310-15 - Organic Lecture Series CH 310 N LECTURE 15...

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Organic Lecture Series 1 CH 310 N LECTURE 15 Textbook Assignment: Chap 16 Finish Homework (for credit): POW 5 posted (Due Feb 28) Today’s Topics: Acidity of α -hydrogens & reductions Notice & Announcements: Exam 1 This Week Check BB for room # Bring #2 pencils Organic Lecture Series 2 Aldehydes Aldehydes And And Ketones Ketones (Chapter 16) (Chapter 16)
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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). O H H H H α α Organic Lecture Series 4 HA + H 2 O H 3 O + A - [H 3 O + ] [A - ] [HA] [H 2 O] K eq = 3 O + - ] [HA] 2 O] K = 3 O + - ] K a = Freshman Flashback!! Note: l and s are not used in K
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Organic Lecture Series 5 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 Organic Lecture Series 6 α -Hydrogens are more acidic because the enolate anion is stabilized by : 1. delocalization of its negative charge (resonance effect) 2. the electron-withdrawing inductive effect of the adjacent electronegative oxygen 3 -C-CH 2 - H O :A - O 3 -C CH 2 O - 3 -C=CH 2 H-A Resonance-stabilized enolate anion + +
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Organic Lecture Series 7 Keto Keto -Enol Enol Tautomerism 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 CH 3 -C-CH 2 3 -C=CH 2 3 2 H-A 3 3 + A - A - + OH O - O *Enol: made from 2 functional groups-alkene and alcohol Organic Lecture Series 8 Keto Keto -Enol Enol Tautomerism 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 - O 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 - 3 -C=CH 2 En ol f o rm + + slow +
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Organic Lecture Series 9 Keto Keto -Enol Enol Tautomerism Tautomerism Keto Keto -enol enol equilibria equilibria for simple aldehydes and ketones lie far toward
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This note was uploaded on 04/24/2011 for the course CH 56255 taught by Professor Colapret during the Spring '11 term at University of Texas.

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310-15 - Organic Lecture Series CH 310 N LECTURE 15...

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