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Org Chem Ch.24
| Question | Answer |
|---|---|
| carbonyl condensation | reactions between 2 carbonyl compounds (occurs at the α carbon) |
| These carbonyl condensation reactions form a new carbon-carbon bond at the ___ carbon to a carbonyl group | occurs at the α carbon |
| enolates react with other carbonyl compounds. Explain how this reaction takes place | one carbonyl serves as the nucleophile and one serves as the electrophile, and a new carbon-carbon bond is formed |
| What determines the structure of the product for these types of reactions? | the presence or absence of a leaving group on the electrophilic carbonyl carbon |
| What are the four types of reactions we will be looking at in ch.24? | aldol, claisen, michael, and robinson annulation |
| Aldol reaction: _____________ or ___________ react with themselves in the presence of BASE to condensate into an aldol | aldehyde or ketone |
| Aldol reaction: Why are the final products called aldols? | because they are part aldehyde and part alcohol |
| Aldol reaction: two molecules of an aldehyde or ketone react with each other in the presence of BASE to form what kind of aldol? | β-hydroxy carbonyl compound |
| The mechanism of the aldol reaction has how many steps? | 3 |
| Aldol Mechanism: Step 1 | Formation of the nucleophilic enolate |
| Aldol Mechanism: Step 1- how is the enolate formed? | the base removes a proton from the α carbon, creating a negative charge (nucleophilic) |
| Aldol Mechanism: Step 2- nucleophilic attack. Explain how this works | the nucleophilic enolate does a back-side attack at the other carbonyl (which acts as an electrophile) |
| Aldol Mechanism: What is formed after the nucleophilic enolate attacks the electrophilic carbonyl? | a tetrahedral intermediate is formed in which the double bond is broken and the oxygen now carries a negative charge |
| Aldol Mechanism: Step 3 | protonate the tetrahedral intermediate |
| Aldol Mechanism: Step 3- What molecule protonates the tetrahedral intermediate? | water |
| Aldol Mechanism: Normally -OH is not a good leaving group. Why, then, is the β-hydroxy carbonyl product easy to dehydrate with an acid? | a highly stable, conjugated product results (and is thus favored) |
| Aldol Mechanism: Explain the dehydration of an aldol product | The product loses the elements of H2O from the α and β carbons, forming an α -β double bond (making it conjugated and stable) |
| An aldol reaction between two DIFFERENT carbonyl compounds is called what? | a crossed aldol (or mixed aldol) reaction |
| Crossed Aldol Reaction: Condensation reactions of two different ketones or aldehydes result in what kind of products? | mixed products |
| Crossed Aldol Reaction: Condensation reactions with two different carbonyls work best when one carbonyl is ____________________ | nonenolizable |
| Crossed Aldol Reaction: Why do condensation reactions with two different carbonyls work best when one carbonyl is nonenolizable? | because the nonenolizable carbonyl must be the electrophile |
| in condensation reactions with asymmetrical ketones, two enolates may be possible. This is where __________ aldol reactions come in hand | directed |
| Directed Aldol Reactions: How can you direct the direction of deprotonation? | use different bases to favor the thernodynamic enolate or the kinetic enolate |
| Directed Aldol Reactions: Thermodynamic enolate is the _________ substituted one | least |
| Directed Aldol Reactions: Thermodynamic enolates are favored by _________ bases | small |
| Directed Aldol Reactions: Kinetic enolate is the _________ substituted one | most |
| Directed Aldol Reactions: Kinetic enolates are favored by _________ bases | bulky |
| Directed Aldol Reactions: Thermodynamic enolates are favored by small bases. What is a base you could use? | NaOH |
| Directed Aldol Reactions: Kinetic enolates are favored by bulky bases. What is a base you could use? | LDA also written as Li+ -N(i-C3H7)2 |
| Crossed Aldol Reaction: When there is an enolizable aldehyde and an enolizable ketone, one product does predominate. The ___________ are more susceptible toward nucleophilic attack and are therefore the electrophile in this reaction | aldehydes |
| Claisen Reaction: in the absence of other electrophiles, _________ will react with themselves (via enolates) | esters |
| Claisen Reaction: Via enolates, esters will react with themselves to form what? | β-ketoesters |
| Claisen Reaction: Because esters have a leaving group on the carbonyl carbon, this reaction is a ________________, not addition | substitution |
| Claisen Reaction: How many steps are in the mechanism? | 3 |
| Claisen Mechanism: Step 1 | Form the enolate |
| Claisen Mechanism: Step 1- How is an enolate formed from the ester? | an alkoxide (OR-) removes an α proton, forming a negative charge on the α carbon |
| Claisen Mechanism: Why do you need to be careful about what kind of alkoxide you use to form the enolate? | you muse use an alkoxide with the SAME R group as the ester, or transesterification will occur |
| Claisen Mechanism: Step 2 | nucleophilic attack of enolate on carbonyl carbon, forming a tetrahedral intermediate |
| Claisen Mechanism: Step 3 | kick out the good leaving group |
| Claisen Mechanism: Step 3- What is the good leaving group? | the OR group from the electrophilic ester |
| Claisen Reaction: This nucleophilic substitution reaction occurs via what reagents? | NaOR (examples NaOEt or NaOMe) -specifically using the same OR group that the ester contains |
| A Claisen reaction between two different carbonyl compounds is called a __________ Claisen reaction | crossed |
| Crossed Claisen Reaction: occurs between two different esters when only one has ___ hydrogens | α |
| Crossed Claisen Reaction: What is the product? | β-dicarbonyl compound (but not necessarily a β-ketoester) |
| Dicarbonyl compounds have to ability to self-condensate through cyclization to form 5- and 6-membered rings. This is called an _______________ condensation | intramolecular |
| Intramolecular Condensation: When a diketone or dialdehyde self-condensate, the resulting product is called a cyclic ___________ | cyclic enone |
| Intramolecular condensation of a diketon or dialdehyde is called what? | an intramolecular aldol reaction |
| Intramolecular Condensation: When a diester self-condensates, the resulting product is called a cyclic ___________ | β-ketoester |
| Intramolecular condensation of a diester is called what? | Dieckmann reaction (this is an intramolecular claisen reaction) |
| How is it that dicarbonyl compounds have the ability to self condensate and form rings? | they have two carbonyl groups- one can form an enolate and act as a nucleophile while the other acts as the electrophile. |
| Once an aldol condensation is completed, an electrophilic carbonyl still remains. Enones remain susceptible to nucleophilic attack. What is an enone and what is another name for an enone? | an enone is an alkene conjugated to a ketone. Also called an α,β-unsaturated carbonyl |
| Enones (or α,β-unsaturated carbonyls) remain susceptible to nucleophilic attack, however they now have two electrophilic regions! explain | due to resonance, the partial positive charge can either be on position 2 or on position 4 (with the negative charge on the oxygen at position 1) |
| 1,2- addition of enones is known as what? | nucleophilic addition |
| 1,4 addition of enones is known as what? | conjugate addition |
| An α,β-unsaturated carbonyl (enone) can undergo either nucleophilic additiion or conjugate addition. What, then, determines the outcome? | the reagents being used |
| What reagents are used for the nucleophilic (1,2-) addition of an α,β-unsaturated carbonyl (enone)? | (1)Grignards (CH3MgBr) (2) organmetallic like (RLi) |
| What reagents are used for the conjugate (1,4-) addition of an α,β-unsaturated carbonyl (enone)? | any nucleophile (CN-, enamine, enolate, etc) |
| Michael Reaction | a 1,4-conjugate addition of an enone with an enolate |
| Michael Reaction: this is basically an ________ reaction x2 | aldol x2 |
| Michael Reaction: Step 1 | enolate formation (α-H removed to form neg. charge) |
| Michael Reaction: Step 2 | nucleophilic attack of the enolate |
| Michael Reaction: Step 2- Where does the enolate add to? | the β carbon (breaking the double bond, resulting in a negative charge) |
| Michael Reaction: Step 3 | protonation of the enolate to form the 1,4-product |
| Michael Reaction: What always forms in this reaction? | 1,5- dicarbonyls |
| 1,5-dicarbonyl compounds have the ability to undergo intramolecular self-condensation into ___-membered rings | 6-membered (Robinson Annulation) |
| Robinson annulation; consists of two parts | a michael addition to the α,β-unsaturated carbonyl to form a 1,5-dicarbonyl compound followed by an intramolecular aldol reaction to form the ring |
| Robinson annulation: The first 3 steps consists of the michael reaction. Briefly explain this reaction | (1) enolate formation (2) nucleophilic attack at the β-carbon (3) protonation |
| Robinson annulation: The next 3 steps consists of an intramolecular aldol reaction. Briefly explain this reaction | (4) enolate formation (5) intramolecular nucleophilic attack (6) protonation |
| Robinson annulation: The next step is dehydration, which occurs in 2 steps | (7) deprotonation (8) loss of -OH |
| Robinson annulation: What does this reaction produce? | cyclic enones (ex: 2-cyclohexenone) |
Created by:
cmccartney2