Date- 24-10-2020
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REACTION OF THE DAY
Chemistry
Aldol Reaction
'Aldol' is an abbreviation of aldehyde and alcohol.
Aldehydes and ketones having atleast one -Hydrogen atom take part in Aldol condensation in the presence of
dilute alkali or acid. Two molecules of carbonyl compound condense to form a -hydroxy aldehyde or -hydroxy
ketone which gets dehydrated on heating into ,-unsaturated aldehyde or ketone.
Acidity of -H atoms of carbonyl compounds is due to strong e– withdrawing effect of carbonyl group and
resonance stabilisation of the conjugate base.
O
C
–
O
C
C
O
–
C
C
C
–
H B
Mechanism of the Aldol Addition
Base Catalyzed:
(1)
Acid Catalyzed :
Aldol Condensation
In some cases, the adducts obtained from the Aldol Addition can easily be converted to , -unsaturated carbonyl compounds. The formation of the conjugated system is the driving force for this spontaneous dehydration.
Mechanism of the Aldol Condensation
Acid Catalyzed condensation :
Base catalyzed condensation :
Cross Aldol Condensation: When Aldol condensation is carried out between two different aldehydes and/or
ketones, it is called cross Aldol condensation. If both the molecules have -H atom, then they can give a mixture
of four-condensation products.
(i) NaOH
Ex. CH3 CHO  CH3 CH2CHO 
(ii) 
Four possible products are formed as following :
(2)
(i) NaOH
2CH3 CHO 
CH3 CH  CH  CHO (Self aldol product)
(ii) 
But-2-enal
(i) NaOH
2CH3 CH2CHO 
CH3 CH2CH  C — CHO (Self aldol product)
(ii) 
|
CH3
2-Methylpent-2-enal
(i) NaOH
CH3 CHO  H.CHCHO 
CH3CH  C — CHO (Cross aldol product)
(ii) 
|
|
CH3
CH3
2-Methylbut-2-enal
(i) NaOH
CH3 — CH2CHO  CH3CHO 
CH3CH2CH  CH — CHO (Cross aldol product)
(ii) 
Pent-2-enal
Hence cross aldol condensation becomes useful when one of the carbonyl compound does not contain H atom.
OH
|
dil. NaOH

Ex. HCHO  CH3 CHO 
 CH2 — CH2CHO 
CH2  CH — CHO
H O
2

Try Yourself
1.
What is the major product in following reaction?
O
CH3 C
(CH2)3
CH3
CH3
C
O
C
O
(1)
O
(2)
CH3
O
(3)
C
O
(4)
CH3
Answer of Question Based on Wurtz Reaction
1.
Answer (1)
(3)
C
H
dil. NaOH
