ALDEHYDES AND KETONES

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ALDEHYDES AND KETONES
Aldehydes and Ketones
H
R
R
C O
Aldehyde
C O
R
Ketone
bond - two overlapping 2p orbitals
H
H
C
O
bond
bond- overlapping 1s
H-orbital and sp2 C-orbital
lone Pairs
H d+ d
118o C O
H3C 121o
H
118o
H
C C
H 121o H
Resonance Structures
H
H
Most Reactive Group –
C O
C O
electrons + polarisation
H3C
H3C
Names
al – aldehydes, one - ketones
H
H
C O
C O
H
Methanal
(formaldehyde)
H
H3C
H
C O
CH3CH2
Propanal
Ethanal
(acetaldehyde)
C O
CH3CH2CH2CH2
Pentanal
Nomenclature of Aldehydes
If the aldehyde group is attached to a ring,
Nomenclature of Ketones
Oxidation
An aldehyde has a greater partial positive charge
on its carbonyl carbon than does a ketone
• Steric factors contribute to the reactivity of an aldehyde
• The carbonyl carbon of an aldehyde is more accessible
to the nucleophile
• Ketones have greater steric crowding in their transition
states, so they have less stable transition states
Reactions
1. Redox reactions
RCH
O
H
RC
O
H2
red.
KMnO4
or CrO3
O
R C
OH
RCH2OH
R
C O
R'
H2
R
H
C
R'
OH
2. Addition: AN
Addition of water
H
OH
H
C O + H2O
R
Aldehyde hydrate
C
R
(ketone:
OH
Addition of bisulfite
R
C
R'
SO3-Na+
R
O + NaHSO3
Bisulfite
C
R'
OH
)
Addition of NH3
H
NH2
H
C O + NH3
R
C
Aldehyde ammonia
(ketone:
R
OH
R
C N
Addition of HCN
R
C
R'
Cyanohydrine
C
O + HCN
R'
OH
)
3. Condensation reactions (addition followed by elimination)
Ammonia derivatives
R
R
H
C
N
O+
Y
N
Y
C
N
R"
R'
H
R'
C
Primary amine
R
C
R'
R
H
N
O+
H
R"
R'
Schiff base
Hydroxylamine
R
R
H
C
N
O+
C
OH
OH
Oxime
R'
H
R'
N
Hydrazine
R
C
R'
H
H
N
O+
H
R
C
N
H
N
NH2
R'
Hydrazone
Phenylhydrazine
R
H
C O+
H
R'
H
N
N
Phenylhydrazone
2,4-dinitrophenylhydrazine
H
R
H
C O+
H
R'
N
N
NO2
NO2
2,4-dinitrophenylhydrazone
Aldol condensation
CH2 C H + CH2 C H
R
O
R'
O
Aldol
OH-
CH2
CH CH C H
R
OH R'
O
Reaction with alcohol
O
R C H + HO R'
OH
R
C
O R'
Hemiacetal
(addition!)
O R'
Acetal
(condensation!)
H
O R
R C
H
H + HO R'
R
C
H
O R'
Keto–Enol Tautomerism
Oxidation of a Primary Alcohol
Reduction by Addition of
a Hydride Ion and a Proton
Aldehydes and ketones react with nucleophiles to form
addition products: nucleophile addition reactions
Water adds to an aldehyde or ketone to form a hydrate
Hydration of Aldehydes and Ketones
R
C
O ••
••
R'
HOH
R
••
HO
••
C
R'
••
O
••
H
Alcohols Under Analogous Reaction
with Aldehydes and Ketones
R
C
O ••
••
R'
R"OH
R
••
R"O
••
C
R'
••
O
••
H

Product is called
a hemiacetal.
Example
O
CH + 2CH3CH2OH
HCl
CH(OCH2CH3)2
+ H2O
Benzaldehyde diethyl acetal (66%)
Reaction with Primary Amines:
Imines
If the nucleophile that adds to the aldehyde or ketone is
an O or an N, a nucleophilic addition–elimination reaction
will occur
Imine (Schiff's Base) Formation
H2N ••
+
C
O ••
••
••
HN
C
••
O
••
R
R
a carbinolamine
••
N
R
C
(imine)
+ H2O
H
Example
O
CH + CH3NH2
CH=NCH3 + H2O
N-Benzylidenemethylamine (70%)
Reaction with Derivatives of Ammonia
H2N
G + R2C
H2N
OH
hydroxylamine
O
R2C
NG + H2O
R2C
NOH
oxime
Reaction with Derivatives of Ammonia
H2N
G + R2C
H2N
OH
O
R2C
NG + H2O
R2C
NOH
hydroxylamine
oxime
H2N
R2C
NH2
hydrazine
etc.
NNH2
hydrazone
Example
O
C
+ H2NNH2
NNH2
C
(73%)
+ H2 O
Example
O
CCH3
+ H2NNH
phenylhydrazine
NNH
CCH3
a phenylhydrazone (87-91%)
+ H2 O
An aldol addition product loses water to form an aldol
condensation product
Aldol Condensation
O
O
NaOH
2RCH2CH
RCH2CH
OH
CHCH
R
Aldol Addition of Acetaldehyde
O
2CH3CH
O
NaOH, H2O
5°C
CH3CH
CH2CH
OH
Acetaldol
(50%)
Hemiacetal reacts further in acid
to yield an acetal
R
••
R"O
••
C
••

OR
••
Product is called
an acetal.
R'
ROH, H+
R
••
R"O
••
C
R'
••
O
••
H
Product is called
a hemiacetal.
Formaldehyde
O
HCH

formaldehyde cannot form an enolate

formaldehyde is extremely reactive
toward nucleophilic addition
Acrolein
O
H2C
CHCH
Acrolein
O
H2C
CHCH
Acrolein
O
H2C
CHCH
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