Chapter 18 Enols and Enolates

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Chapter 18Enols and Enolates
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18.1The a-Carbon Atom and its Hydrogens
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Terminology
a
b
g

• The reference atom is the carbonyl carbon.
• Other carbons are designated a, b, g, etc. on
  the basis of their position with respect to the
  carbonyl carbon.
• Hydrogens take the same Greek letter as the
  carbon to which they are attached.

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18.2 a Halogenation ofAldehydes and Ketones
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General Reaction


X2
HX

• X2 is Cl2, Br2, or I2.
• Substitution is specific for replacement of a
  hydrogen.
• Catalyzed by acids. One of the products is an
  acid (HX) the reaction is autocatalytic.
• Not a free-radical reaction.

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Example
H2O
(61-66)
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Example
CHCl3


HBr
Br2
(80)

• Notice that it is the proton on the a carbon
  that is replaced, not the one on the carbonyl
  carbon.

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18.3 Mechanism of a Halogenation ofAldehydes
and Ketones
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Mechanism of a Halogenation
Experimental Facts

• specific for replacement of H at the a carbon
• equal rates for chlorination, bromination, and
  iodination
• first order in ketone zero order in halogen

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Mechanism of a Halogenation
Experimental Facts

• specific for replacement of H at the a carbon
• equal rates for chlorination, bromination, and
  iodination
• first order in ketone zero order in halogen

Interpretation
no involvement of halogen until after
therate-determining step
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Mechanism of a Halogenation
Two stages

• first stage is conversion of aldehyde or ketone
  to the corresponding enol is rate-determining
• second stage is reaction of enol with halogen
  is faster than the first stage

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Mechanism of a Halogenation
slow
X2
RCHCR'
fast
X
enol

• Enol is key intermediate

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Mechanism of a Halogenation
Two stages

• first stage is conversion of aldehyde or ketone
  to the corresponding enol is rate-determining
• second stage is reaction of enol with halogen
  is faster than the first stage

examine second stage first
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Reaction of enol with Br2


Br
Br




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Reaction of enol with Br2


Br
Br

• carbocation is stabilized by electron release
  from oxygen

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Reaction of enol with Br2


Br
Br

• carbocation is stabilized by electron release
  from oxygen

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Reaction of enol with Br2


Br
Br




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Loss of proton from oxygen completes the process

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18.4 Enolization and Enol Content
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Mechanism of a Halogenation
Two stages

• first stage is conversion of aldehyde or ketone
  to the corresponding enol is rate-determining
• second stage is reaction of enol with halogen
  is faster than the first stage

now examine first stage
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Mechanism of Enolization(In general)
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Mechanism of Enolization(In general)
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Mechanism of Enolization(Acid-catalyzed)
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Mechanism of Enolization(Acid-catalyzed)
H


O
H
O


H
R2C
CR'
H
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Mechanism of Enolization(Acid-catalyzed)


O
H
R2C
CR'
H
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Mechanism of Enolization(Acid-catalyzed)


O
H
R2C
CR'
H
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Mechanism of Enolization(Acid-catalyzed)
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Enol Content
enol
keto

• percent enol is usually very small
• keto form usually 45-60 kJ/mol more stablethan
  enol

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Enol Content
K 3 x 10-7
K 6 x 10-9
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18.5 Stabilized Enols
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2,4-Cyclohexadienone

• keto form is less stable than enol form
• keto form is not aromatic
• enol form is aromatic

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1,3-Diketones(also called b-diketones)
Example 2,4-pentanedione
(20)
(80)

• keto form is less stable than enol form

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Enol form of 2,4-pentanedione
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Enol form of 2,4-pentanedione
intramolecular hydrogen bond
103 pm
166 pm
H
O
O
133 pm
124 pm
C
C
H3C
C
CH3
134 pm
141 pm
H
CC and CO are conjugated