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Published byMarjorie Leonard Modified over 9 years ago
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REACTIONS OF -HYDROGENS : REACTIONS OF -HYDROGENS : ALDOL AND CLAISEN ALDOL AND CLAISEN CONDENSATION REACTIONS CONDENSATION REACTIONS
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Nu: :B nucleophilic addition.. - removal of -H TYPES OF REACTIVITY FOR ALDEHYDES AND KETONES Good nucleophiles add. Strong bases remove -hydrogens. Often, both processes compete.
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ALDOL CONDENSATION
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The Aldol Condensation base an aldol ( -hydroxyaldehyde) ald + ol H3O+H3O+ - H 2 O , -unsaturated aldehyde aldols easily lose water to form a double bond
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Aldol Condensation -- Mechanism fast slow enolate ion forms new C-C bond
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The Bond Forming Step nucleophile (donor) carbonyl (acceptor) enolate
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Ketones Also Give Aldol Condensations.. NaOH “aldol” -H 2 O -
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“CROSSED” ALDOL CONDENSATIONS CONDENSATIONS
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Crossed Aldol Condensations KETONE + ALDEHYDE.. a “chalcone” - H 2 O NaOH Works best to if an aldehyde is the “acceptor”, since they are more reactive; and works really well if the aldehyde has no -H. The ketone should have the -H. aldehyde ketone -
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IMPORTANT GUIDELINES Aldehyde carbonyl groups are more reactive toward nucleophilic addition than ketone carbonyl groups. ++ -- ++ -- +I Nu: MORE REACTIVE Alkyl groups deactivate the carbonyl ( + I ) to addition. 1.
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H-C-HH-C-HCH 3 - C - HCH 3 - C - CH 3 OOO RELATIVE REACTIVITY OF C=O GROUPS Density - LUMO plots ( color scale 0.000 to 0.030 ) MORE REACTIVE LESS REACTIVE THE EFFECT OF ALKYL SUBSTITUTION
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Ketones form enolate ions more easily than aldehydes. -- ::::.. More substituents on the double bond more stable Which enolate will form fastest?.. :: :: : : - - - - disubstituted monosubstituted 2. aldehyde enolateketone enolate
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In “mixed” reactions the ketone enolate usually adds to the aldehyde. ALDEHYDE + KETONE ? The ketone forms the lower energy enolate (forms faster) and it adds to the aldehyde (more reactive C=O).
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WHAT ABOUT WHAT ABOUT TWO DIFFERENT KETONES ?
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HOW MANY PRODUCTS WITH THIS ONE ? x2 8 POSSIBLE PRODUCTS ! two different self dimers four mixed products A B abcd aB, bB, cA, dA aA, bAcB, dB ….. which enolate do you think will form preferentially?
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FORMATION OF RINGS
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Formation of Rings NaOH : - 11 22 Why don’t 2 hydrogens react ?
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KOH EtOH TETRAPHENYLCYCLOPENTADIENONE
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1) O 3 2) H 3 O + KOH Aldol H 2 SO 4 - H 2 O OH - An Interesting Sequence
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CLAISEN CONDENSATIONS
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The Claisen Ester Condensation a -ketoester CH 3 CH 2 OH Notice that the base, the solvent and the leaving group CH 3 CH 2 O - Na +, CH 3 CH 2 OH, CH 3 CH 2 O - all match (this is required in most cases).
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Claisen Ester Condensation Mechanism
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Dieckmann Condensation.. A CYCLIC CLAISEN CONDENSATION NaOMe MeOH
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PATTERNS 3-hydroxyaldehyde or 3-hydroxyketone (H) -hydroxy to C=O , -unsaturated C=O 2-propen-1-al or 2-propen-1-one ALDOL CLAISEN -keto ester Type of Condensation Reaction -H 2 O (with loss of H 2 O)
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SYNTHESIS
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Synthesis of a Perfumery Compound Why don’t the other sets of -H react? cis-Jasmone Scent of Jasmine in perfumes. Aldol Condensation Dehydration cis 11 22 33 44
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Biological Synthesis of Fructose
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Synthesis of an Insect Repellent
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