Aldol Condensation

1. BY
Dr. G.C.Wadhawa
DEPARTMENT OF CHEMISTRY
K. B. P. College,Vashi,Navimumbai
Aldol Condensation

2. 1. Aldol Condensation
 Aldehydes having α- hydrogen undergo self-condensation on
warming with dilute or mild base to give β-hydroxy aldehydes,
called aldols (aldehyde+alcohol). This reaction is known as aldol
condensation.

3. The reagent such as dilute sodium hydroxide, aqueousalkali
carbonate, alkali metal alkoxide, etc., may be used.
Aldol condensation has broad scope. It can occurbetween
1. two identical or differentaldehydes.
2. Two identical or different ketones.
3. An aldehyde and aketone.
4. Aldehyde or Ketone may aromatic or aliphatic
When the condensation is between two differentcarbonyl
compounds, it is called as crossed aldol condensation.

4. Mechanism reactions as follows

6. Reagents: commonly a base such as
NaOH or KOH is added to the aldehyde.
The reaction involves an aldehyde enolate
reacting with another molecule of the
aldehyde.
 Remember enolates are good
nucleophiles and carbonyl C are good
electrophiles.
 Since the pKa of an aldehyde is close to
that of NaOH, both enolate and aldehyde
are present simultaneously.
 The products of these reactions are β-
hydroxyaldehydes or aldehyde-alcohols =
aldols.

7. Why formal aldehyde does not give aldol
reaction in alkaline medium?
 For a carbonyl compound to undergo aldol condensation, the
necessary condition is that it MUST possess at-least 1 "alpha-
hydrogen" (an alpha-hydrogen is a hydrogen attached to an alpha-
carbon, while an alpha carbon is the carbon attached DIRECTLY to
the functional group) since formaldehyde(i-e HCHO) has only 1
carbon which is the carbonyl carbon (i-e the functional group) and
there is no alpha-carbon present and hence no alpha-hydrogen, so
formaldehyde doesn't undergo aldol condensation. e.g Acetaldehyde
can undergo aldol condensation since it has alpha-hydrogens. - The
2 hydrogens present in the formaldehyde molecule are not alpha-
hydrogens; they are simply the hydrogens attached to the functional
group.

8. What is an alpha hydrogen atom needed for
aldol condensation reaction to occur?
 Due to the acidic nature of α-hydrogen of aldehydes and ketones,
they undergo a number of reactions. This acidity is because of the
strong electron withdrawing nature of the carbonyl groups and
resonance stabilization of the conjugate base. - Because, the C-H
proton is not acidic enough.
 Further, carbonyl group of aldehyde is highly electrophilic (hard
electrophile) hence the base (hard nucleophile) prefers to attack the
carbonyl carbon leading to alcohol & acid. - So, in-order to get the
aldol product, the presence of alpha hydrogens is necessary.
 These hydrogens are more acidic than the hydrogen attached to the
carbonyl group.

9. Formaldehyde
O
HCH
formaldehyde cannot form an enolate
formaldehyde is extremely reactive toward
nucleophilic addition

10. Formaldehyde
O
HCH + (CH3)2CHCH2CH
O
3 2
(CH ) CHCHCH
O
CH2OH
(52%)
K2CO3
water-
ether

11. Aromatic Aldehydes
CH3O CH
O
aromatic aldehydes cannot form an enolate

12. Aromatic Aldehydes
CH3O CH
O
+ CH3CCH3
O
NaOH, H2O 30°C
CH3O CH CHCCH3
O
(83%)

16. What is the product?
There are 4 possibilities because the
reaction mixture contains the two
aldehydes plus the enolate of each
aldehyde.
NaOH
CH3CH
O O
+ CH3CH2CH

17. Intramolecular Aldol
Condensation
Na2CO3, H2O
heat
O
O
O
O
(96%)
via:
OH

19. References:
http://en.wikipedia.org/wiki/Aldol_condensa
tion
http://www.organic-
chemistry.org/namedreactions/aldol-
condensation.shtm
http://www.chem.ucalgary.ca/courses/350/
Carey5th/Ch18/ch18-3-4.html
http://www.usask.ca/chemistry/courses/255
/laboratory/aldol.htm