Chapter18 - Chapter 18 Chapter 18 Reactions at the C-atoms...

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Chapter 18 1 Chapter 18 - Reactions at the C-atoms alpha to the carbonyl groups. There are TWO potential sites for attack in carbonyl compounds. To this point we have been concerned with attack at the carbonyl carbon. A second point of potential reaction is the alpha-hydrogens. R C C O R H H carbonyl addition here "relatively acidic hydrogen" SKIP the following sections: 18.11, 18.16, 18.17, 18.19, 1821, 18.22 Do problems: 48a,c,d,e; 49b, c, e, g, I, j; 50, 52a,b, 53, 57a, b, c; 60, 62b, 64, 68a, b, c, CH 2 COCH 2 CH 3 O H pKa 25 CH 2 C N H CH 2 CCH 3 O H 25 20 CH 2 CH O H pKa 17 CH 3 CHNO 2 H 8.6 PhCCHCCH 3 O O H 9.4 CH 3 O H (15) HC C H (25) H 2 C CCH 3 O CH 2 CCH 3 O B: CH 2 CCH 3 O H Reason for stability β -ketoesters and β -diketones are more acidic than simple ketones (pKa, 9 - 11) . R O O OR H R O O OR H R O O OR H Keto-Enol Tautomerism H 2 C C CH 3 H O H 2 C C CH 3 OH keto form enol form R O O OR H H B: R O O OR H α β
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Chapter 18 2 Tautomerism see p269 Tautomers are constitutional isomers. The equilibrium is also called ENOLIZATION and is catalyzed by acids and bases. H O B O an enolate ion Base Catalyzed H OH OH + OH enol tautomer Acid Catalyzed H O H + H + H OH OH 2 OH + H 3 O + O Take a closer look at the enolate ion: O Two different nucleophilic sites. An AMBIIDENT NUCLEOPHILE. Which position will react with an electrophile (C or O) depends on reaction conditions and the electrophile. 1. Protons react at Oxygen
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This note was uploaded on 04/25/2008 for the course CHEM 343 taught by Professor Spaziano during the Spring '08 term at Saint Louis.

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Chapter18 - Chapter 18 Chapter 18 Reactions at the C-atoms...

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