Download ALDEHYDES AND KETONES:

Page 1 of 18
ALDEHYDES AND KETONES:
ch.19\p.503
O
Carbonyl group is a functional group
C
O
O
CH3C
C
H
Acetaldehyde
(An aliphatic)
H
Benzaldehyde
(An aromatic aldehyde)
The ald. Group is often written RCHO for convenience
O
CH3C CH2CH3
O
O
C
C
CH3
2-Butanone
(An aliphatic ketone)
Acetophenone
Benzophenone
(An aryl alkyl ketone) (An aromatic ketone)
-Naming: Common name, or IUPAC names:
Common:
CHO
CH3CH2CHO
Propionaldehyde
CH3CH2CH2CHO
Benzaldehyde
Butyraldehyde
1
Page 2 of 18
-The position of substituents in the aldehyde can be
indicated in the common name by Greek letters starting
with ∝ at the position adjacent to the carbonyl group:
δ 𝛾 𝛽 ∝
O
C C C C C
𝛾 𝛽 ∝
∝
Br CH2CH2CH2CHO
CH3CHCHO
H
-For example:
cl
∝-Chloropropionaldehyde
Ȣ-Bromobutyraldehyde
IUPAC names: replacing -e with -al.
CH3
CH3‫ــ‬CH‫ــ‬CHO
CH3CHO
2
Ethanol
CH3
1
clCH2CH‫ــ‬CH2CHO
4
3
2
1
2-methylpropanal 4-chloro-3-methyl butane
(Acetaldehyde)
(Isobutyraldehyde)
H.W: Ex.19.1, 19.2, 19.3 Ex. 19.4, 19.5 P.506
The common name of Ketones: (the name are written as
separate words) for example:
2
Page 3 of 18
O
O
O
CH3CH2C CH3
CH 2 C CH3
CH3CH2C CH2CH3
Ethyl methyl ketone
Diethyl ketone
O
Benzyl methyl ketone
O
CH3C CH3
O
C
C
CH3
Acetone
Acetophenone
Benzophenone
-The IUPAC names of ketones: -e replaced with –one
O
CH3CH2CH2CCH3
5
4
3
CH3
O
C CHCCH3
2 1 5 CH3 4
2-pentanone
O
3
21
cyclohexane
4-methyl-3-penten-2-one
H.W: Ex. 19.6, 19.7, 19.8, 19.9
P.508
Physical properties: Read Table 19-1 P.509. Most simple
aldehydes and Ketones are polar molecule strong
intermolecular attractions in these compounds account
for their boiling points, which intermediate between HC
& alcohol (similar M.Wt).
3
Page 4 of 18
Preparing Aldehydes and Ketones:
1° alcohols oxid. Aldehydes further oxid. Carboxylic acids
2° alcohols oxid. Ketones (method of preparing ketones)
OH
O
CH3‫ـــ‬CH‫ـــ‬CH2CH3
2-Butanol
𝐾2Cr 2O7
𝐻+,𝐻2𝑂
OH
CH3‫ـــ‬C‫ــ‬CH2CH3
O
Butanone
𝐾𝑀𝑛𝑂 4
𝐻+,𝐻2𝑂
Cyclohexanol
cyclohexanone
Preparing aldehydes: oxidation of 1° alcohols as a
method of preparing aldehydes, it’s necessary to stop the
reaction at the intermediate aldehyde stage.
This can be done by removing the aldehyde from the
reaction as soon as it’s formed quite easily by distillation,
because aldehydes have lower boiling points than that of
the starting alcohol. By carrying out the oxidation at a
temperature slightly above the boiling point of the
aldehyde, it can be removed by distillation as its forms
and before it can be further oxidized.
4
Page 5 of 18
CH3CH2CH2OH
𝐶𝑟𝑂3,𝐻+
65℃,𝐻2𝑂
1-propanol (b.p., 97℃ )
CH3CH2CHO
propanaldehyde (b.p., 49℃)
The fact that aldehydes are easily oxidized to carboxylic
acids is used as a method of testing for the presence of
an aldehyde.
H.W.: Ex. 19-10 P.510
Tests for Aldehyde: Most of the reactions of
aldehydes and ketones are very similar, but their ability
to be oxidized differs greatly. Ketones are oxidized only
under the most vigorous conditions, whereas aldehydes
are easily oxidized, (ald. Are slowly oxidized by air).
The ease of oxidation is the basis of several tests for
aldehyde.
1-Tollen’s test: (an alkaline solution of di (ammine)
silver (I) ion, Ag (NH3)2+ , to aldehyde.
2Ag (NH3)2++ RCHO+3OHSilver ion
RCO2-+2Ag +4NH3+2H2O
silver metal
The silver metal coats the glass surface and forms a silver
mirror, this reaction called silver mirror test for
aldehydes.
5
Page 6 of 18
2-Fehling’s solution: (an alkaline solution of cupric ion
(Cu+2) complexed with tartrate ion).
Aldehyde oxidized
carboxylate ion
Cupric ion reduced
cupric oxide (Cu2O) ppt. as brick-red solid
Because alcohols, alkenes, ketones, and carbocyclic acids
and their derivatives are not oxidized by either the
Tollen’s or the Fehling’s reagent, then tests are highly
specific for aldehyde.
ADDITION REACTION OF ALDEHYDES & KETONES:
:‫تفاعالت االضافة‬
X ‫ ــ‬C ‫ ــ‬O ‫ــ‬Y
C=O + X‫ــ‬Y
Cδ+=Oδ-
(the C=O is made up of two atoms of different electronegativities.
(Diff. in polarity)).
As a result of polarity, the carbon of carbonyl group
forms bonds with the more nucleophilic part of the
reagent X‫ـــ‬Y.
𝛿+
𝛿-
C
X
C ‫ ــــ‬X
O
Y
O ‫ــــ‬Y
𝛿-
𝛿+
6
Page 7 of 18
Addition of water: Most aldehydes react with water
to form an equilibrium mixture of the aldehyde and an
aldehyde hydrate: - R H
R H
C
OH
C ‫ ــــ‬OH
O
H
O ‫ ــــ‬H
Aldehyde
aldehyde hydrate1, 1-diols
They are usually too unstable to isolate and purify
because the easily lose water to reform the aldehyde.
In certain special cases, the 1, 1-diol is sufficiently stable
that it can be isolated. The carbonyl carbon is bonded to
one or more highly electronegative group.
Ex.:
OH
OH
CF3 C CF3
Ccl3 CH
OH Hexafluoroacetone hydrate
OH chloral hydrate
Addition of alcohols: ald. +alcohols
CH3
H
CH3
C
O
O
H
Acetaldehyde
methanol
CH3
H+
𝑎𝑐𝑖𝑑
hemiacetal
H
C ‫ ـــ‬OCH3
O ‫ـــ‬H
Hemiacetal
7
Page 8 of 18
In the presence of excess alcohol, a Hemiacetal can react to
form an Acetal and water.
OCH3
OCH3
CH3CH + CH3OH
H+
CH3 ‫ ـــ‬CH + H2O
OH methanol
OCH 3
Hemiacetal
Acetal
2steps summarized:H+
CH3CHO + 2CH3OH
CH3CH (OCH3)2 + H2O
Have 2 alkoxyl gr.
In addition to forming Hemiacetals between 2 different
molecules, such a reaction can occur within a molecule.
6 5 4
3 2
1 O
CH3CHCH2CH2CH2C
OH
H
3CH3
H+
4CH2
6CH3
2CH2
5C
H
1CHOH
O
Carbohydrate: are compounds that form cyclic Hemiacetal.
CH3
CH3 OCH3
C O + CH3OH
CH3
Acetone
H+
C + CH3OH
CH3 OH
methanol
Hemikketal
CH3 OCH3
H+
C
CH3
+ H2O
OCH3
Ketal
8
Page 9 of 18
If adiol is used as the alcohol component, the acetal or
the ketal will have a cyclic structure, for ex.:
C O+
CH2 H+
𝐶
H+ C
𝐶𝐻2
CH2
+H2O
H.W: Ex. (19.11, 19.12, 19.13) P.514
Addition of ammonia and its derivatives :( Schiff bases)
C
NH2
O
H
𝐶 ‫𝐻𝑁 ــــ‬2
-H2O
C
NH
Ketone or aldehyde Ammonia
RHC==NR`
Aldimines
R2C==NR`
Ketimines
An imine
Schiff bases
H.W:19.14 P516
Reduction of aldehydes and ketones:
Reduction
RCHO
Aldehyde
O
RCH2OH
oxidation
RCR
OH
R CH R
primary alcohol ketone oxidation 2°alcohol
9
Page 10 of 18
CH3
𝛿+
CH3
𝑁𝑎𝐵𝐻 4
C==O
CH3
Al ‫ــــ‬H
CHOH
𝑜𝑟 𝐿𝑖𝐴𝑙𝐻 4
CH3
Acetone
𝛿-
polarization of AL‫ــ‬H bond
2-propanol
O
C
+ H2
Pt.
CH2OH
Pt.: metal catalyst
H
ADDITION REACTIONS OF ALDEHYDES AND KETONES IN
LIVING SYSTEM:
OO=C=O+H2O
𝐸𝑛𝑧𝑦𝑚𝑒
(Carbon dioxide)
O=C +H+
OH (bicarbonate ion)
The enzyme that catalyzes this reaction is widely
distributed in mammals. (Active in tissues that are
involved in respiration), such as red blood cells.
The formation of Schiff bases is an important reaction in
the formation of many compounds in living systems. One
example is the transamination reaction.
10
Page 11 of 18
CH3
CO2H
CHNH2 + O=C
CO2H
CH3
𝐸𝑛𝑧𝑦𝑚𝑒
CH2
CO2H
C=O
+NH2CH
CO2H
CH2
CH2CO2H
Alanine (A.A)
CH2CO2H
𝛼-Ketoglutaric acid pyruvic acid
Glutamic acid
(An 𝛼-Keto acid)
In such reactions, the 𝛼-amino group of an A.A is
transferred to the 𝛼-carbon of an 𝛼-Keto acid.
The first step in this transformation is the reaction of the
amino group of the amino acid (A.A) with pyridoxal
phosphate, the reactive part of the enzyme, to form a
Schiff base.
O
O-‫ـــ‬P‫ ـــ‬O-
O
O-‫ــ‬P‫ــ‬O-
CHO
OCH3
CH3
CO2H O‫ــ‬CH2 CH=NCHCO2H
OH+NH2CH
N
CH3
CH3
Pyridoxal phosphate
Alanine
OH +H2O
N
CH3
Schiff base
This Schiff base is the amino group carrier between the
amino acid and the Keto acid.
11
Page 12 of 18
The enzyme- catalyzed reduction of aldehydes and
ketones occurs frequently in biological reactions. In these
reactions, the same enzyme frequently catalyzes the
oxidation of alcohols to aldehydes and ketones.
The oxidation of L-malate is an example of such an
oxidation-reduction reaction:
H
H
-
O2C C‫ــ‬OH+
CH2CO2-
L-malate
O
C
N+
O CO2-
H H
C +C=O
𝑂𝑥𝑖𝑑 .𝑜𝑓 𝑚𝑎𝑙𝑎𝑡𝑒
𝑟𝑒𝑎𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑜𝑥𝑎𝑙𝑜𝑎𝑐𝑒𝑙𝑎𝑡𝑒
NH2
N
NH2 CH2
R
R
CO2-
NAD
NADH
Oxidation is very stereospecific (only L-malate is oxidized).
CONDENSATION REACTIONS
In condensation reactions, two compounds are joined
together (or condensed) to form one larger compound.
1) aldol condensation. 2) claisen condensation.
Aldehyde and alcohol (the name derived from the structure of product)
12
Page 13 of 18
1) An aldol condensation reaction: - is a reaction in which
the carbonyl carbon of one molecule forms a bond with
the 𝛼 carbon of another carbonyl-containing molecule.
O
H
O
C
CH2 C
O‫ــ‬H O
C‫ــ‬CH2 C R
R
Carbonyl carbon
Ex.
O
H
CH3C
O‫ــ‬H
OH-
H
Acetaldehyde
𝛼 carbon
O
CH2C
H
Aldol condensation product
O
CH3C‫ـــ‬CH2C
H
H
Aldol
The reaction is reversible. The reverse reaction is called a
retro-aldol condensation. Most aldehydes and many
ketones that have a hydrogen undergo the aldol
condensation. All the products of an aldol condensation
reaction have a common structural feature,
A 𝛽-hydroxy carbonyl skeleton:
OH
O
C CC
This structure is the result of the way the two pieces join together. The
𝛼 carbon of one aldehyde molecule becomes bonded to what was the
13
Page 14 of 18
original carbonyl carbon of the second aldehyde molecule. For
example:-
O
CH3CH2 C
H
O H
C HCHO
H
CH3CH2 C C H CHO
CH3
H CH3
The aldol condensation is an equilibrium. Formation of
product is favorable for ald. than ketone.
O
OH O
2 CH3 C CH3
CH3
C CH2 C CH3
CH3
The following reaction is an example of mixed aldol
condensation reaction:O
C
OH
O
C CH 2CCH3
H +CH3 C CH3 OHBenzaldehyde Acetone
O
H
mixed aldol
Condensation product
Mixed aldol condensation reactions are successful only when the 𝛼 carbon of a
ketone adds to the carbonyl group of an aldehyde that does not have 𝛼
hydrogens.
14
Page 15 of 18
Claisen condensation:Esters also undergo
condensation reactions when treated with base. For
example:
O
O
CH3C + CH3C
O
NaOCH2CH3
CH3C
OCH2CH3 OCH2CH3
O + CH3CH2OH
CH2C
OCH2CH3
Notice that in this reaction the ‫ـــ‬OCH2CH3 group of one ester molecule
is replaced by the 𝛼 carbon of another ester molecule (the overall
result of a claisen condensation is the transfer of an acyl group from
one ester molecule to another).
H.W: ex. 19.17 P 526
Condensation reactions in living systems:
Carbohydrates are prepared in living systems by an enzyme-catalyzed
aldol condensation.
O
HC
C
H
HO
HO H
CHCCH2O P
C
OH
CH2
OP
HO CH2O P
D-Glyceraldehyde
3-phosphate
1, 3- Dihydroxyacetone
phosphate
H
C
OH
CHOH
C=O
CH2O P
D-fructose 1, 6 diphosphate
15
Page 16 of 18
Show stereo specificity of enzyme.
O
‫ـــ‬P‫ـــ‬O-
P
OThis condensation reaction again shows the
stereospecificity of enzyme-catalyzed reactions.
The acetyl group of acetyl coenzyme A also undergoes
claisen condensations. An example is the formation of
acetoacetyl coenzyme A by an enzyme-catalyzed reaction
of acetyl coenzyme A:
O
CH3C
O
O
CH2C ENZYME CH3C
O + H‫ــ‬SCOA
S H
SCOA
CH2C
COA
Acetyl
Coenzyme A
SCOA
Acetyl
coenzyme A
Acetoacetyl
coenzyme A
coenzyme A
Acidity of 𝜶 hydrogens: - saturated hydrocarbons are
generally unreactive compounds. C – H bond is not acidic
in these compounds, Ka = 10-40 for methane (CH4). Thus,
there is little tendency for methane to ionize to form a
proton at the ion –CH3 in which carbon bears –ve charge.
Such anions are called carbanions.
16
Page 17 of 18
Carbanions: - a compound that contains a carbon atom
bearing a negative charge and bonded to three groups of
atoms. However, aldehydes, ketones, & esters do react
with strong bases to form carbanions. The hydrogen 𝛼 to
the carbonyl group is more acidic than most C – H bonds.
The reason for this is that the enolate anion formed by
loss of a 𝛼 hydrogen of an aldehyde or ketone is
stabilized by resonance.
The presence of ‫ ـــ‬C ‫ ـــ‬group makes the C – H bond 𝛼
to it much more acidic than usual. The reason for this is
that the carbanion can be stabilized by resonance: for
example:
O
CH3 C CH3 + OH-
𝑂
−CH2 C CH3
𝑂−
CH2 = C CH3
Resonance-stabilized enolate anion
This enolate anion is a resonance hybrid of 2 structures.
One structure has the negative charge on carbon,
whereas the other has the negative charge on oxygen.
[Molecules or ions that can be written as a resonance
hybrid of 2 or more structures are stabilized]. This
stabilization of the enolate anion is the reason why the H
𝛼 to a carbonyl gr. is lost to a strong base.
17
Page 18 of 18
H.W.: Ex.19.19 p 528
In condensation reactions, the nucleophile is the enolate
anion formed by a strong base removing the proton 𝛼 to
the carbonyl group. The carbonyl group serves two
functions in condensation reactions. It serves as the site
where a nucleophile adds, and it makes its 𝛼 hydrogens
more acidic than usual for a C – H bond.
Many of the reactions of aldehyde and ketones in the
laboratory are similar to their reactions in living system.
18