318-13 - CH 318 N LECTURE 13 Textbook Assignment: Chapter...

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CH 318 N LECTURE 13 Textbook Assignment: Chapter 17 (Begin) Homework (for credit): POW 7 posted Today’s Topics: Acidity of α -Hydrogens; Oxidations & Reductions; Carboxylic Acids Notice & Announcements: Grades posted to BB (Pick up exams in undergrad office after this class) Organic Lecture Series 2 Aldehydes Aldehydes And And Ketones Ketones
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Organic Lecture Series 3 Acidity of Acidity of α -Hydrogens Hydrogens Hydrogens alpha to a carbonyl group are more acidic than hydrogens of other hydrocarbons (e.g. alkanes, alkenes, aromatic). The acidity is measured as K a O H H H H α α Organic Lecture Series 4 Acidity of Acidity of α -Hydrogens 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 CH 3 2 O- H O 3 CCH 2 - H 2 =CH- H 3 2 - H 3 CC - H Type of Bond pK a 16 20 25 44 51 pKa = -log Ka
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Organic Lecture Series 5 α -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 CH 3 -C-CH 2 - H O :A - O 3 -C CH 2 O - 3 -C=CH 2 H-A Resonance-stabilized enolate anion + + Organic Lecture Series 6 Keto Keto -Enol Tautomerism Enol Tautomerism – protonation of the enolate anion on oxygen gives the enol form* ; protonation on carbon gives the keto form Enolate anion Enol form Keto form - O 3 -C-CH 2 3 -C=CH 2 3 2 3 3 + A - A - + OH O - O *Enol: made from 2 functional groups-alkene and alcohol
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Organic Lecture Series 7 Keto Keto -Enol Tautomerism Enol Tautomerism acid acid -catalyzed 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 - O CH 3 -C-CH 3 H-A O 3 -C-CH 3 H A - + + + Keto form The conjugate acid of the ketone fast and reversible O 3 -C-CH 2 -H H :A - OH 3 -C=CH 2 H-A En ol f o rm + + slow + Organic Lecture Series 8 Keto Keto -Enol Tautomerism Enol Tautomerism Keto Keto -enol enol equilibria equilibria for simple aldehydes and ketones lie far toward the keto form O O 3 2 =CH 3 CCH 3 Keto form Enol form % Enol at Equilibrium 6 x 10 -5 3 C=CH 2 6 x 10 -7 O O 1 x 10 -6 4 x 10 -5
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Organic Lecture Series 9 Keto Keto -Enol Tautomerism Enol Tautomerism For certain types of molecules, however, the enol is the major form present at equilibrium –fo r β -diketones, the enol is stabilized by conjugation of the pi system of the carbon-carbon double bond and the carbonyl group – for acyclic β -diketones, the enol is further stabilized by hydrogen bonding (i.e.1,3 diones) R C OH O R C OR' O R C H O R C R' R C O H Carboxylic Acids & Esters Ketones Aldehydes Alcohols R C H H Hydrocarbon (lowest oxidation) Oxidation Reduction
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Organic Lecture Series 11 Oxidation of Aldehydes Oxidation of Aldehydes • Aldehydes are oxidized to carboxylic acids by a variety of oxidizing agents, including *H 2 CrO 4 • They are also oxidized by Ag(I) – in one method, a solution of the aldehyde in aqueous ethanol or THF is shaken with a slurry of silver oxide CHO H
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318-13 - CH 318 N LECTURE 13 Textbook Assignment: Chapter...

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