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Organic Chemistry > Chapter 21: Alpha Carbons > Flashcards

Chapter 21: Alpha Carbons Flashcards

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1
Q

Nucleophilic position of an enol

A

The alpha position of enols/enolates is nucleophilic

Enolates are negative, so more nucleophilic than enols

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2
Q

Drawing enolates

A
  1. Identify all α protons
  2. Remove the proton at the α position and draw the resulting anion
  3. Draw the resonance structure showing the negative charge on the oxygen atom
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3
Q

Alpha halogenation of enols and enolates

Acid catalyzed

A

Reagtents

[H3O+] & X2
Cl2, Br2, and I2 (not fluorine)

Mechanism

Results in halogen addition at the α position of and aldehyde or a ketone

If the ketone/aldehyde is asymmetrical, halogen adds to the more substituted alpha carbon

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4
Q

Formation of an α,β-unsaturated ketone

A

Reagents

  1. [H3O+] & Br2
  2. Pyridine or t-ButO
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5
Q

Hell-Volhard-Zelinsky reaction

A

Reagents

  1. Br2 & PBr3 (phosphorus tribromide)
  2. H2O

Mechanism

Performs α-halogenation of a carboxylic acid

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6
Q

Alpha halogenation of enols and enolates

Base catalyzed

A

Reagents

NaOH & Br2

Mechanism

Product formed is reactive, so poly-halogenation readily occurs

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7
Q

Haloform reaction

A

Reagents

  1. NaOH & Br2
  2. H3O+

Mechanism

Converts methyl ketones into carboxylic acids

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8
Q

Aldol addition reaction

Same reactants

A

Reagents

NaOH & H2O

Mechanism

An aldehyde treated with a strong base is converted to an enolate which exists in equilibrium and attack another aldehyde

Enolate attacks at the α position

Product features both aldehyde and alcohol groups with the −OH group will be on the beta carbon

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9
Q

Aldol condensation reaction

Same reactants

A

Reagents

[H3O+ or OH−] & heat

Mechanism

When aldol product is heated water leaves and a double bond forms

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10
Q

Aldol condensation vs. addition yields

A

The condensation yields typically better than addition yields

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11
Q

Drawing the product of an aldol condensation

A
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12
Q

Crossed aldol reactions

A

Reagents

  1. NaOH
    or
  2. LDA

Mechanism

Two ways to limit outcomes:

  1. One reactant is unhindered without alpha protons
  2. Use LDA as a base to completely convert one ractant to an enolate
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13
Q

Intramolecular aldol reactions

A

Reagents

NaOH & heat

Mechanism

Intramolecular aldol reactions form cyclic compounds

ONLY 5 and 6 membered rings will form as others are too unstable

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14
Q

Identifying reagents for a crossed aldol reaction

A
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15
Q

Claisen condensations

Same reactants

A

Reagents

  1. RO−
  2. H3O+

Mechanism

Condensation reaction involving an ester

  1. Starting ester needs 2 alpha protons to work
  2. NO hydroxide can be used because hydrolysis will result
  3. Alkoxide must match –OR of ester to avoid transesterification
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16
Q

Crossed Claisen condensations

A

Reagents

  1. Strong alkoxy base
    or
  2. LDA

Mechanism

Addition of two different esters

Must meet one of two conditions:

  1. One ester must have no alpha protons
  2. Directed Claisen is possible with LDA as base
17
Q

Dieckmann cyclization

A

Reagents

  1. RO−
  2. H3O+

Mechanism

Intramolecular Claisen condensation

Only favorable for 5 or 6 membered rings

18
Q

α alkylation of an aldehyde or ketone

A

Reagents

Kinetic product

  1. LDA, −78°C
  2. RX

Thermodynamic product

  1. NaH, 25°C
  2. RX

Mechanism

Two-step mechansim produces an enolate that attacks the alkyl group in an SN2 reaction

Regioselectivity is an issue with asymmetrical ketones and choice of base determines product

19
Q

Acetoacetic ester synthesis

A

Reagents

  1. Ethyl acetoacetate
  2. NaOEt
  3. RX
  4. H3O+, heat

Mechanism

Installs an acetone goup onto the R group as a terminal methyl ketone

Works best with primary alkyl halides

Disubstituted products can also be obtained via two successive akylation reactions

20
Q

Malonic ester synthesis

A

Reagents

  1. Diethyl malonate
  2. NaOEt
  3. RX
  4. H3O+, heat

Mechanism

Installs a carboxylic acid goup onto the R group with an additional carbon atom between the R group and the carbonyl

Works best with primary alkyl halides

Disubstituted products can also be obtained via two successive akylation reactions

21
Q

α,β-unsaturated aldehyde or ketone conjugate addition

Grignard reagents

A

Reagents

  1. RMgBr
  2. H3O+

Mechanism

Grignard reagents typically attack the carbonyl carbon resulting in a 1,2 addition

22
Q

α,β-unsaturated aldehyde or ketone conjugate addition

Gillman reagents

A

Reagents

  1. R2CuLi (dialkyl lithium cuparate)
  2. H3O+

Mechanism

Gillman reagents typically attack the β carbon resulting in a 1,4 addition

23
Q

Conjugate addition reactions

Reagent choice

A

Stronger nucleophile gives 1,2 addition

Grignard reagent

Weaker / less reactive gives 1,4 addition

Gilman reagent

Stabilized enolates (Michale reaction) give 1,4 addition exclusively

24
Q

Michael reaction

A

Conjugate 1,4 addition that takes place between a Michael donor and a Michael acceptor

  • Michael donor- nucleophile that does conjugate addition
  • Michael acceptor- the α, β-unsaturated carbonyl
25
Q

Stork enamine synthesis

A

Reagents

  1. R2NH, [H+]
  2. α,β-unsaturated aldehyde or ketone
  3. H3O+

Mechanism

Performs a conjugate 1,4 addition when the starting reagent is NOT a Michael donor through the formation of an enamine intermediate

26
Q

Robinson annulation

A

Reagents

  1. NaOH & Michael addition
  2. NaOH, heat & Aldol condensation

Mechanism

Conjugate addition which combines Michael addition followed by two intramolecular aldol condensations

27
Q

Product of an aldol addition

A

Beta-hydroxy aldehydes or ketone

28
Q

Product of a Claisen reaction

A

Beta-keto ester

29
Q

Product of a conjugate addition reaction

A

Beta-substituted aldehydes or ketone

30
Q

Product of an enolate alkylation reaction

A

Alpha-substituted aldehydes or ketone

Organic Chemistry (18 decks)

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