Aldehydes & Ketones
The Carbonyl Group:
In this chapter we study the physical and chemical properties of classes of compounds
containing the carbonyl group, C=O
• aldehydes and ketones
• carboxylic acids ,
• Carboxylic acid derivatives: acid halides, acid anhydrides, esters, amides.
The carbonyl group consists of
• one sigma bond formed by the overlap of sp2 hybrid orbitals, and
• one pi bond formed by the overlap of parallel 2p orbitals

C

pi bonding MOs
formaldehyde
O
Structure
The functional group of an aldehyde is a carbonyl group bonded to a H atom and a carbon
atom .The functional group of a ketone is a carbonyl group bonded to two carbon atoms.
O
HCH
Methanal
(Formaldehyde)
O
CH3 CH
Ethanal
(Acetaldehyde)
O
CH3 CCH3
Propanone
(Acetone)
Nomenclature: Aldehydes
IUPAC names:
 the parent chain is the longest chain that contains the functional group
 for an aldehyde, change the suffix from -e to -al
 for an unsaturated aldehyde, show the carbon-carbon double bond by changing the
infix from -an- to -en-; the location of the suffix determines the numbering pattern
 for a cyclic molecule in which -CHO is bonded to the ring, name the compound by
adding the suffix -carbaldehyde
1
O
O
H
H
3-Methylbutanal
1
5
7
8
2-Propenal
(Acrolein)
6
4
CHO
2,2-Dimethylcyclohexanecarbaldehyde
H
2
(2E)-3,7-Dimethyl-2,6-octadienal
(Geranial)
CHO
CH3
2 CH
3
O
1
3
CHO
C6 H5
Benzaldehyde
t rans -3-Phenyl-2-propenal
(Cinnamaldehyde)
Nomenclature: Ketones
IUPAC names:
 select as the parent alkane the longest chain that contains the carbonyl group
 indicate its presence by changing the suffix -e to -one
 number the chain to give C=O the smaller number
O
1
O
O
1
Propanone
(Acetone)
5
3
5
6
5-Methyl-3-hexanone
1-Phenyl-1-pentanone
Order of Precedence
Increasing precedence
For compounds that contain more than one functional group indicated by a suffix
Functional
Group
Suffix If Higher
in Precedence
Prefix If Lower
in Precedence
- COOH
-oic acid
- CH O
-al
oxo-
-one
oxo-
- OH
-ol
hydroxy-
- SH
-thiol
- NH2
-amine
-sulfanyl
-amino
C= O
2
Common Names
•
•
for an aldehyde, the common name is derived from the common name of the
corresponding carboxylic acid
for a ketone, name the two alkyl or aryl groups bonded to the carbonyl carbon and
add the word ketone
O
H
H
Formaldehyde
O
O
O
H
OH
Formic acid
H
Acetaldehyde
OH
Acetic acid
O
O
O
Ethyl is opropyl ketone
Diethyl ketone
Dicyclohexyl ketone
Physical Properties
Oxygen is more electronegative than carbon (3.5 vs 2.5) and, therefore, a C=O group is polar
•
C
O: –
More important
contributing
structure
Polarity of a
carbonyl group
•
+
O:
:
C
O
: :
+ C
aldehydes and ketones are polar compounds and interact in the pure state by dipole-dipol
interaction
they have higher boiling points and are more soluble in water than nonpolar compounds
of comparable molecular weight
Reaction Themes

One of the most common reaction themes of a carbonyl group is addition of a nucleophile
to form a tetrahedral carbonyl addition compound.
:
:O :
R
+
C
R
O:
:
N u: -
Nu
-
C
R
R
Tetrahedral carbonyl
addition compound
3

A second common theme is reaction with a proton or Lewis acid to form a resonancestabilized cation.
R
O: + H- B
+
C O H + :B
:
:
C
R
fast
R
R
protonation in this manner increases the electron deficiency of the carbonyl carbon and
makes it more reactive toward nucleophiles.
I. Addition of C Nucleophiles
Addition of carbon nucleophiles is one of the most important types of nucleophilic additions
to a C=O group; a new carbon-carbon bond is formed in this process.
We study addition of these carbon nucleophiles:
A) Grignard Reagents:
Given the difference in electronegativity between carbon and magnesium (2.5 - 1.3), the CMg bond is polar covalent, with C- and Mg+
in its reactions, a Grignard reagent behaves as a carbanion.
Carbanion: an anion in which carbon has an unshared pair of electrons and bears a negative
charge .a carbanion is a good nucleophile and adds to the carbonyl group of aldehydes and
ketones.
1) Addition of a Grignard reagent to: formaldehyde followed by H3O+ gives a 1° alcohol
-
O
+
CH 3 CH2 -MgBr + H- C-H
ether
+
Formaldehyde
-
O [ Mg Br ]
CH 3 CH2 -CH 2
A magnesium
alkoxide
+
HCl
OH
CH3 CH 2 -CH2 + Mg 2+
H2 O
1-Propanol
(a primary alcohol)
2) Addition to any other RCHO gives a 2° alcohol
4
O
-
O [ Mg Br ]
- +
ether
Mg Br + CH3 - C-H
+
Acetaldehyde
(an aldehyde)
+
CHCH3
A magnes ium
alkoxide
OH
HCl
H2 O
2+
CHCH3 + Mg
1-Cyclohexylethanol
(a secondary alcohol)
3) Addition to a ketone gives a 3° alcohol

O
 
C6 H5 Mg Br + CH3 -C- CH3

Acetone
O - [ MgBr ] +
C6 H5 CCH3
ether
OH
HCl
H2 O
CH3
A magnesium
alkoxide
C6 H5 CCH3 + Mg 2 +
CH3
2-Phenyl-2-propanol
(a tertiary alcohol)
B)Addition of an acetylide anion:
Addition of an acetylide anion followed by H3O+ gives an -acetylenic alcohol.
O
+
HC C: N a +
Cyclohexanol
HC C O - N a+
HC C OH
HCl
H2 O
A s odium
alkoxide
1-Ethynylcyclohexanol
5
Oxidation of terminal alkyne:
C) Addition of HCN:
 HCN adds to the C=O group of an aldehyde or ketone to give a cyanohydrin
 Cyanohydrin: a molecule containing an -OH group and a -CN group bonded to the same
carbon
OH
CH 3 C-C N
H
2-Hydroxypropanenitrile
(Acetaldehyde cyanohydrin)
O
CH3 CH + HC N
Mechanism of cyanohydrin formation:
H3 C
H3 C
••
C O +
-
C N
C
H3 C
H3 C
O:-
H3 C
C N
H3 C
+
C
H3 C
O:-
H C N
C N
O-H
+ - :C N
C
H3 C
C N
The value of cyanohydrins
 acid-catalyzed dehydration of the 2° or 3° alcohol
OH
CH3 CHC N
acid
catalyst
2-Hydroxypropanenitrile
(Acetaldehyde cyanohydrin)
CH2 = CHC N + H2 O
Propenenitrile
(Acrylonitrile)
6
•
catalytic reduction of the cyano group gives a 1° amine
OH
OH
Ni
CHC N + 2 H2
Benzaldehyde cyanohydrin
CHCH2 N H2
2-Amino-1-phenylethanol
II.Addition of S Nucleophiles
Thiols, like alcohols, add to the C=O of aldehydes and ketones to give tetrahedral carbonyl
addition products.The sulfur atom of a thiol is a better nucleophile than the oxygen atom of
an alcohol .A common sulfur nucleophile used for this purpose is 1,3-propanedithiolthe
product is a 1,3-dithiane.
O
RCH
+
An aldehyde
HS
SH
H
+
1,3-Propanedithiol
R
S3
C2
+ H2 O
1
H S
A 1,3-dithiane
(a cyclic thioacetal)
III.Addition of N Nucleophiles

Ammonia, 1° aliphatic amines, and 1° aromatic amines react with the C=O group of
aldehydes and ketones to give imines (Schiff bases).
O
+
Cyclohexanone
N H3
+
Ammonia
O
CH3 CH + H2 N
Acetaldehyde
H
Aniline
N H + H2 O
An imine
(a Schiff bas e)
H
+
CH3 CH =N
+ H2 O
An imine
(a Schiff base)
Formation of an imine occurs in two steps :
Step 1: carbonyl addition followed by proton transfer
7
:
C
O H
O:- H
+
C N -R
O
+ H 2 N- R
C
N -R
H
A tetrahedral carbonyl
addition compound
H
Step 2: loss of H2O and proton transfer to solvent
H
+
O H +
H
:O
H
H
+
H
O
C N -R + H2 O
C N -R
H
C N -R
H
:O
H
An imine
H
A value of imines is that the carbon-nitrogen double bond can be reduced to a carbonnitrogen single bond
O +
Cyclohexanone
H+
- H2 O
H 2N
Cyclohexylamine
H
H2 / N i
N
(An imine)

N
Dicyclohexylamine
Secondary amines react with the C=O group of aldehydes and ketones to form
enamines
O
Cyclohexanone
+
H-N
H
+
Piperidine
(a secondary amine)
N
+ H2 O
An enamine
8
The mechanism of enamine formation involves formation of a tetrahedral carbonyl addition
compound followed by its acid-catalyzed dehydration

The carbonyl group of aldehydes and ketones reacts with hydrazine and its derivatives
in a manner similar to its reactions with 1° amines
O + H2 NNH2
NNH2
Hydrazine
•
+ H2 O
A hydrazone
hydrazine derivatives include
H 2 N-OH
Hydroxylamine
H2 N-NH
Phenylhydrazine
IV. III.Addition of O
ucleophiles
A) Addition of H2O:
Addition of water (hydration) to the carbonyl group of an aldehyde or ketone gives a gemdiol, commonly referred to as a hydrate.
when formaldehyde is dissolved in water at 20°C, the carbonyl group is more than 99%
hydrated
O
OH
HCH + H2 O
HCOH
H
Formaldehyde
hydrate
(>99%)
Formaldehyde
The equilibrium concentration of a hydrated ketone is considerably smaller.
H3 C
H3 C
C
O + H2 O
H3 C
Acetone
(99.9%)
OH
C
H3 C
OH
2,2-Propanediol
(0.1%)
9
b) Addition of Alcohols
Addition of one molecule of alcohol to the C=O group of an aldehyde or ketone gives a
hemiacetal
Hemiacetal: a molecule containing an -OH and an -OR or -OAr bonded to the same carbon
H
O
OH
CH3 CCH3 + OCH2 CH3
CH3 COCH 2 CH 3
CH3
A hemi acetal
Formation of a hemiacetal is base catalyzed

Step 1: proton transfer from HOR gives an alkoxide
B: - + H OR

Step 2: Attack of RO- on the carbonyl carbon
O
CH3 - C-CH3 +

B H + - :OR
–
:O- R
O:–
CH3 - C-CH3
OR
Step 3: proton transfer from the alcohol to O- gives the hemiacetal and generates a
new base catalyst
O:–
CH3 - C-CH3 + H–OR
OR
OH
CH3 - C-CH3 +
–
:O- R
OR
10
Formation of a hemiacetal is also acid catalyzed
 Step 1: proton transfer to the carbonyl oxygen
+ H
O
CH3 - C-CH3 + :A
O:
CH3 - C-CH3 + H- A

Step 2: attack of ROH on the carbonyl carbon
+
O
H
:OH
CH3 - C-CH 3
+
O
H
R
:
CH3 - C-CH 3 + H- O-R

Step 3: proton transfer from the oxonium ion to A- gives the hemiacetal and
generates a new acid catalyst
OH
CH3 - C-CH3
+
A :- H O R
OH
CH3 - C-CH3
: OR
+ H- A
Hemiacetals react with alcohols to form acetals
Acetal: a molecule containing two -OR or -OAr groups bonded to the same carbon
OH
+
H
CH3 COCH 2 CH 3 + CH 3 CH 2 OH
CH 3
OCH 2 CH 3
A hemiacetal
CH 3 COCH 2 CH 3
+
H2 O
CH3
A diethyl acetal
With ethylene glycol, the product is a five-membered cyclic acetal.
O + HOCH 2 CH2 OH
H
+
O
CH2
O CH2
A cyclic acetal
+ H2 O
11
V. -Halogenation:
-Halogenation: aldehydes and ketones with at least one -hydrogen react at  -carbon with
Br2 and Cl2 reaction is catalyzed by both acid and base
O
CCH3 + Br2
CH3 COOH
Acetophenone
O
CCH2 Br + HBr
Haloform Reaction
In the presence of base, a methyl ketone reacts with three equivalents of halogen to give a
1,1,1-trihaloketone, which then reacts with an additional mole of hydroxide ion to form a
carboxylic salt and a trihalomethane
O
RCCH3
O
O
NaOH
+
RCCBr3
RCO Na + CHBr3
3 Br2
3 NaOH
Tribromomethane
(Bromoform)
O
O
1 . Cl 2 / N aOH
2 . HCl/ H2 O
5-Methyl-3-hexen-2-one
OH
4-Methyl-2-pentenoic
acid
+
CHCl 3
Trichloromethane
(Chloroform)
The final stage is divided into two steps
Step 1: addition of OH- to the carbonyl group gives a tetrahedral carbonyl addition
intermediate and is followed by its collapse
: O-
O
RC- CBr 3 + - :OH
O
RC
RC- CBr 3
+
-
:CBr 3
OH Conjugate base
of bromoform
OH
Step 2: proton transfer from the carbonyl group to the haloform anion
O
O
RC- O- H +
-
:CBr 3
RC- O: - + H- CBr 3
Bromoform
12
VI. a)Oxidation of Aldehydes

Aldehydes are oxidized to carboxylic acids by a variety of oxidizing agents, including
H2CrO4
H2 CrO4
CHO
Hexanal


COOH
Hexanoic acid
They are also oxidized by Ag(I)
in one method, a solution of the aldehyde in aqueous ethanol or THF is shaken with a
slurry of silver oxide
O
CH
CH3 O
HO
T HF, H 2 O
+ A g2 O
N aOH
HCl
H2 O
Vanillin

O
COH
CH3 O
+ Ag
HO
Vanillic acid
Aldehydes are oxidized by O2 in a radical chain reaction
liquid aldehydes are so sensitive to air that they must be stored under N2
O
2
O
CH
+ O2
2
Benzaldehyde
COH
Benzoic acid
b)Oxidation of Ketones
•
•
ketones are not normally oxidized by chromic acid
they are oxidized by powerful oxidants at high temperature and high concentrations of
acid or base
O
OH
O
HN O 3 HO
Cyclohexanone
(keto form)
Cyclohexanone
(enol form)
OH
O
Hexanedioic acid
(Adipic acid)
13
VII. 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
Aldehydes
Can Be
Reduced to
Ketones
RCH2 OH
O
Can Be
Reduced to
OH
O
RCH
RCHR'
RCR'
RCH3
RCH2 R'
a)Catalytic Reduction
Catalytic reductions are generally carried out at from 25° to 100°C and 1 to 5 atm H2
OH
O
+
H2
Pt
25 o C, 2 atm
Cyclohexanone
Cyclohexanol
A carbon-carbon double bond may also be reduced under these conditions
• by careful choice of experimental conditions, it is often possible to selectively reduce a
carbon-carbon double in the presence of an aldehyde or ketone.
O
2 H2
H
trans- 2-Butenal
(Crotonaldehyde)
Ni
OH
1-Butanol
b) Metal Hydride Reduction
The most common laboratory reagents for the reduction of aldehydes and ketones are NaBH4
and LiAlH4 .Both reagents are sources of hydride ion, H:-, a very powerful nucleophile
H
Na
+
H- B- H
H
Sodium
borohydride
H
Li
+
H- A l- H
H
Lithium aluminum
hydride (LAH)
H:
Hydride ion
14
•
•
reductions with NaBH4 are most commonly carried out in aqueous methanol, in pure
methanol, or in ethanol
one mole of NaBH4 reduces four moles of aldehyde or ketone
O
methanol
4 RCH + NaBH4
-
+
( RCH2 O) 4 B Na
A tetraalkyl borate
H2 O
4 RCH2 OH +
borate
salts
The key step in metal hydride reduction is transfer of a hydride ion to the C=O group to form
a tetrahedral carbonyl addition compound
H
O BH3 N a
O
+
N a H- B- H + R- C-R'
+
R- C-R'
H
H2 O
H
OH
from the hydride
reducing agent
from
water
R- C-R'
H
LiAlH4 Reduction
• unlike NaBH4, LiAlH4 reacts violently with water, methanol, and other protic solvents
• reductions using it are carried out in diethyl ether or tetrahydrofuran (THF)
O
ether
4RCR + LiAlH4
(R2CHO)4Al- Li+
H2O
OH
4RCHR + aluminum salts
A tetraalkyl aluminate
Metal Hydride Reduction
• metal hydride reducing agents do not normally reduce carbon-carbon double bonds, and
selective reduction of C=O or C=C is often possible
O
RCH= CHCR'
1 . Na BH 4
2 . H2 O
O
RCH= CHCR'
+
H2
Rh
OH
RCH= CHCH R'
O
RCH2 CH 2 CR'
15
Clemmensen Reduction
• refluxing an aldehyde or ketone with amalgamated zinc in concentrated HCl converts the
carbonyl group to a methylene group
OH O
OH
Zn( H g) , HCl
Wolff-Kishner Reduction
• in the original procedure, the aldehyde or ketone and hydrazine are refluxed with KOH in
a high-boiling solvent
• the same reaction can be brought about using hydrazine and potassium tert-butoxide in
DMSO
O
+ H2 NN H2
Hydrazine
KOH
diethylene glycol
(reflux)
+ N 2 + H2 O
16