Rules of the day 3rd midterm

Rules of the day 3rd midterm - Rules of the Day 3-24-08 1....

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Unformatted text preview: Rules of the Day 3-24-08 1. There are four new concepts associated with chapter 19. Click on these one at a time to get more information. A) Reactions favor the formation of weaker bases at equilibrium (provides a motive), so you need to have a Table of pKa values B) Enolates as nucleophiles C) Beta-Dicarbonyl species are especially acidic D) Conjugate addition to alpha,beta unsaturated carbonyl species. 2. Aldols between two different aldehydes can be run in high yield if one of the carbonyls is a ketone and the other is an aldehyde that cannot make an enolate . This is generally only relevant to formaldehyde and benzaldehyde. 3. A Claisen reaction is an "aldol with esters", also known as Dr. Evil meets an ester . At least 0.5 equivalents of the alkoxide corresponding to the alcohol portion of the ester is used to create an enolate that reacts with another molecule of ester via Mechanism B . For a movie of the Claisen reaction, click here. Claisen reactions are favorable (have a strong motive) because of the last deprotonation step. 4. The amount of base you add matters !! Only a catalytic amount of hydroxide is needed for an aldol to work, but at least 0.5 equivalents of alkoxide are needed for a Claisen to go to completion. 5. WARNING Cyclic aldol and Claisen reactions (called a Dieckmann reaction) can be hard to figure out. They work best when forming 5 or 6 membered rings and the best thing for you to do is to number your carbon atoms. Rules of the Day 3-26-08 1. There are four new concepts associated with chapter 19. Click on these one at a time to get more information. A) Reactions favor the formation of weaker bases at equilibrium (provides a motive), so you need to have a Table of pKa values B) Enolates as nucleophiles C) Beta-Dicarbonyl species are especially acidic D) Conjugate addition to alpha,beta unsaturated carbonyl species. 2. Aldols between two different aldehydes can be run in high yield if one of the carbonyls is a ketone and the other is an aldehyde that cannot make an enolate . This is generally only relevant to formaldehyde and benzaldehyde. 3. LDA is a wicked strong base that is not nucleophilic. It will quantitatively convert a carbonyl species into an enolate, so it can be used for crossed aldol or Claisen reactions. The amount you add matters!! You need to add 1.0 equivalent of LDA to make 1.0 equivalent of enolate from an ester or aldehyde. 4. The acetoacetate synthesis provides a simple way to make complex methyl ketones. The enolate is formed using 1.0 equivalent of strong base such as NaOEt, followed by 1) alkylation with an alkyl halide, 2) acid catalyzed hydrolysis of the ester and heating (loss of CO 2 ). The initial acetoacetate can actually be alkylated twice if necessary....
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Rules of the day 3rd midterm - Rules of the Day 3-24-08 1....

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