cl
O
CH3
H
O
CH3© 2014 by John Wiley & Sons, Inc. All rights reserved.
6
v Reduction of an Acyl Chloride to an AldehydeLiAlH(OtBu)3
R CCl
OR C
Cl
O Li+ Al(OtBu)3
H
R CCl
OH
Li
Al(OtBu)3R CCl
OHAl(OtBu)3
Li
R CH
O Al(OtBu)3-LiCl
R CH
OH2O
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v Aldehydes from esters and nitriles: RCO2R’ ® RCHORC≡N ® RCHO● Both esters and nitriles can be
reduced to aldehydes by DIBAL-H
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v Reduction of an ester to an aldehyde
R COR'
O
H
Al(i-Bu)2 R COR'
O Al(i-Bu)2H
R COR'
OHAl(i-Bu)2
R CH
O H2O
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v Reduction of a nitrile to an aldehyde
R C N
H
Al(i-Bu)2 Al(i-Bu)2H
NCR
R CN
H
Al(i-Bu)2R C
H
O H2O
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7
v Examples
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5A. Ketones from Alkenes, Arenes,and 2o Alcohols
v Ketones (and aldehydes) by ozonolysis of alkenes
5. Synthesis of Ketones
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v Examples
1. O3
2. Me2S
O
O
(i)
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v Ketones from arenes by Friedel–Crafts acylations
O
R Cl
AlCl3 R
O
+ HCl+
an alkyl arylketone
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8
v Ketones from secondary alcohols by oxidation
OH
R R'
O
R R'
H2CrO4
or PCC
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5B. Ketones from Nitriles
R C N1. R'-M, Et2O
N M
R'R
2. H3O+
O
R'R
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v Examples
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v Suggest synthesis ofO
from andBr
HO
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9
v Retrosynthetic analysisO
HO
need to add one carbon
5 carbons here 4 carbons here
Þ
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v Retrosynthetic analysis
NC
Br
+
HO
disconnection
disconnection
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v Synthesis
O
HO Br
CN
PBr3
NaCNDMSO
1.
2. H3O+
Et2O
MgBr
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v Suggest synthesis ofO
from andBr
HO
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10
v Retrosynthetic analysisO
HO
no need to add carbon
5 carbons here
Þ
5 carbons here
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v Retrosynthetic analysis
MgBr+
H
O
HO
disconnection
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v Synthesis
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v Structure
● Carbonyl carbon: sp2 hybridized● Trigonal planar structure
Nu⊖
6. Nucleophilic Addition to theCarbon–Oxygen Double Bond
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11
Nucleophilic Addition Reactions of Aldehydes and Ketones
Aldehydes more reactive toward nucleophilic addition than ketones
• Aldehydes less sterically hindered than ketones
v Polarization and resonance structure
CO
d+
OC
d-
● Nucleophiles will attack the electrophilic carbonyl carbon
● Note: nucleophiles usually do not attack a non-polarized C=C bond
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v With a strong nucleophile:
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12
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v Also would expect nucleophilic addition reactions of carbonyl compounds to be catalyzed by an acid (or Lewis acid)
● Note: full positive charge on the carbonyl carbon in one of the resonance formst Nucleophiles readily attack
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13
+ A:C OHR
R'C OH
R
R'
v Mechanism
d+ d-C O
R
R'H A+(or a Lewis acid)
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+ A:C O
RR'
NuH
H
C OHR
R':Nu H
v Mechanism
C O
RR'
:NuH
H A+
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6B. Relative Reactivity: Aldehydes,Ketones, and Esters
O
R H
O
R R'
O
R OR'> >
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large
small
O
R H
O
RNu
H
Nu
O
R R'
O
RNu
R'
Nu
v Steric factors
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OC
R H
OC
R R'
d-
d+
d-
d+> >< <
v Electronic factors
(positive inductive effect from only one R group)
(positive inductive effect from both R & R' groups) Þ carbonyl carbon less d+ (less nucleophilic)
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6C. Addition Products Can Undergo Further Reactions
v Butstable product: isolable
unstable© 2014 by John Wiley & Sons, Inc. All rights reserved.
v Hemiacetal & Acetal Formation: Addition of Alcohols to Aldehydes
R R'
O
R R'
R"O OHH+
R R'
R"O OR"
+ R"OH
H+
R"OH
hemi-acetal (R' = H)hemi-ketal (R' = alkyl)
acetal (R' = H)ketal (R' = alkyl)
Catalyzed by acid
7. The Addition of Alcohols:Hemiacetals and Acetals
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Reactions with Alcohol
Elimination of water prevents O-alkylated intermediate from reverting to reactant
15
v Note: All steps are reversible. In the presence of a large excess of anhydrous alcohol and a catalytic amount of acid, the equilibrium strongly favors the formation of acetal(from aldehyde) or ketal (from ketone).
v On the other hand, in the presence of a large excess of H2O and a catalytic amount of acid, the acetal or ketal will hydrolyze back to aldehyde or ketone. This process is called hydrolysis.
© 2014 by John Wiley & Sons, Inc. All rights reserved.
v Acetals and ketals are stable in neutral or basic solution, but are readily hydrolyzed in aqueous acid
H+OR"R OR"
R'H2O
O
R R'+ + 2 R"OH
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v Aldehyde hydrates: gem-diols
H2O+OH
H3C
H
H3C O
O
H
H
Acetaldehyde Hydrate(a gem-diol)
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16
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d+ d-C O
H
H3C OH2
v Mechanism
OH2H3C
O:H
OHH3C
OHH
OHHO
HR
O
R H+ H2O
distillation
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HO
O
O
O
O
OH
H
Butanal-4-ol
A cyclichemiacetal
Hemiacetal: OH & OR groups bonded to the same carbon
7A. Hemiacetals
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17
7B. Acetals
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+O
R R'HO
OHH3O+
O O
R R'
+ H2O
Ketone (excess) Cyclic acetal
v Cyclic acetal formation is favored when a ketone or an aldehyde is treated with an excess of a 1,2-diol and a trace of acid
© 2014 by John Wiley & Sons, Inc. All rights reserved.
+
O
R R'
HOOH
H3O+O O
R R'
+ H2O
v This reaction, too, can be reversed by treating the acetal with lots of an aqueous acid
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7C. Acetals Are Used as Protecting Groups
v Although acetals are hydrolyzed to aldehydes and ketones in aqueous acid, acetals are stable in basic solutions
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18
v Acetals are used to protect aldehydes and ketones from undesired reactions in basic solutions
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O
OH
Br
O
Attempt to synthesize:
from:
v Example
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O
O
OH
BrMg
O
+
● Synthetic plan
t This route will not work© 2014 by John Wiley & Sons, Inc. All rights reserved.
BrMg
Od+ d-
Reason:(a) Intramolecular nucleophilic addition
(b) Homodimerization or polymerization
BrMg
O
BrMg
O
BrMg
O
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19
t Thus, need to “protect” carbonyl group first
Br
O O
HOOH
, H+
(ketal)
BrMg
O O
MgEt2O d+
d- O
OMgBr
O O
© 2014 by John Wiley & Sons, Inc. All rights reserved.
7D. Thioacetalsv Aldehydes & ketones react with thiols
to form thioacetalsEtS SEt
R H
O
R H
2 EtSH
HA+ H2O
Thioacetal
O
R R' BF3+ H2O
S S
R R'
HSSH
Cyclicthioacetal
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v Thioacetal formation with subsequent “desulfurization” with hydrogen and Raney nickel gives us an additional method for converting carbonyl groups of aldehydes and ketones to –CH2–groups
H2, Raney Ni
+ NiS
S S
R R'HS
SHR R'
H H+
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20
© 2014 by John Wiley & Sons, Inc. All rights reserved.
DesulfurizationofthioacetalusingRaneyNiv Aldehydes & ketones react with 1o
amines to form imines and with 2o
amines to form enaminesFrom a 1o amine From a 2o amine
8. The Addition of Primary andSecondary Amines
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8A. Iminesv Addition of 1o amines to aldehydes &
ketones
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H2NR"
v Mechanism
R R'
O H3O+
R R'
OH O
RR'
H
N R"H
H
-H+
O
RR'
H
NHR"
(amino alcohol)
H+OH2
RR'
NHR"
H2O
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21
v Similar to the formation of acetals and ketals, all the steps in the formation of imines are reversible. Using a large excess of the amine will drive the equilibrium to the imine side
v Hydrolysis of imines is also possible by adding excess water in the presence of catalytic amount of acid
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8B. Oximes and Hydrazonesv Imine formation – reaction with a 1o amine
C O H2N R C N+R
+ H2O
a 1o amine an imine[(E) & (Z) isomers]
aldehydeor ketone
C O H2N OH C N+OH
+ H2O
hydroxylamine
an oxime[(E) & (Z) isomers]
aldehydeor ketone
v Oxime formation – reaction with hydroxylamine
© 2014 by John Wiley & Sons, Inc. All rights reserved.
There are some special amines thatyield insoluble products (imines) that are easy to crystallize …..
CRYSTALLINE IMINES
:NH2OH
R-NH-NH2
hydroxylamine
varioushydrazinecompounds
NHNH2
NO2
O2N2,4-dinitrophenyl-
hydrazine
C NHNH2
ONH2 semicarbazine
..
....
shownbelow
v Hydrazone formation – reaction with hydrazine
C O H2NNH2 C N+NH2
+ H2O
hydrazine a hydrazonealdehydeor ketone
N R C C+N
+ H2O
2o amine
cat. HAOC
CH R
H
RR
enamine
v Enamine formation – reaction with a 2o
amine
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22
8C. The Wolff-Kishner Reduction
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v Mechanism
N
R'R
NH
H
- N2
H
R R'
OH
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8D. Enamines
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N R+
OC
CH R
H
v Mechanism
C CH
ONR
RH
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23
C CH
ON RR
H
A H +
v Mechanism (Cont’d)
C CH
ON RR
HH
iminium ionintermediate
CH
CNR
R:A + H2O +
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CH
CNR
R
A:
v Mechanism (Cont’d)
enamine
CH
CN
R
R
+ H A
© 2014 by John Wiley & Sons, Inc. All rights reserved.
C
R
R
N G
O H
H
..R C C R
H
R
OH
NR2
imine enamine
..
PRIMARY AMINES SECONDARY AMINES
-H2O -H2O no hydrogenon nitrogenhydrogen
on thenitrogen
COMPARISON
hydrogen on theadjacent carbon
When there is no hydrogen onnitrogen, one is lost from carbon.
carbinolamine intermediates
Quiz 1
24
Quiz 2 Quiz 3
Quiz 4 Quiz 5
25
v Addition of HCN to aldehydes & ketones
R R'
O HCN OH
RR'
CN
O
RR'
CNH+CN
(cyanohydrin)
9. The Addition of HydrogenCyanide: Cyanohydrins
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R R'
O CN
v Mechanism
O
RR'
CN(slow)
NC H
OH
RR'
CN
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v Slow reaction using HCN since HCN is a weak acid and a poor source of nucleophile
v Can accelerate reaction by using NaCN or KCN and slow addition of H2SO4
R R'
O
NaCN
O Na
RCN
R'
OH
RR'
CNH2SO4
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R'
O HCNR
R'R
HO CN
R'R
COOH95% H2SO4
heat
HCl, H2O
heat R'R
HO COOH
1. LiAlH4
2. H2O R'R
HO NH2
(a-hydroxy acid)
(a,b-unsaturated acid)
(b-aminoalcohol)
v Synthetic applications
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26
R
R'O
aldehydeor ketone
+ (C6H5)3P CR"
R"C C
R'
R
R"
R"
O P(C6H5)3+
phosphorus ylide(or phosphorane)
alkene[(E) & (Z) isomers]
triphenyl-phosphine
oxide
10. The Addition of Ylides:The Wittig Reaction
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YlideA compound or intermediate with both a positive and a negative charge on adjacent atoms.
X Y..- +
Betaine or Zwitterion
A compound or intermediate with both a positive and a negative charge, not on adjacent atoms, but in differentparts of the molecule. X
-Y
+
:
BOND
MOLECULE
v Phosphorus ylides(C6H5)3P C
R"
R"(C6H5)3P C
R"
R"
(C6H5)3P CR"'
R"H :B + H:B(C6H5)3P C
R"'
R"
a phosphorusylide© 2014 by John Wiley & Sons, Inc. All rights reserved.
C
(C6H5)3P C
R
R(C6H5)3P C
R
R+ _ ..
Resonance in Ylides
..
3d 2p
dp-pp BACKBONDING
Remember that Phosphorousis a Period III element (d orbitals).
Backbonding to phosphorousreduces the formal chargesand stabilizes the negativecharge on carbon.
P
27
v Example
(C6H5)3P CH3(C6H5)3P: Br+
Methyltriphenylphos-phonium bromide
(89%)
CH3BrC6H6
(C6H5)3P CH3
Br
+ C6H5Li (C6H5)3P CH2:
+ + LiBrC6H6
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v Mechanism of the Wittig reaction
+CO
R R'R":C R"'P(C6H5)3: :
aldehydeor ketone
ylide
R'C CR
:O
R"R"'
P(C6H5)3oxaphosphetane
:
C CR
R'
R"
R"O P(C6H5)3 +
alkene(+ diastereomer)
triphenylphosphineoxide
::
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10A. How to Plan a Wittig Synthesis
v Synthesis of
using a Wittig reaction
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v Retrosynthetic analysis
disconnection
O
Ph3P+route 1
BrPh3P: +route 2
PPh3
O+
Br+ :PPh3
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28
v Synthesis – Route 1
O
Ph3PBr:PPh3 Br
nBuLi
Ph3P
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v Synthesis – Route 2
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10B. The Horner–Wadsworth–Emmons Reaction: A Modification of the Wittig Reaction
P OEt
O
OEt
NaH
+ H2
P OEt
O
OEt
a phosphonateester
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29
P OEt
O
OEt
X
OEtP
EtO OEt+
EtX +
Triethyl phosphite
v The phosphonate ester is prepared by reaction of a trialkyl phosphite [(RO)3P] with an appropriate halide (a process called the Arbuzov reaction)
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Arbuzov reaction
11. Oxidation of Aldehydes
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12. The Baeyer-Villiger Oxidation
O
+
O
OOH
O
R OH
(a peroxy-carboxylic acid)
Acetophenone Phenyl acetate
O
O
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30
v Mechanism
O
O ArO
O
R'R
H
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13A. Derivatives of Aldehydes & Ketones
13. Chemical Analyses for Aldehydes and Ketones
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R H
O O
R O-
Ag(NH3)2+
H2O+ Ag
silvermirror
13B. Tollens’ Test (Silver Mirror Test)
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31
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14A. IR Spectra of Aldehydes and Ketones
Range (cm-1)R CHOAr CHO
C CCHO
C CCOR
RCORArCOR
Compound Range (cm-1)Compound
CyclohexanoneCyclopentanoneCyclobutanone
171517511785
1720 - 17401695 - 1715
1680 - 1690
1705 - 17201680 - 1700
1665 - 1680
C=O Stretching Frequencies
14. Spectroscopic Properties of Aldehydes and Ketones
Table 16.3
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32
v Conjugation of the carbonyl group with a double bond or a benzene ring shifts the C=O absorption to lower frequencies by about 40 cm-1
O Osingle bond
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14B. NMR Spectra of Aldehydes andKetones
v 13C NMR spectra● The carbonyl carbon of an aldehyde
or ketone gives characteristic NMR signals in the d 180–220 ppmregion of 13C spectra
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v 1H NMR spectra● An aldehyde proton gives a distinct 1H
NMR signal downfield in the d 9–12 ppmregion where almost no other protons absorb; therefore, it is easily identified
● Protons on the a carbon are deshielded by the carbonyl group, and their signals generally appear in the d 2.0–2.3 ppmregion
● Methyl ketones show a characteristic (3H) singlet near d 2.1 ppm
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33
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O
OHR1. RM
2. H3O+
OHH
1. LiAlH4 or NaBH42. H3O+
OHCN
1. NaCN2. H3O+
RR PPh3
RO OR
2 ROH, H+
NR
R-NH2, H+
R2NHH+
NR2
15. Summary of Aldehyde and Ketone Addition Reactions
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