9_Aldehyde_Ketone

9_Aldehyde_Ketone - Chem 267 Basic Organic Chemistry II...

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Prof. Dr. Xiangdong Fang University of Waterloo Chem 267 Basic Organic Chemistry II 1
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Both aldehyde and ketone contain carbonyl group “carbonyl group = CO double bond” 2
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Reactivity in Aldehydes and Ketones 1) oxygen of carbonyl group serves as a Lewis base (electron-rich; attacked by electrophiles) 2) carbon of carbonyl group is attacked by nucleophiles (Lewis acid; electron deficient) 3) hydrogen at α -carbon is acidic and can isomerize or be easily removed with a base; C=O chemistry α -C chemistry (similar to benzylic) 3
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C=O chemistry is mainly about ionic addition “implies that the addition reagent (X-Y) with a polar bond adds to C=O following Coulomb’s Law and fundamental principles of Lewis acid-base interactions” H O LiAlH 4 ,E t 2 O then H 2 O OH The hydride reagents, e.g. NaBH 4 and LiAlH 4 , only add to polar C=O bond 90% H O H O H O major minor very minor polar character only restricts to C=O group 4
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5 Addition of strong nucleophile, e.g. Nuc: , to C=O group is irreversible
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6 Addition of weak nucleophile such as Nuc-H is under equilibrium (reversible) 1) nucleophilic addition-protonation: if the reaction is run in basic condition 2) electrophilic protonation-addition: if the reaction is run in acidic condition
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7 Water adds to C=O to give germinal diols (hydrates, usually not isolable) “since water is a weak nucleophile, the addition is reversible and is catalyzed by either acid or base”
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8 Electronic Factor versus Steric Factor C=O hydration is reversible and is greatly influenced by C=O substituents: 1) electron-withdrawing group makes carbonyl carbon more carbocationic (favours hydration); electron-donating group makes carbonyl carbon less carbocationic (disfavours hydration); 2) bulkier group makes carbonyl carbon less accessible (disfavours hydration); smaller group makes carbonyl carbon more accessible (favours hydration);
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This note was uploaded on 07/07/2010 for the course CHEM 267 taught by Professor Fang during the Winter '09 term at Waterloo.

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9_Aldehyde_Ketone - Chem 267 Basic Organic Chemistry II...

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