18.9 the aldol condensation. a basic solution contains comparable amounts of the aldehyde and its...
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18.918.9The Aldol CondensationThe Aldol Condensation
A basic solution contains comparable amounts A basic solution contains comparable amounts of the aldehyde and its enolate.of the aldehyde and its enolate.
Aldehydes undergo nucleophilic addition.Aldehydes undergo nucleophilic addition.
Enolate ions are nucleophiles.Enolate ions are nucleophiles.
What about nucleophilic addition of enolate to What about nucleophilic addition of enolate to aldehyde?aldehyde?
RCHRCH22CHCH
OO
++ ••••OHOH••••
••••––
RCHCHRCHCH
OO
++ ••••HOHHOH
••••––••••ppKKaa = 16-20 = 16-20 ppKKaa = 16 = 16
Some thoughts...Some thoughts...
••••RCHCHRCHCH
OO•••• ••••
––
RCHRCH22CHCH
OO••••
••••
••••RCHCHRCHCH
OO•••• ••••
––
RCHRCH22CHCH
OO••••
••••
––
RCHRCH22CHCH
OO••••
••••
•••• ••••
RCHCHRCHCH
OO
••••
RCHRCH22CHCH
OO••••
•••• ••••
RCHCHRCHCH
OO
•••• HH
••••RCHCHRCHCH
OO•••• ••••
––
RCHRCH22CHCH
OO••••
••••
––
RCHRCH22CHCH
OO••••
••••
•••• ••••
RCHCHRCHCH
OO
••••••••
RCHRCH22CHCH
OO
•••• ••••
RCHCHRCHCH
OO
•••• HH
2RCH2RCH22CHCH
OONaOHNaOH
RCHRCH22CHCH
OHOH
CHCHCHCH
OO
RR
product is called an "aldol" because it is product is called an "aldol" because it is both an aldehyde and an alcoholboth an aldehyde and an alcohol
Aldol AdditionAldol Addition
RCHRCH22CHCH
OHOH
CHCHCHCH
OO
RR
Aldol Addition of AcetaldehydeAldol Addition of Acetaldehyde
AcetaldolAcetaldol(50%)(50%)
NaOH, HNaOH, H22OO
5°C5°C2CH2CH33CHCH
OO
CHCH33CHCH
OHOH
CHCH22CHCH
OO
Aldol Addition of ButanalAldol Addition of Butanal
(75%)(75%)
KOH, HKOH, H22OO 6°C6°C
2CH2CH33CHCH22CHCH22CHCH
OO
CHCH33CHCH22CHCH22CHCH
OHOH
CHCHCHCH
OO
CHCH22CHCH33
2RCH2RCH22CHCH
OONaOHNaOH
RCHRCH22CHCH
OHOH
CHCHCHCH
OO
RR
Aldol CondensationAldol Condensation
2RCH2RCH22CHCH
OONaOHNaOH
RCHRCH22CHCH
OHOH
CCHHCHCH
OO
RR
Aldol CondensationAldol Condensation
2RCH2RCH22CHCH
OONaOHNaOH
RCHRCH22CHCH
OHOH
CCHHCHCH
OO
RR
Aldol CondensationAldol Condensation
heatheat
RCHRCH22CHCH CCHCCH
OO
RR
2RCH2RCH22CHCH
OONaOHNaOH
RCHRCH22CHCH
OHOH
CCHHCHCH
OO
RR
Aldol CondensationAldol Condensation
heatheat
RCHRCH22CHCH CCHCCH
OO
RR
NaOHNaOHheatheat
Aldol Condensation of ButanalAldol Condensation of Butanal
NaOH, HNaOH, H22OO 80-100°C80-100°C
2CH2CH33CHCH22CHCH22CHCH
OO
Aldol Condensation of ButanalAldol Condensation of Butanal
(86%)(86%)
NaOH, HNaOH, H22OO 80-100°C80-100°C
2CH2CH33CHCH22CHCH22CHCH
OO
CHCH33CHCH22CHCH22CHCH CCHCCH
OO
CHCH22CHCH33
dehydration of dehydration of -hydroxy aldehyde can be-hydroxy aldehyde can becatalyzed by either acids or basescatalyzed by either acids or bases
Dehydration of Aldol Addition ProductDehydration of Aldol Addition Product
CC OO
CC
CCOHOH
HHCC OO
CC
CC
in base, the enolate is formed in base, the enolate is formed
Dehydration of Aldol Addition ProductDehydration of Aldol Addition Product
OHOH
HHCC OO
CC
CC
NaOHNaOH
OHOH
CC OO
CC
CC
••••––
the enolate loses hydroxide to form the the enolate loses hydroxide to form the ,,-unsaturated aldehyde-unsaturated aldehyde
Dehydration of Aldol Addition ProductDehydration of Aldol Addition Product
OHOH
HHCC OO
CC
CC
NaOHNaOH
OHOH
CC OO
CC
CC
••••––
Aldol reactions of ketonesAldol reactions of ketones
the equilibrium constant for aldol addition the equilibrium constant for aldol addition reactions of ketones is usually unfavorablereactions of ketones is usually unfavorable
2%2%
98%98%2CH2CH33CCHCCH33
OO OO
CHCH33CCHCCH22CCHCCH33
OHOH
CHCH33
Intramolecular Aldol CondensationIntramolecular Aldol Condensation
NaNa22COCO33, H, H22O O
heatheat
OO
OO
OO
OHOH
OO
(96%)(96%)
via:via:
Intramolecular Aldol CondensationIntramolecular Aldol Condensation
NaNa22COCO33, H, H22O O
heatheat
OO
OO
OO
(96%)(96%)
even ketones give good yields of aldol even ketones give good yields of aldol condensation products when the reaction is condensation products when the reaction is intramolecular intramolecular
18.1018.10Mixed Aldol CondensationsMixed Aldol Condensations
What is the product?What is the product?
There are 4 possibilities because the There are 4 possibilities because the reaction mixture contains the two reaction mixture contains the two aldehydes plus the enolate of each aldehydes plus the enolate of each aldehyde.aldehyde.
NaOHNaOHCHCH33CHCH
OO
CHCH33CHCH22CHCH
OO
++
What is the product?What is the product?
CHCH33CHCH
OO
CHCH33CHCH22CHCH
OO
++
CHCH22CHCH
OO
CHCH33CHCHCHCH
OO
••••––
••••
––
CHCH33CHCH
OHOH
CHCH22CHCH
OO
What is the product?What is the product?
CHCH33CHCH
OO
CHCH33CHCH22CHCH
OO
++
CHCH22CHCH
OO
CHCH33CHCHCHCH
OO
••••––
••••
––
CHCH33CHCH22CHCH
OHOH
CHCHCHCH
OO
CHCH33
What is the product?What is the product?
CHCH33CHCH
OO
CHCH33CHCH22CHCH
OO
++
CHCH22CHCH
OO
CHCH33CHCHCHCH
OO
••••––
••••
––
CHCH33CHCH
OHOH
CHCHCHCH
OO
CHCH33
What is the product?What is the product?
CHCH33CHCH
OO
CHCH33CHCH22CHCH
OO
++
CHCH22CHCH
OO
CHCH33CHCHCHCH
OO
••••––
••••
––
CHCH33CHCH22CHCH
OHOH
CHCH22CHCH
OO
In order to effectively carry outIn order to effectively carry outa mixed aldol condensation:a mixed aldol condensation:
need to minimize reaction possibilitiesneed to minimize reaction possibilitiesusually by choosing one component that cannot usually by choosing one component that cannot form an enolateform an enolate
FormaldehydeFormaldehyde
formaldehyde cannot form an enolateformaldehyde cannot form an enolateformaldehyde is extremely reactive toward formaldehyde is extremely reactive toward nucleophilic additionnucleophilic addition
OO
HCHHCH
FormaldehydeFormaldehyde
OO
HCHHCH ++ (CH(CH33))22CHCHCHCH22CHCH
OO
(CH(CH33))22CHCHCHCHCHCH
OO
CHCH22OHOH
(52%)(52%)
KK22COCO33
water-water-etherether
Aromatic AldehydesAromatic Aldehydes
CHCH33OO CHCH
OO
aromatic aldehydes cannot form an enolatearomatic aldehydes cannot form an enolate
Aromatic AldehydesAromatic Aldehydes
CHCH33OO CHCH
OO
++ CHCH33CCHCCH33
OO
NaOH, HNaOH, H22OO 30°C30°C
CHCH33OO CHCH
CHCCHCHCCH33
OO
(83%)(83%)
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