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CH10136 > Carbonyl chemistry- enol/enolates > Flashcards

Carbonyl chemistry- enol/enolates Flashcards

1
Q

What is a tautomer of a ketone and imine

A
  1. Enol

2. Enamine

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2
Q

Why do ketones and imines exist in two forms

A
  1. Carbonyl compounds containing alpha-hydrogens exist in two forms
  2. Only normally see the ketone and imine form as th enol and enamine only appear for a very short time
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3
Q

What happens to the tautomerisation in 1,3-dicarbonyl compounds

A
  1. The equilibrium is shifted
  2. The additional carbonyl groups offers additional stability by conjugation
  3. A h-bond forms between the alcohol and ketone group to form a 6-membered ring
  4. The enol form can now be viewed spectroscopically
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4
Q

How can an enol be forced from a ketone

A
  1. Via acid catalysis

2. Replaced ketone with alcohol and C-C double bond forms + H2O(+)-H

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5
Q

Describe enol reactivity

A
  1. Similar to alkenes

2. Lone pair on alcohol can donate electrons into the system so enols can react with electrophiles

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6
Q

What is acid strength affected by

A
  1. Strength of H-X bond

2. Stability of conjugate base (most important for organic acids)- lower pKa

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7
Q

Describe acidity of carbonyls

A
  1. Alpha- hydrogen is acidic compared to normal sp3 C-H and so is readily deprotonated
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8
Q

What needs to happen for deprotonation of ketones

A
  1. C-H needs to be aligned in a specific orientation

2. Needs to be aligned with pi bond of carbonyl system- 90 degrees

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9
Q

What happens when a base is added to a ketone

A
  1. The alpha hydrogen is deprotonated and an enolate is formed
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10
Q

What can form enolates

A
  1. Any compounds with electron withdrawing group so alpha hydrogen is acidic
  2. E.g CN group, ketones etc
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11
Q

What two products are formed from the alpha-deprotonation of an unsymmetrical carbonyl compound

A
  1. 2 possible enolates form
  2. The kinetic enolate
  3. The thermodynamic enolate
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12
Q

What is the kinetic enolate

A
  1. C-C double bond forms on less substituted carbon

2. Less stable

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13
Q

When does the kinetic enolate form

A
  1. Prefers a strong hindered base- conjugate acids with much higher pKa than carbonyl
  2. Prefers low temperatures -78 degrees
  3. Prefers short reaction times
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14
Q

Name 2 strong hindered bases used to form the kinetic enolate

A
  1. Lithium diisopropyl amine (LDA)

2. Lithium Hexamethyl disilazide (LiHMDS)

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15
Q

What is the thermodynamic enolate

A
  1. More substituted

2. More stable

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16
Q

When does the thermodynamic enolate form

A
  1. Prefers excess ketone
  2. Prefers higher temperature- rt
  3. Prefers long reaction times
  4. Base with conjugate acid that is more acidic than carbonyl itself- alkoxide bases
  5. It is very reversible- only forms a bit of enolate as base not strong enough to completely deprotonate
17
Q

What can enolisation be used for

A
  1. Create a more stable molecule
  2. Can form conjugated molecules- so increases stability
  3. Racemisation
18
Q

What happens to stereogenic centres in enolisation

A
  1. Any stereogenic centre next to a carbonyl group would be destroyed by enolisation
  2. Sp3 –> sp2
19
Q

What type of nucleophile is an enolate

A
  1. An ambident nucleophile- two nucleophilic sites
  2. Choice of electrophile can direct addition towards either site
  3. Can add to O- or C=C bond
20
Q

What electrophiles cause addition to oxygen in enolates

A
  1. Hard electrophiles

2. MeSiCl, Me3O+, AcCl, Tf2O

21
Q

What electrophiles cause addition of C=C of enolates

A
  1. Soft electrophiles
22
Q

What does addition to oxygen of enolates produce

A
  1. Can create stable enolate equivalents

2. Enol ether and silyl enol ether

23
Q

Describe what takes place in alpha-halogenation of ketones under acidic conditions

A
  1. Results in mono alpha-halogenation - only halogenates once
  2. Enol formed as intermediate
24
Q

Describe what takes place in alpha-halogenation of ketones under basic conditions

A
  1. Multiple alpha-halogenation
  2. Enolate forms as intermediate
  3. After first halogenation there is still an alpha proton which is now more acidic than in the starting material so the OH- attacks again
  4. This happens again producing a C-X3
  5. The halogens can then be removed and replaced with OH to form a carboxylic acid

CH10136 (30 decks)

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