Chapter22 - Chapter 22. Carbonyl Alpha-Substitution...

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Unformatted text preview: Chapter 22. Carbonyl Alpha-Substitution Reactions Based on McMurrys Organic Chemistry , 6 th edition 2003 Ronald Kluger Department of Chemistry University of Toronto Based on McMurry, Organic Chemistry, Chapter 22, 6th 2 The Position The carbon next to the carbonyl group is designated as being in the position Electrophilic substitution occurs at this position through either an enol or enolate ion Based on McMurry, Organic Chemistry, Chapter 22, 6th 3 22.1 KetoEnol Tautomerism A carbonyl compound with a hydrogen atom on its a carbon rapidly equilibrates with its corresponding enol Compounds that differ only by the position of a moveable proton are called tautomers Based on McMurry, Organic Chemistry, Chapter 22, 6th 4 Tautomers Are Not Resonance Forms Tautomers are structural isomers Resonance forms are representations of contributors to a single structure Tautomers interconvert rapidly while ordinary isomers do not Based on McMurry, Organic Chemistry, Chapter 22, 6th 5 Enols The enol tautomer is usually present to a very small extent and cannot be isolated However, since it is formed rapidly, it can serve as a reaction intermediate Based on McMurry, Organic Chemistry, Chapter 22, 6th 6 Acid Catalysis of Enolization Brnsted acids catalyze keto-enol tautomerization by protonating the carbonyl and activating the protons Based on McMurry, Organic Chemistry, Chapter 22, 6th 7 Base Catalysis of Enolization Brnsted bases catalyze keto-enol tautomerization The hydrogens on the carbon are weakly acidic and transfer to water is slow In the reverse direction there is also a barrier to the addition of the proton from water to enolate carbon Based on McMurry, Organic Chemistry, Chapter 22, 6th 8 Acid Catalyzed Enolization The addition of a proton to the carbonyl oxygen makes the C-H more acidic, reducing the barrier to the enol The enol then can react with another electrophile Based on McMurry, Organic Chemistry, Chapter 22, 6th 9 22.2 Reactivity of Enols: The Mechanism of Alpha-Substitution Reactions Enols behave as nucleophiles and react with electrophiles because the double bonds are electron- rich compared to alkenes Based on McMurry, Organic Chemistry, Chapter 22, 6th 10 General Mechanism of Addition to Enols When an enol reacts with an electrophile the intermediate cation immediately loses the OH proton to give a substituted carbonyl compound Based on McMurry, Organic Chemistry, Chapter 22, 6th...
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Chapter22 - Chapter 22. Carbonyl Alpha-Substitution...

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