lecture 22 - Chapter 22 Carbonyl Alpha-Substitution...

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Chapter 22. Carbonyl Alpha-Substitution Reactions Based on McMurry’s Organic Chemistry , 7 th edition
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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
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3 Why this Chapter? Many schemes make use of carbonyl a- substitution reactions. These reaction are one the few general methods for making C-C bonds.
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4 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
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5 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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6 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
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7 Acid Catalysis of Enolization Brønsted acids catalyze keto-enol tautomerization by protonating the carbonyl and activating the protons
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8 Base Catalysis of Enolization Brønsted 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
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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
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