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J: Chem > Enolate Chemistry > Flashcards

Enolate Chemistry Flashcards

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1
Q

Characteristics of an enolate

A
  • Base with negative charge
  • Pos charge on Oxygen
  • Neg charge on alpha carbon
  • nucleophile
  • intermediate in keto-enol tautamerization under basic conditions
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2
Q

What favors a kinetic product?

A
  • low temp

- large bulky base (LDA)

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3
Q

Tautomer definition and how it differs from resonance

A
  • two molecules that can be easily interconverted at equilibrium
  • resonance is showing two different forms of charge delocalization
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4
Q

Breakdown what an enol is:

A
  • ene = C=C

- ol = OH group

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5
Q

Acid Catalyzed Keto-enol Tautomerism

A
  • add strong acid, protonate carbonyl O
  • goal: now to remove pos charge that discretes
  • water (as weak base) removes one of the acidic alpha hydrogens
  • electron (formerly bound to hydrogen) forms double bond with carbonyl carbon
  • allows electrons in one of carbonyl bonds to be pushed up to oxygen
    result: enol with OH
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6
Q

Base Catalyzed keto enol Taut

A
  • strong base attack alpha H directly
  • C=C bond between alpha carbon and carbonyl carbon
  • carbonyl O is now negative, pulls proton from H to regenerate OH-
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7
Q

Keto vs Enol tautomerism

A

keto > enol by 99:1

- stabilize enol structure through resonance and H-bond

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8
Q

Outcome of asymmetric ketones: kinetic vs thermodynamic

A

Kinetic: C=C with less substituted alpha cabon

  • form quickly > less stable long term
  • low temp
  • rapid, irreversible Rx
  • strong bulkly base, less likely to go for hydrogen on alpha carbon

Thermodynamic; C=C carbonyl carbon and more substituted alpha carbon

  • form slowly > more stable long term
  • high temp, weaker less hinder base
  • more slowly, promote reversibility
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9
Q

What is an aldol

A
  • combine features of aldehyde (or ketone) and OH alcohol
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10
Q

Base catalyzed aldol condensation

A
  • base removes hydrogen from aldehyde > enolate
  • neg charged alpha carbon attacks carbonyl C of another aldehyde C-C
  • neg charged oxygen need protonation > aldol
  • spont dehydration can occur removing OH group
  • strong base removes another H from same alpha carbon > C=C from alpha carbon and C-OH
  • forces OH off > alpha, beta unsatured aldehyde
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11
Q

Acid catalyzed aldol condensation

A
  • acid protonates carbonyl O > pos charge
  • enol thru keto-enol taut
  • 2nd aldehyde protonated by acid to make better nucleophile > pos charge on O
  • enol > carbonyl C of activated aldehyde
  • C-C alpha C of enol and carbonyl C of activated aldehyde > aldol
  • typically break down into alpha, beta unsaturated aldehyde
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12
Q

What happens with asymmetric ketone aldol condensation?

A
  • alpha carbons are no symmetric > two different aldol products based on two different enolates that can be produced
  • both can be dehydrated into alpha, beta unsaturated ketones
  • possibility of 4 different products > difficult to control
  • control: use ketone without hydrogens on other alpha carbon > only one enolate form
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13
Q

What is a cross aldol condensation?

A
  • aldol condensation with two diff ketones/aldehydes or one of each
  • start with only one aldehyde/ketone (only hydrogen on one alpha carbon)
  • LDA (strong hindered base) > rapid deprotonation > every ketone becomes enolate (neg charge)
  • now add 2nd ketone/aldehyde (electrophile)
  • produce very specific aldol product
  • can even avoid dehydration step
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14
Q

What is a keypoint of enolates including how carbonyls can behave differently?

A

enolates can act as nucleophiles

carbonyls carbon act as electrophiles

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15
Q

Famous example of retro-aldol condensation?

A

glycolysis:

- F1,6BP > GAP + DHAP

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16
Q

what two sites on aldols are electrophilic?

what is this due to?

A

Carbonyl carbon and beta carbon

-resonance: major and minor structure, puts pos charge on beta carbon

17
Q

Michael addition reaction

A

Substrates: Enolate and alpha, beta unsaturated aldehyde/ketone

  • enolate neg > attacks beta carbon > C=C alpha carbon and carbonyl > neg charge on carbonyl O
  • double bond is removed, during protonation which removes carbonyl O neg charge
  • 2 carbonyl groups separated by number of carbons
  • Michael addition: Alcohol, amine, cynade, beta dicarbonyl are nucleophile
  • commonality is nuc addition to beta carbon of alpha, beta unsaturated aldehyde/ketone
18
Q

Robinsinon annulation is what two reactions

A

michael addition > aldol condensation

19
Q

Robinson annulation product

A

2,6 dicarbonyl of 7 carbon structure

20
Q

Robinson annulation steps

A
  • starting with end product of michael addition: 1,5 dicarbonyl
  • we need an enolate > strong base pull hydrogen from terminal alpha carbon
  • neg charge attack carbonyl carbon on other side: C6
  • cyclic structure with carbonyl from beginning unchanged, C6 with methyl and hydroxyl
  • hydroxyl can be pushed off via dehydration > alpha, beta unsaturated ketone but cyclic
21
Q

what biological importance does robinson annulation have?

A

formation of 6 membered ring > similar to ringed structures in hormone synthesis

22
Q

important takeaways from michael addition and robinson annulation:

A
  • enolates make good nucleophiles

- carbonyl containing compounds can have several electrophilic sites

23
Q

Base catalyzed keto enol Taut

A
  • base is strong enough to attack alpha hydrogen directly and create C=C bond
  • compared to acid hydrolyzed, since it wasnt protonated first this adds neg charge on oxygen
  • neg charge pulls proton from water > enol
24
Q

What is more predominate usu: Keto or enol?

what can stabilize the less predominate species?

A

Keto by a lot

  • resonance and H-bonding
  • also solvent, but to less extent as other two

J: Chem (18 decks)

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