Download - Organic chemistry for medicine and biology students Organic B Chapter 18 Aldehydes and Ketones
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Organic chemistry for medicine and biology students
Organic B
Chapter 18Aldehydes and Ketones
By Prof. Dr.Adel M. Awadallah
Islamic University of Gaza
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Nomenclature of Aldehydes and KetonesCommon aldehydes
H H
O
CH3 H
O
CH3CH2CH2 H
O
CH3CH2 H
O
H
O
HO HO
OH
HO
OH
OMe
Methanl ethanl propanal butanal(formaldehyde) (acetaldehyde) (propionaldehyde) (n-butyraldehyde)
benzaldehyde salicylaldehyde(2-hydroxybenzenecarbaldehyde) Vanillin
cyclopentanecarbaldehyde
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Common Ketones
CH3 CH3
O
CH3
O
CH3
O
CH3CH3
OOCH3
O
propanone (acetone)
2-butanone(ethyl methyl ketone)
3-pentanone(diethyl ketone)
cyclohexanone acetophenone(methyl phenyl ketone)
benzophenone(diphenyl ketone)
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Nomenclature of aldehydes and ketones
(al) aldehyde, (one) ketonealkanes < alkenes < OH < ketone < aldehyde < acid < esterExamples
OOH
H
OO
CH3
CH3
O
CH3 O
HCH3
Cl O
CH3CH3
12
3451
23
4
123
456
4-hydroxy-2-pentanone 3-oxobutanl 3-hexen-2-one
1
23
41 2
3
45
6
2-cholro-3-methylbutanal 2,4-dimethyl-3-hexanone
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Acyl groups
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Common aldehydes and ketones
CH3OH CH2 = O + H2
Ag
600 - 700 oC
Formaldehyde
Formaldehyde is a gas (b. p. -21 oC) Formalin (37% aqueous solution of formaldehyde)
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Acetaldehyde (Wacker synthesis)
2 CH2 = CH2 + O22 CH3CH = O
Pd - Cu
100 - 130 oC (bp 20 oC)
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O
OH
CH3 CH3
O
Acetone (Wacker synthesis)
2 CH3CH2 = CH2 + O22 CH3 C CH3
Pd - Cu
100 - 130 oC (bp 56 oC)
From isopropylbenzene
1) O2
2) dil H2SO4
+
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Synthesis of aldehydes and Ketones
1) Oxidation of Alcohols
primary gives aldehydes using PCC
secondary gives ketones
2) Friedel-Crafts
3) From Alkynes
OCH3
CH3COCl
AlCl3
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4- Aldehydes from Acid Chlorides
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5 -Ketones from Acid Chlorides
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Synthesis of Alkylbenzenes
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Reduction of Acids and their derivatives by Lithium Aluminum hydride (LAH)
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Naturally occuring aldehydes and Ketones
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The carbonyl group
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Reactions of the carbonyl group
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Remember: Generation of a chiral center
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A. Hydration and Hemiacetal Formation• Water adds rapidly to the carbonyl function of aldehydes and ketones. In most
cases the resulting hydrate (a geminal-diol) is unstable relative to the reactants and cannot be isolated.
• Exceptions to this rule exist, • one being formaldehyde (a gas in its pure monomeric state). • Thus, a solution of formaldehyde in water (formalin) is almost exclusively the
hydrate, or polymers of the hydrate. • Another is chloral hydrate
CCl3 OH
OH
HChloral hydrate
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Addition of Alcohols
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Examples
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Addition of hydrogen cyanide to aldehydes and ketones
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Reducing Agents
The reduction of an aldehyde
You get exactly the same organic product whether you use lithium tetrahydridoaluminate or sodium tetrahydridoborate.
For example, with ethanal you get ethanol:
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The reduction of a ketone
• Again the product is the same whichever of the two reducing agents you use.
• For example, with propanone you get propan-2-ol:• Reduction of a ketone leads to a secondary alcohol.
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REACTION OF ALDEHYDES AND KETONES WITH GRIGNARD REAGENTS
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The reaction between Grignard reagents and methanal
1) EtMgBr
2) H2O, H+
1) EtMgBr
2) H2O, H+
The reaction between Grignard reagents and other aldehydes
The reaction between Grignard reagents and ketones
1) EtMgBr
2) H2O, H+
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Reaction with Acetylides
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OXIDATION OF ALDEHYDES AND KETONES
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6) Cannizzaro reaction. (self oxidation/reduction)
a reaction of aldehydes without α-hydrogens
CHO
Br
conc. NaOH
CH2OH COO-
Br Br
+
CH3OH + HCOO-H2C=Oconc. NaOH
The Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro, is a chemical reaction that involves the base-induced disproportionation of an aldehyde.[1][2] Cannizzaro first accomplished this transformation in 1853, when he obtained benzyl alcohol and potassium benzoate from the treatment of benzaldehyde with potash (potassium carbonate). More typically, the reaction would be conducted with sodium or potassium hydroxide:
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Mechanism of the Cannizzaro Reaction
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Crossed Cannizzaro:
Formaldehyde is the most easily oxidized aldehyde. When mixed with another aldehyde that doesn’t have any alpha-hydrogens and conc. NaOH, all of the formaldehyde is oxidized and all of the other aldehyde is reduced.
CH=O
OCH3
OH
vanillin
+ H2C=Oconc. NaOH
CH2OH
OCH3
OH
+ HCOO-
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ADDITION-ELIMINATION REACTIONS OF ALDEHYDES AND KETONES
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with hydroxylamineThe product is an "oxime" - for example, ethanal oxime.
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Formation of Imines and Related Compounds
The reaction of aldehydes and ketones with ammonia or 1º-amines forms imine derivatives, also known as
Schiff bases, (compounds having a C=N function).
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Wittig Reaction
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Keto-Enol Tautomerism• Keto-enol tautomerism refers to a chemical equilibrium between a
keto form (a ketone or an aldehyde) and an enol. The enol and keto forms are said to be tautomers of each other. The interconversion of the two forms involves the movement of a proton and the shifting of bonding electrons; hence, the isomerism qualifies as tautomerism.
• A compound containing a carbonyl group (C=O) is normally in rapid equilibrium with an enol tautomer, which contains a pair of doubly bonded carbon atoms adjacent to a hydroxyl (−OH) group, C=C-OH. The keto form predominates at equilibrium for most ketones. Nonetheless, the enol form is important for some reactions. Furthermore, the deprotonated intermediate in the interconversion of the two forms, referred to as an enolate anion, is important in
carbonyl chemistry, in large part because it is a strong nucleophile.
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Acidity of α-Hydrogen
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• Examples of -Hydrogen exchange
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The Aldol Condensation
The name aldol is derived from "aldehyde" and "alcohol". An aldol is a β-hydroxycarbonyl compound.
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