Week 3 - Organic Chemistry II UC-Berkeley Extension x36B...

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Organic Chemistry II UC-Berkeley Extension x36B Fall 2011 September 19, 2011 acetals as protecting groups, imines/Schiff bases, acidity of ! -hydrogens, keto-enol- tautomerism, racemization, ! halogenation, oxidations, hydride reductions, catalytic reduction. Carboxylic Acids: nomenclature, physical properties, pKa, reactions with bases, synthesis from Grignard reagents, reduction, NADH, Fischer esterification, acid chlorides, decarboxylation.
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Week 2 Acetals as Protecting Groups ! Suppose you wish to bring about a Grignard reaction between these compounds. 2
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Week 2 Acetals as Protecting Groups ! The Grignard reagent prepared from 4-bromobutanal will self-destruct! To avoid this, ! First protect the -CHO group as an acetal. ! Then do the Grignard reaction. ! Hydrolysis (not shown) gives the target molecule. 3
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Week 2 Addition of Nitrogen Nucleophiles ! Ammonia, 1° aliphatic amines, and 1° aromatic amines react with the C=O group of aldehydes and ketones to give imines ( Schiff bases ). 4
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Week 2 Addition of Nitrogen Nucleophiles ! Formation of an imine occurs in two steps: Step 1: Carbonyl addition followed by proton transfer. Step 2: Loss of H 2 O and proton transfer to solvent. 5
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Week 2 Addition of Nitrogen Nucleophiles ! A value of imines is that the carbon-nitrogen double bond can be reduced to a carbon-nitrogen single bond. 6
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Week 2 Addition of Nitrogen Nucleophiles ! Secondary amines react with the C=O group of aldehydes and ketones to form enamines. ! The mechanism of enamine formation involves formation of a tetrahedral carbonyl addition compound followed by its acid- catalyzed dehydration. 7
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Week 2 Acidity of ! -Hydrogens ! Hydrogens alpha to a carbonyl group are more acidic than hydrogens of alkanes, alkenes, and alkynes but less acidic than the hydroxyl hydrogen of alcohols. 8
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Week 2 Acidity of ! -Hydrogens ! ! -Hydrogens are more acidic because the enolate anion is stabilized by: 1. Delocalization of its negative charge. 2. The electron-withdrawing inductive effect of the adjacent electronegative oxygen. 9
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Week 2 Keto-Enol Tautomerism ! Protonation of the enolate anion on oxygen gives the enol form; protonation on carbon gives the keto form. 10
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Week 2 Keto-Enol Tautomerism ! Acid-catalyzed equilibration of keto and enol tautomers occurs in two steps. Step 1: Proton transfer to the carbonyl oxygen. Step 2: Proton transfer to the base A - . 11
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Week 2 Keto-Enol Tautomerism ! Keto-enol equilibria for simple aldehydes and ketones lie far toward the keto form. 12
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Keto-Enol Tautomerism ! For certain types of molecules, however, the enol is the major form present at equilibrium: ! For " -diketones, the enol is stabilized by conjugation of the pi system of the carbon-carbon double bond and the carbonyl group. ! For acyclic
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Week 3 - Organic Chemistry II UC-Berkeley Extension x36B...

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