Download Handout 7

Handout # 7
Winter 2015/2016
(N. Noureldin)
Aldehydes and Ketones
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Aldehydes and Ketones (Alkanal & Alkanone)
3.1 Carbonyl group: structure, reactivity and molecular orbital model,
polarity and polarization
 
C O
Carbonyl group
C
O
p-Orbital Picture
Resonance Picture
Typical methods of the synthesis of Aldehydes and Ketones (Review)
Oxidation of alcohols
Secondary alcohols can be oxidized to ketones
OR KMnO4
Primary alcohols can be oxidized to aldehydes (absence of water!)
Ozonolysis of alkenes gives aldehydes or ketones
Hydration of terminal alkynes gives methyl ketones.
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The preparation of alcohols from aldehydes or ketones with the same number and
arrangement of carbon atoms.
O
C
R
O
OH
H
Na BH4
Na BH4
O
CH3CCH2CH2CH3
C
LiAlH4
LiAlH4
C
H
H
O
CH3CH2CH2CH2CH
R
OH
R
OH
R
\
R
C
R
\
H
CH3CH2CH2CH2CH
OH
H
CH3CCH2CH2CH3
H
3.2 General array of reactivities
Generally speaking, aldehydes are less stable than ketones hence, aldehydes are more
reactive. Among others, the steric factors contribute to the greater reactivity of an aldehyde
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3.3 Nomenclature and trivial names of aldehydes and ketones
O
Aldehydes:
O
RCH i.e. RCHO
; Ketones:
Nomenclature:
1)
Give #1 to
Formula
C
IUPAC
CH3CHO
Methanal
Ethanal
CH3CH2CHO
Propanal
HCHO
ClCH2CH2CHO
give
Formula
IUPAC
O
O
H
Common
Formaldehyde
Acetaldehyde
Propanaldehyde
C
the lowest #
Common
Acetone
Methyl ethyl ketone
O
(2
CH3C CH2 C CH3
O
Propanone
Butanone
Alkanedione
CH3CO CO CH2CH3
R\ i.e. RCOR\
3-Chloropropanal
2) Ketones (alkanone)
CH3CH2COCH3
C
O
Aldehydes (alkanal)
CH3COCH3
R
C
groups)
2, 3-Pentanedione
2, 4-Pentanedione
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O
O
CH3
1, 4-Cyclohexanedione
CH2CH2CH3
CH3CHCHCH2COCH2CHCH3
CH3
CH3COCH2CH CH2
2, 3, 7-Trimethyl-5-decanone
(Alkenone,CO lowest #) 4-Penten-2-one
O
CH3CH2CCH2CH3
3-Pentanone
CH3CH2CH2CCH3
2-Pentanone
O
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3.4 Physichochemical properties
 
C O
Carbonyl group
They are strongly polar compounds hence, their B.P. are higher than
corresponding alkanes and ethers. However, due to the absence of hydrogen
bonding, B.P. of aldehydes and ketones are lower than corresponding alcohols
and carboxylic acid.
3.5 Enolisation (Tautemerization)
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3.6 Some typical reactions of aldehydes and ketones:
Nucleophilic Additions Reactions

O
O-
B-
C
nucleophile
C
A+
B
Intermediate
 
(A B)
OA
C
B
Product
3.6.1
3.6.2
Hemiacetals and Hemiketals
Under anhydrous conditions, dissolving an aldehyde in an alcohol causes the
establishment of an equilibrium between the two and a new compound called
“hemiacetal”.
The essential structure features of a hemiacetal are an –OH and –OR
groups attached to the same carbon atom (and since this carbon
atom came from an aldehyde, this carbon also has one hydrogen atom
attached to it).
very high
concentration
Most open-chain hemiacetals are not sufficiently stable to isolate. However,
cyclic ones with 5 or 6 membered rings are usually much more stable.
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Most simple sugars exist primarily in a cyclic hemiacetal form. Glucose is an
example.
Hemiketals
Ketones undergo similar reactions when are dissolved in an alcohol producing also
unstable products in open-chain compounds “hemiketals”
Conversion of Aldehydes / Ketones to Acetals / Ketals through
Hemiacetals / Hemiketals intermediates
It should be emphasized that acid catalysis is required to make
an acetal OR a ketal.
Why? A: the mechanism
Mechanism of hemiketal formation from ketone is similar to the formation of
hemiacetal from an aldehyde.
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Mechanism
1) Formation of Hemiacetals/Hemiketals
All steps are reversible
2) Hemiacetals /Hemiketals to Acetals/Ketals
As expected, excess ROH and /or removal of water shifts equilibrium to
product. On the other hand addition of water converts the product back
into the carbonyl group.
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Mechanism
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3.6.3 Acetals as protecting groups
It should be noted that Ketal formation is not favoured when ketones are treated
with simple alcohol and gaseous HCl. Fortunately, cyclic ketal formation is favoured
when a ketone is treated with an excess of an anhydrous1,2 diol and trace of
anhydrous acid.
In conclusion, all steps included in the conversion of an aldehyde or ketone to acetal
or ketal via hemiacetal or hemiketal as intermediates, are reversible. Performing
the reaction in large excess of an anhydrous alcohol and a small amount of an
anhydrous acid will strongly favour the formation of acetals or ketal. On the other
hand, in the presence of large amounts of water and small amount of an acid,
acetals or ketal could be converted back to the carbonyl compounds.
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Acetals / Cyclic Ketals are inert to basic and reducing reagents.
Because of this property, acetals and cyclic ketals give us a convenient method for
protecting aldehyde and ketone groups from undesired reactions in basic solutions.
We can convert an aldehyde or ketone to acetal or cyclic ketal, carry out a reaction
on some other part of the molecule, and then hydrolyze the acetal or ketal with
aqueous acid. As an example, let us consider the problem of converting
Keto groups are more easily reduced than ester groups. Any reducing agent such as
LiAlH4 or H2/Ni that will reduce the ester of A will reduce the keto group as well.
However, if we protect the keto group by converting it to a cyclic ketal (the ester
group does not react), we can reduce the ester group in basic solution without
affecting the cyclic ketal. Then hydrolysis the ketal yields the desired product as
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follows:
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3.6.4
Addition of Derivatives of Ammonia
General Equation:
NH3  ammonia; NH2G  derivative of ammonia
C
O + H2N
G
H+
-H 2O
C
G
N
(solid products used for identification purposes)
Some reactions of ammonia (primary amine)derivatives with carbonyl
compounds.
H+
C O + H2NOH
C NOH
-H 2O
Hydroxyl amine
an oxime
+
H
C O + H2N NH2
C NNH2
-H 2O
hydrazine
hydrazone
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Mechanism- Imines formation; compounds with C=N.
Mechanism
Step 1: Addition
Step 2: Acid-catalyzed E1 dehydration
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3.6.5
3.6.6 Beckmann Rearrangement: conversion of Oximes to Amides
Beckmann rearrangement is an acid-induced rearrangement of oximes
to give amides.
In this reaction an electropositive nitrogen is formed that initiates an
alkyl migration (rearrangement).
Oximes generally have a high barrier to inversion, and accordingly
this reaction is envisioned to proceed by protonation of the oxime
hydroxyl, followed by migration of the alkyl substituent.
Mechanism
*
H
OH
N
+
H
N
O
+
H
*
H2O
Alkyl migration
- H+
Recall the addition of water to Alkynes : Tautomerism
R C C R
[H+/H2O]
R
H
O
C
C R
H
H
RCH
O
C R\