ochem chapters 19-22 - Organic Chemistry Study Guide...

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Organic Chemistry Study Guide Chapter 22 22.1 Introduction 2 new types of reactions Alpha substitution Substitution at the carbon next to the carbonyl carbon Replacement of the H on a carbon by some other group Generally takes place when the compound is converted to its enolate ion or enol tautomer Carbonyl Condensation A substitution where the electrophile is another carbonyl compound 22.2 Enols and Enolate Ions 22.2A Keto-Enol Tautomerism In the presence of a strong acid, Ketones and Aldehydes loose a protone on the a carbon and thereby acting as a weak acid to form a resonance stabilized enolate ion Protonation can occur on the a carbon returning to the keto form, or on the oxygen giving a vinyl alcohol, the enol form. In simple ketones and aldehydes the keto form predominates Isomerization in which a proton and a double bond move is called tautomerism Keto-enol tautomerism is catalyzed by acid and base. There are two different mechanisms generally In acid 1) protonate at new location 2) deprotonate old location In base The opposite of in acid 22.2B Formation and Stability of Enolate Ions Ketons and Aldehydes are much more acidic than alkenes or alkynes due to the carbonyl group and the electronegative oxygen atom. They are still less acidic than water or an alcohol. A base such as hydroxide or alkoxide will create an enolate ion, but in low concentrations However, the addition of an electrophile to react with the enolate ion will cause the carbonyl compound to react completely. One problem.. The base may react with thte electrophile faster than the enolate and this will stop the reaction
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We must find a base that will force the carbonyl compound to completely react before the addition of an electophile Strong bases must be used to force the carbonyl compound to completely go to the enolate The most common base is Litium diisopropylamide (LDA) MECHANISM: 22.3 Alpha Halogenation of Ketons 22.3A Base-Promoted a Halogenation When a ketone is treated with a ahalogen and base, an a-halogenation reaction occurs: MECHANISM: Base-promoted halogenation often does not stop at just one replacement of hydrogen The a-haloketone is more reactive toward further halogenation because the electronwithdrawing halogen stabilizes the enolate ion to cause another substitution. Base-promoted halogenation is rarely used to prepare monohalo ketones. 22.3B The Haloform Reaction When a methyl ketone reacts through base promoted halognetion, the result is a trihalomethylketone. The trihalide can participate in nucleophilic acyl substitution. This reaction is called a Halofrom reaction MECHANISM: 22.3C Acid-Catalyzed a-Halogenation Acid Catalyzed a halogenation can replace just one hydgrogen or more by using the correct amoung of the halogen.
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