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Questions and Answers

What is produced when an enolate reacts with an electrophile?

  • An ester
  • An alcohol
  • A ketone
  • A carbonyl compound (correct)
  • Which reaction specifically results in the formation of haloform?

  • Halogenation of enolates
  • Halogenation of carboxylic acids
  • Halogenation of esters
  • Halogenation of aldehydes (correct)
  • Racemization of chiral enolates leads to the formation of which type of compound?

  • Optically active compounds
  • Stereoisomers
  • Achiral compounds (correct)
  • Diastereomers
  • What is the role of a base in the formation of enolates?

    <p>To deprotonate the carbonyl compound (C)</p> Signup and view all the answers

    Which of the following reactions involves the use of base in the formation of an enolate?

    <p>Michael addition (B)</p> Signup and view all the answers

    What compound is produced from the reaction of a carboxylic acid with X2 and P?

    <p>A bromo acid (C)</p> Signup and view all the answers

    What component is typically used to stabilize the resonance of an enolate?

    <p>A carbonyl compound (A)</p> Signup and view all the answers

    Which type of bond does an enolate primarily form with an electrophile?

    <p>Covalent bond (B)</p> Signup and view all the answers

    What is a key characteristic of direct alkylation via lithium enolates?

    <p>It requires the use of lithium diisopropylamide (LDA). (C)</p> Signup and view all the answers

    In acetoacetic ester synthesis, what role does heat play in the reaction process?

    <p>It is necessary for the elimination of carbon dioxide. (A)</p> Signup and view all the answers

    During the malonic ester synthesis, which compound is primarily being produced following the initial reactions?

    <p>A ketone. (B)</p> Signup and view all the answers

    What temperature is commonly used during the formation of the kinetic enolate in direct alkylation?

    <p>-78 °C (D)</p> Signup and view all the answers

    In direct alkylation of esters, what is the purpose of adding LDA in THF?

    <p>To generate the enolate intermediate. (C)</p> Signup and view all the answers

    What is the correct sequence of steps in acetoacetic ester synthesis?

    <p>Forming the enolate, alkylating, and then treating with acid. (C)</p> Signup and view all the answers

    Which reagent is commonly used in malonic ester synthesis to generate the enolate?

    <p>NaOEt (A)</p> Signup and view all the answers

    Which statement regarding the direct alkylation of esters is true?

    <p>It can yield a variety of alkylation products. (A)</p> Signup and view all the answers

    Flashcards

    Enolate Formation

    Formation of an enolate involves reaction of a carbonyl compound with a base, creating a resonance-stabilized enolate intermediate.

    Enolate Chemistry

    Enolate chemistry focuses on the reactions of enolates, often in organic synthesis.

    Regioselective Enolate Formation

    A reaction process leading to a predictable preference for one particular enolate over others.

    Halogenation of Aldehydes/Ketones

    Treatment of aldehydes or ketones with halogens (X2) under acidic or basic conditions to add a halogen atom.

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    Haloform Reaction

    A specific halogenation reaction where a methyl ketone is converted to a carboxylic acid and a haloform.

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    HVZ Reaction

    A reaction involving halogenation of carboxylic acids.

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    Racemization

    Conversion of a chiral molecule into a racemic mixture, meaning equal amounts of both enantiomers.

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    Enolate stability

    The stability of enolates is affected by resonance stabilization, with more resonance contributors leading to greater stability.

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    Direct Alkylation via Lithium Enolates

    A method for adding alkyl groups to carbonyl compounds using lithium enolates, often at low temperatures like -78°C.

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    Kinetic Enolate Formation

    The formation of an enolate intermediate at low temperatures (-78°C) to control the reaction outcomes and prevent competing reactions.

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    Direct Alkylation of Esters

    A method for alkylating esters using LDA to form an enolate followed by alkyl halide attack.

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    Acetoacetic Ester Synthesis

    A method to synthesize ketones with a specific carbon chain structure using the ester and alkyl halide to form a new carbon chain.

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    Malonic Ester Synthesis

    A method to introduce longer carbon chains using a malonic ester and alkyl halide followed by hydrolysis and decarboxylation, resulting in the formation of dicarboxylic acids.

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    LDA

    Lithium diisopropylamide; a strong, non-nucleophilic base commonly used in organic synthesis.

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    Enolate

    A resonance-stabilized carbanion intermediate derived from ketones or aldehydes through deprotonation.

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    Decarboxylation

    The removal of a carboxyl group from a molecule as carbon dioxide during a reaction

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    Hydrolysis

    A chemical reaction in which water is used to break down a chemical compound.

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    Study Notes

    Reactions at the α Carbon of Carbonyl Compounds

    • Reactions at the α carbon of carbonyl compounds arise from the weak acidity of hydrogen atoms adjacent to the carbonyl group.
    • These hydrogen atoms are called α hydrogens.
    • The carbon to which they are attached is called the α carbon.
    • α hydrogens are weakly acidic (pKa = 19-20).

    The Acidity of α Hydrogens of Carbonyl Compounds: Enolate Anions

    • α hydrogens of carbonyl compounds are unusually acidic for hydrogen atoms attached to carbon.
    • The pKa values for α hydrogens of simple aldehydes or ketones are around 19-20.
    • This is more acidic than ethyne (pKa= 25), ethene (pKa = 44), or ethane (pKa = 50).
    • The carbonyl group is strongly electron withdrawing.
    • When an α proton is lost, the resulting anion (enolate) is stabilized by delocalization.
    • Two resonance structures can be written for an enolate, one with the negative charge on carbon and the other on oxygen.
    • Although A is favored due to the stronger carbon-oxygen bond, resonance structure B makes a larger contribution due to oxygen being electronegative allowing it to better accomodate the negative charge.
    • The enolate hybrid shows a delocalized negative charge.

    Keto and Enol Tautomers

    • Keto and enol forms of carbonyl compounds are constitutional isomers; they are easily interconverted in the presence of acids and bases.
    • Interconvertible keto and enol forms are called tautomers, and their interconversion is called tautomerization.
    • Under most circumstances, keto-enol tautomers exist in equilibrium.
    • For simple carbonyl compounds like acetone and acetaldehyde, the amount of the enol form is very small.
    • The greater stability of the keto form in these compounds is due to the stronger carbon-oxygen bond compared to the carbon-carbon bond. (~364 versus ~250 kJ mol⁻¹).
    • In β-dicarbonyl compounds, the enol form is significantly more stable due to resonance stabilization and hydrogen bonding.

    Racemization

    • Racemization at alpha carbons occurs in the presence of acids or bases.
    • The keto form reversibly converts to the enol form.
    • The enol form is achiral, leading to the production of equal amounts of the two enantiomers when it reverts to the keto form.
    • A base catalyzes the formation of an enol via an enolate anion intermediate.

    Halogenation at the α Carbon

    • Carbonyl compounds with an α hydrogen can undergo halogen substitution in the presence of an acid or base.
    • In the presence of a base, halogenation happens through the slow formation of an enolate anion or enol, followed by a rapid reaction of the enolate or enol with a halogen.
    • The reaction produces a racemic mixture of products.

    The Haloform Reaction

    • Methyl ketones react with halogens in the presence of excess base to produce multiple halogenations at the methyl group.
    • The first halogenation makes the remaining alpha hydrogens more acidic.
    • A resulting CX3 group (chloroform, bromoform, or iodoform) attached to the carbonyl carbon can act as a leaving group, leading to a carboxylate salt and a haloform.
    • The trihalomethyl anion, unusually stable, is the leaving group.
    • The reaction can be used to convert methyl ketones into carboxylic acids.

    α-Halo Carboxylic Acids: The Hell-Volhard-Zelinski Reaction

    • Carboxylic acids reacting with bromine or chlorine in presence of phosphorous yields alpha-halo carboxylic acids in the Hell-Volhard-Zelinski (HVZ) reaction.
    • This reaction is suitable for introducing a halogen at the alpha position of carboxylic acids.

    Lithium Enolates

    • Lithium diisopropylamine (LDA) is a strong base for converting carbonyl compounds to enolates.
    • The equilibrium position depends on the base strength.
    • A stronger base shifts the equilibrium toward the enolate form.
    • Preparation involves dissolving diisopropylamine in a solvent such as diethyl ether or THF and treating it with an alkyllithium to generate LDA.

    Regioselective Formation of Enolates

    • The stability of the enolate depends on the substitution pattern of the double bond. More substituted alkenes are generally more stable.
    • The equilibrium generally favors the more stable enolate. These reactions provide useful means for controlling regioselectivity.

    Direct Alkylation of Ketones via Lithium Enolates

    • Lithium enolates provide a way to alkylate ketones selectively.
    • Reaction of LDA followed by methyl iodide or benzyl bromide can yield alkylated ketones.

    Direct Alkylation of Esters

    • Direct alkylation of esters.
    • LDA can be used to alkylate esters.

    Enolates of β-Dicarbonyl Compounds

    • β-dicarbonyl compounds (compounds with two carbonly groups separated by a single carbon atom) exhibit enhanced acidity of their α hydrogens because the negative charge can be delocalized to the two carbonyl oxygens.
    • They have pKa values in the range of 9-11, showing increased acidity compared to single carbonyl groups (18-20).

    Synthesis of Methyl Ketones: The Acetoacetic Ester Synthesis

    • Acetoacetic ester, a β-dicarbonyl compound, can easily be converted to an enolate using sodium ethoxide.
    • This enolate can then be alkylated with an alkyl halide to produce a monoalkylacetoacetic ester.
    • Using primary alkyl halides or methyl halides provides optimal yields; secondary halides result in lower yields, and tertiary halides yield mostly elimination.
    • Further alkylation is possible by using a stronger base than ethoxide ion.

    Synthesis of y-keto acids and y-diketones

    • Acetoacetic ester synthesis can be used for synthesizing y-keto acids using halo esters.
    • The reaction proceeds through halogenation at the alpha position, basic hydrolysis of the ester, acidification, and heating to cause decarboxylation.

    Acylation

    • Anions from acetoacetic esters can undergo acylation with acyl chlorides or acid anhydrides in aprotic solvents.

    Synthesis of Substituted Acetic Acids: The Malonic Ester Synthesis

    • The malonic ester synthesis is a method for preparing mono- and disubstituted acetic acids.
    • The starting compound is diethyl malonate (a dicarboxylic acid diester)

    Further Reactions of Active Hydrogen Compounds

    • Active hydrogen compounds—compounds with electron-withdrawing groups on the same carbon atom—have increased acidity in their alpha positions.
    • Compounds like ethyl cyanoacetate are examples of active methylene compounds. Ethyl cyanoacetate can be dialkylated with isopropyl iodide.

    Synthesis of Enamines: Stork Enamine Reactions

    • Enamines are formed from ketones or aldehydes and secondary amines.

    The Nucleophilicity of Enamines

    • Enamines can undergo a variety of reactions including acylation and alkylation, as well as Michael additions.

    Synthesis of y-keto esters

    • Enamines can be used in the synthesis of y-keto esters.

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