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ALDEHYDES AND KETONES
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CARBONYL COMPOUNDS ALDEHYDES KETONES
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EXAMPLES Formaldehyde Acetaldehyde Acetone
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MOLECULAR MODELS Formaldehyde Acetaldehyde Acetone
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INDUSTRIAL PRODUCTION
Catalytic dehydrogenation (oxidation) of alcohols
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Common names of simple carbonyl compounds
Formula Common name Systematic name HCHO CH3CHO CH3CH2CHO CH3CH2CH2CHO CH3CH2CH2CH2CHO H2C=CHCHO PhCHO CH3COCH3 CH3COCH2CH3 CH3CH2COCH2CH3 Formaldehyde Acetaldehyde Propionaldehyde Butyraldehyde Valeraldehyde Acrolein Benzaldehyde Acetone Methyl ethyl ketone Ethyl ketone Methanal Ethanal Propanal Butanal Pentanal 2-Propenal Benzenecarbaldehyde Propan-2-one Butan-2-one Pentan-3-one
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Physical properties of aldehydes and ketones
More polar than alkanes, higher melting and boiling points No hydrogen bonds formation – lower boiling points than alcohols Solubility in water – only formaldehyde, acetaldehyde and acetone
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Physical properties of carbonyl compounds
M.p (°C) B.p (°C) Formaldehyde Acetaldehyde Propanal Butanal Pentanal Benzaldehyde Acetone 2-Butanone 2-Pentanone 3-Pentanone Cyclohexanone -92 -121 -81 -99 -26 -95 -86 -78 -40 -16 -21 21 49 76 103 178 56 80 102 156
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Preparation of aldehydes
Oxidation of primary alcohols Aldehyde which boils at lower temperature than alcohol is distilled off the reaction mixture immediately after formation
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Preparation of aldehydes
Ozonolysis of di- or trisubstituted alkenes
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Preparation of aldehydes
Reduction of carboxylic acids esters DIBAH – diisobutylaluminum hydride (aldehyde is not reduced further to primary alcohol)
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Preparation of aldehydes
Reduction of carboxylic acids chlorides 8 October 2018 Tri-tert-butoxylithiumaluminum hydride
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Preparation of aldehydes
Oxidation of methylarenes
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Preparation of ketones
Oxidation of secondary alcohols 90% yield
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Preparation of ketones
Ozonolysis of alkenes
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Preparation of ketones
Friedel-Crafts acylation of arenes (electrophilic aromatic substitution) 95% yield
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Preparation of ketones
Hydration of alkynes (terminal or symmetric) 78% yield
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Preparation of ketones
Reaction of acid chloride and diorganocopper reagent 81% yield
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Oxidation of aldehydes and ketones
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Oxidation of aldehydes and ketones
Tollens oxidation Reaction used as laboratory test to distinguish aldehyde and ketone
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Reaction limited to symmetric cyclic ketones
Oxidation of ketones Reaction limited to symmetric cyclic ketones
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Nucleophilic addition reactions of aldehydes and ketones
Alcohol Alcohol Cyanohydrin Alkene Imine Alkane Acetal Enamine
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Aldehydes are more reactive than ketones Nu
Formaldehyde Acetaldehyde Acetone Steric factor Access of nuclephile to carbonyl carbon is less hindered in aldehyde (hydrogen is smaller than any alkyl substituent)
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Aldehydes are more reactive than ketones
Electronic factor Positive charge on carbon is stronger stabilized by inductive effect of two alkyl groups Ketones are more stable – less reactive
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Nucleophilic addition of H2O
(hydration) A gem-diol A gem-diol
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Base-catalyzed addition of H2O
Hydroxide anion is more reactive nucleophile than neutral water
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Acid-catalyzed addition of H2O
Protonated carbonyl is more electrophilic and more reactive
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Nucleophilic addition of HCN
(cyanohydrins) In practice HCN is generated during reaction by adding acid (like H2SO4) to a mixture of carbonyl compound and NaCN (or KCN). Cyanide anion is nucleophile
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Reactions of cyanohydrins
Cyanohydrin formation from ketone or aldehyde provides compounds with new functional groups while lenghtening the carbon chain by one unit
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Nucleophilic addition of Grignard reagents
(alcohol formation) New alcohol with larger hydrocarbon framework is obtained
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Nucleophilic addition of hydride
(reduction) Alcohol with the same hydrocarbon framework as starting ketone or aldehyde is formed
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Nucleophilic addition of amines to carbonyl group
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Water elimination from carbinolamine
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Crystalline imines
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Crystalline imines m. p. 126°C
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Nucleophilic addition of hydrazine (Wolff-Kishner reaction)
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Nucleophilic addition of alcohols (acetal formation)
Protonated carbonyl group is strongly electrophilic and highly reactive towards nucleophiles
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Nucleophilic addition of alcohols
(acetal formation)
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Mechanism of acetal formation
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Acetal as carbonyl protective group
How to reduce ester carbonyl without reducing ketone carbonyl?
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Acetal as carbonyl protective group
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Nucleophilic addition of thiols (thioacetal formation)
Conversion of carbonyl to thioacetal and subsequent desulfurization is a method for reducing C=O to CH2
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Nucleophilic addition of phosphorus ylides (The Wittig reaction)
New molecule containing C=C bond instead of carbonyl group is synthesized
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Conjugate nucleophilic addition to ,-unsaturated carbonyl
,-unsaturated carbonyl compounds possess 2 electrophilic carbons Conjugate addition product
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Conjugate nucleophilic addition to ,-unsaturated carbonyl
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Some biological nucleophilic additions Synthesis of -amino acid
from -ketoacid in living cells
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Biological reaction reverse to nucleophilic addition
Millipede Apheloria corrugata and its predator – an ant
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The Cannizzaro reaction
The only example when hydride ion is expelled from aldehyde as leaving group (like in nucleophilic acyl substitution) Nucleophilic acyl substitution
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The Cannizzaro reaction (disproportionation) -carbon react this way
Only aldehydes without protons at -carbon react this way in the presence of base
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The Cannizzaro reaction as model for biological reductions
NADPH functions as hydride donor in biological reductions
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The Cannizzaro reaction as model for biological reductions
NADPH reduction of carbonyl to hydroxyl – one of the key steps during fatty acid biosynthesis
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-Substitution reactions of aldehydes or ketones
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Keto-enol tautomerism Concentration of enol form at equilibrium
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Enol formation is catalyzed by base or acid
Acid-catalyzed enol formation
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Enol formation is catalyzed by base or acid
Base-catalyzed enol formation
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Only the protons on the position are acidic
Not acidic Not acidic acidic Not acidic
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Mechanism of -substitution in aldehydes or ketones
Net effect – substitution of -hydrogen by group E
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-Substitution in aldehydes or ketones
Examples: -halogenation
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-Substitution in aldehydes or ketones
-Halogenation of enolate ions (iodoform reaction) Bromine and chlorine react in the same way
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Two modes of enolate ion reactivity
An enol derivative An -substituted carbonyl compound More commonly followed path
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Condensation of aldehydes and ketones
(aldol condensation) Nucleophilic donor Mechanism Electrophilic acceptor Aldol - -hydroxyaldehyde
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Condensation of aldehydes
Examples:
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Condensation of ketones
Examples:
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Mixed aldol reactions (useless)
Symmetrical products Mixed products
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Mixed aldol reactions (useful)
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Brief summary Aldol self-condensation cannot occur for aldehydes or ketones without -hydrogens Aldehydes containing -hydrogens are more reactive than ketones Aldol equilibrium is favorable for aldehydes without branching at -carbon Aldol equilibrium is not favorable for -branched aldehydes Mixed aldol reaction between two different carbonyl partners with -hydrogens leads to a mixture of products (useless as synthetic method) Mixed aldol reaction is very efficient when one partner is an unusally good nucleophilic donor or is a good electrophilic acceptor 29 October 2018
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1,3-Dicarbonyl compounds are excellent nucleophilic donors
Enolate ion stabilized by three resonance structures
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Acidity constants for some organic compounds
Compound type Formula pKa Carboxylic acid 1,3-Diketone 1,3-Ketoester 1,3-Dinitrile 1,3-Diester WATER Primary alcohol Acid chloride Aldehyde Ketone Ester Nitrile Dialkylamide AMMONIA Dialkylamine CH3COOH CH2(COCH3)2 CH3COCH2CO2C2H5 CH2(CN)2 CH2(CO2C2H5)2 H2O CH3CH2OH CH3COCl CH3CHO CH3COCH3 CH3CO2C2H5 CH3CN CH3CON(CH3)2 NH3 HN(i-C3H7)2 5 9 11 13 16 17 19 25 30 35 40
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Dehydration of aldol products
NOT FORMED
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Dehydration of aldol products
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Recognizing aldol products (Retrosynthetic analysis)
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Intramolecular aldol reactions
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Intramolecular aldol reactions
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