Chemical Properties of Aldehydes and Ketones
Oxidation of Aldehydes
Aldehydes are very easily oxidized to carboxylic acids.
R C O HH
HR C O H
OR C
OH
primary alcohol aldehyde carboxylic acid
[O][O]
From the standpoint of the organic component, an oxidation can involve the removal of hydrogen atoms OR the addition of oxygen to the structure.
Oxidation reagents: MnO4-, Cr2O72-, Ag+, Cu2+ Usually, the metal ion is “reduced” in the process.
Chemical Properties of Aldehydes and Ketones
Oxidation of AldehydesOxidation of Aldehydes forms the basis of two useful identification tests
Tollen’s Test:
Benedict’s Test:
RC
H
O+ Ag+ + 3 OH-
RC
O-
O+ Ag0 + 2 H2O
RC
H
O+ 2 Cu2+ + 5 OH-
RC
O-
O+ + 3 H2OCu2O
Oxidation of Aldehydes
+ ++ +++
The degree of reduction varies with time and the concentration of the aldehyde.
Chemical Properties of Aldehydes and Ketones
Addition Reactions to Carbon-Oxygen Double Bonds
REVIEW:Reactions of Carbon-Carbon Double Bonds
“A” and “B” can be the same or different atoms or groups of atoms.
In some cases, the identity of A and B and the groups attached to the double bond influence the orientation of the addition.
C
C
A
B
C
C
A
B C
C
A
B
Chemical Properties of Aldehydes and Ketones
Addition Reactions to Carbon-Oxygen Double Bonds
Reactions of the Carbon-Oxygen double bond can be viewed in a similar manner.
“A” and “B” can be the same or different atoms or groups of atoms.
The identity of A and B and the polar nature of the carbon-oxygen double bond influence the orientation of the addition.
O
C
δ−
δ+
A
B δ−
δ+O
CA
B
O
C
A
B
δ−
δ++
δ−
δ+
Addition Reactions to Carbon-Oxygen Double Bonds
Reduction (Hydrogenation) of Aldehydes and Ketones
O
C
H
H O
C
H
H
O
C
H
H
Bonds Break
Bonds Form
Aldehyde or Ketone Alcohol
Aldehyde or Ketone Alcohol
REMEMBER:
OXIDATION (DEHYDROGENATION)
Addition Reactions to Carbon-Oxygen Double Bonds
Reduction (Hydrogenation) of Aldehydes and Ketones
2,3-dimethylbutanal 2,3-dimethyl-1-butanol
H2 Ni (catalyst)
H3CCH
CHC
O
H
H3C
CH3H3C
CHCH
CH2
OHH3C
CH3
C6H12O C6H14O
H3CCH
CH2C
O
CH3
F
H3CCH
CH2 HC
OH
CH3
F
H2 Ni (catalyst)C5H9OF C5H11OF
4-fluoro-2-pentanone 4-fluoro-2-pentanol
Addition Reactions to Carbon-Oxygen Double Bonds
Hydrates, Hemiacetals, Hemiketals, Acetals, and Ketals
O
C
H
O X
O
C
δ−
δ+ δ−
δ+ H
O X
X = H, R
If X=H, H-O-X is water;
If X=R, H-O-R is an alcohol.
aldehyde hydrate ketone hydrate
hemiacetal hemiacetal (hemiketal)
Addition Reactions to Carbon-Oxygen Double Bonds
Hydrates, Hemiacetals and Hemiketals
O
C
H
O H R H
O
C
H
O H R Rʼ
O
C
H
O Rʼ R H
O
C
H
O Rʼʼ R Rʼ
O
C?
O
?
O
C? ?
O
CCH3 H
H
O H + O
C
H
O H
HCH3
O
CCH3 CH3
H
O H + O
C
H
O H
CH3 CH3
acetaldehyde acetaldehyde hydrate
acetone acetone hydrate
Addition Reactions to Carbon-Oxygen Double Bonds
Addition of Water - Hydrate Formation
Addition Reactions to Carbon-Oxygen Double Bonds
Addition of an Alcohol - Hemiacetal Formation
O
CCH3CH2 H
H
O CH3 + O
C
H
O CH3
H
O
CCH3
H
O CH3 + O
C
H
O CH3
CH3
propionaldehyde propionaldehyde methyl hemiacetal
butanone butanone methyl hemiacetal
CH3CH2
CH3CH2
CH3CH2
Addition of an Alcohol - Hemiacetal Formation
C O
C
C C
C O
H
HCH2HO
HO
OH
OHH
H HH
CCCCCCH2
OHHO
OHO H
HO
HHH
OH
H
D-Glucose
Addition of an Alcohol - Hemiacetal Formation
CCCCCCH2
OHHO
OHO H
HO
HHH
OH
HC
H O
O H
123456
123
456
Further Reactions of Hemiacetals
Reaction with an Alcohol - Acetal Formation
O
C
H
O CH3
HCH3CH2
O
C
CH3
O CH3
HCH3CH2
H O CH3
propionaldehyde propionaldehyde methyl hemiacetal methyl acetal
++ H2O?
O
C
H
O CH3
CH3CH3CH2
O
CO CH3
CH3CH3CH2
CH3H O CH3
butanone butanone methyl hemiacetal methyl acetal
+ ? + H2O
Note: Acetal formation from a hemiacetal is another example of an
INTERMOLECULAR DEHYDRATION or CONDENSATION reaction.
O
C
H
O Rʼ R H
O
C
H
O Rʼʼ R Rʼ
O
C
Rʼʼ
O Rʼ R H
O
C
Rʼʼʼ
O Rʼʼ R Rʼ
Further Reactions of Hemiacetals
Reaction of a Hemiacetal with an Alcohol - Acetal Formation
O RʼʼH O RʼʼʼH
H2O H2O
CH3 CH2 CO
H
CH3 CH2 COH
OCH3
H CH3 CH2 COCH3
OCH3
H
CH3OH, H+ CH3OH, H+
H2O
CH3OH
H2O, H+
H+
CH3OH
Aldehyde Hemiacetal Acetal
Most hemiacetals are unstable and cannot be isolated.
The equilibrium forming a hemiacetal lies far to the left, or to the reactant side of the reaction.
Hemiacetals and Acetals
The equilibrium forming an acetal from a hemiacetal lies far to the right, if excess alcohol is present.
Hemiacetals and Acetals
CH3 CH2 CO
H
CH3 CH2 COH
OCH3
H CH3 CH2 COCH3
OCH3
H
Aldehyde Hemiacetal Acetal
Addition Condensation
Elimination Hydrolysis
In the presence of excess alcohol and acid, an aldehyde or ketone is converted into its corresponding acetal.
In the presence of excess water and acid an acetal is converted into its parent aldehyde or ketone.
Hemiacetal and acetal formations are reversible reactions:
CH3OH, H+ CH3OH, H+
H2O
CH3OH
H2O, H+
H+
CH3OH