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

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1 Chapter 22. Carbonyl Alpha-Substitution Reactions Based on McMurry’s Organic Chemistry , 6 th edition ©2003 Ronald Kluger Department of Chemistry University of Toronto Based on McMurry, Organic Chemistry, Chapter 22, 6th edition, (c) 2003 2 The α Position ± The carbon next to the carbonyl group is designated as being in the α position ± Electrophilic substitution occurs at this position through either an enol or enolate ion Based on McMurry, Organic Chemistry, Chapter 22, 6th edition, (c) 2003 3 22.1 Keto–Enol 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 edition, (c) 2003 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
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2 Based on McMurry, Organic Chemistry, Chapter 22, 6th edition, (c) 2003 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 edition, (c) 2003 6 Acid Catalysis of Enolization ± Brønsted acids catalyze keto-enol tautomerization by protonating the carbonyl and activating the α protons Based on McMurry, Organic Chemistry, Chapter 22, 6th edition, (c) 2003 7 Base Catalysis of Enolization ± Brønsted bases catalyze keto-enol tautomerization ± The hydrogens on the α 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 edition, (c) 2003 8 Acid Catalyzed Enolization ± The addition of a proton to the carbonyl oxygen makes the α C-H more acidic, reducing the barrier to the enol ± The enol then can react with another electrophile
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3 Based on McMurry, Organic Chemistry, Chapter 22, 6th edition, (c) 2003 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 edition, (c) 2003 10 General Mechanism of Addition to Enols ± When an enol reacts with an electrophile the
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Chapter22 - Chapter 22. Carbonyl Alpha-Substitution...

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