An Aldol Condensation Reaction: The Synthesis of
Tetraphenylcyclopentadienone—An
Example of a Double-Crossed Aldol
Addition Reactions of Carbonyl Groups
The chemical reactivity of aldehydes and ketones is closely associated with
the presence of the carbonyl group in their structures. For example, both aldehydes
and ketones undergo addition reactions such as the addition of a Grignard reagent
to the carbonyl group as shown in Figure 1.
O
RCH + R'MgX
aldehyde
O
RCR'' + R'MgX
ketone
addition
Grignard
reagent
OMgX
RCH
R'
salt
addition
Grignard
reagent
OMgX
RCR''
R'
salt
H+
H2O
acidification
H+
H2O
acidification
OH
RCH
R'
IIo alcohol
OH
RCR''
R'
IIIo alcohol
Figure 1. Addition reactions.
The reactions in Figure 1 differ only because the pink H of an aldehyde is
replaced by R′′ in the ketone. The addition reaction occurs at the carbonyl group.
The carbonyl group is polarized so that the carbon atom bears a partial positive
charge and the oxygen atom bears a partial negative charge. The R′ group of the
Grignard reagent is negative relative to the positive Mg atom. Thus, the negative R′
group bonds to the positive carbon atom, and the negative oxygen and metallic
magnesium form an ionic bond, yielding a salt in each reaction. The addition
product is acidified in each case to make a covalent alcohol. The aldehyde
produces a IIo alcohol; whereas, the ketone produces a IIIo alcohol owing to the R′′
group.
The two equations for addition reactions in Figure 1 are summarized in
Figure 2. A nucleophile (negative species) bonds to the carbonyl carbon (positive),
breaking the  bond of the carbonyl group.
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O-
O
:Nu
C
C
Nu
Figure 2. Addition of a nucleophile to a carbonyl group.
Figure 2 focuses our attention on the salient part of an addition reaction that
involves either a ketone or an aldehyde. A nucleophile bonds to the carbonyl
carbon. In an aldol addition reaction, the nucleophile is an enolate formed from an
aldehyde or ketone by the removal of a hydrogen atom next to the carbonyl group.
The enolate (negative nucleophile) then adds to a carbonyl group of another
aldehyde or ketone as shown in Figure 1.
Formation of an Enolate from an Aldehyde or Ketone
Aldehydes and ketones that possess alpha hydrogen atoms can form
enolates. The Greek alphabet (, , etc.) is used by chemists to identify carbon
atoms in relation to the carbon atom of a carbonyl group. An alpha carbon atom is
a carbon atom that is bonded directly to the carbon atom of a carbonyl group. A
beta carbon atom is the second carbon atom from the carbonyl carbon, a gamma
carbon is the third carbon away from the carbonyl, etc. Likewise, hydrogen
atoms are named according to the name of the carbon atom to which
they are bonded. A hydrogen atom bonded to an alpha carbon is called an alpha
hydrogen, etc. Figure 3 shows these relationships for acetaldehyde and 2pentanone.
 carbon
3  hydrogens
H O
H C C H
H
acetaldehyde
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2  hydrogens
3  hydrogens
-C
-C
H H H O H
H C C C C C H
H H H
H
2-pentanone
2
Figure 3. Alpha hydrogens.
Alpha hydrogen atoms are important because they can be removed by a
base. Thus, the hydrogen atoms shown in blue and pink in Figure 3 can be
removed in base. What is the first thing that happens when acetaldehyde is placed
in base? The base removes or abstracts a hydrogen atom. The base forms a new
bond with a pair of electrons from the base, so the hydrogen atom leaves its
electrons with the alpha carbon atom. Thus, the base abstracts H+ or a proton,
leaving C- or a carbanion. Compounds that contain only one kind of equivalent
alpha hydrogens (e.g., acetaldehyde has 3 equivalent blue H atoms) are preferred
over compounds that contain two kinds of equivalent alpha hydrogens (e.g., 2pentanone has 2 pink and 3 blue equivalent H atoms). Figure 4 shows the first step
when acetaldehyde reacts with hydroxide ion, which may come from sodium
hydroxide or potassium hydroxide.
H O
H C C H
H
-OH
step 1
acetaldehyde
O
H C C H
+ HOH
H
carbanion
Figure 4. Formation of carbanion.
A base abstracts an alpha hydrogen atom to form a carbanion. The carbanion
is a nucleophile, which can react with a carbonyl group as shown in Figure 2. The
carbanion shown in Figure 4 is also an enolate because of resonance. Figure 5
shows how the initially formed carbanion from acetaldehyde is resonance
stabilized via the enolate. Thus, alpha hydrogen atoms are acidic because
the resulting anion is resonance stabilized.
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O
H C C H
OH
C C H
H
H
carbanion
enolate
Figure 5. Resonance structures of carbanion and enolate.
Self-Aldol Addition Reaction
The carbanion or enolate of Figure 5 is a nucleophile, which can add to a
carbonyl group of an aldehyde or ketone. When an enolate made from an aldehyde
such as acetaldehyde reacts with another molecule of acetaldehyde, the addition
product is a dimer and the reaction is called a self-aldol addition reaction. Figure 6
shows the carbanion (enolate) of Figure 5 adding to a second molecule of
acetaldehyde.
H O
H C C H
+
H
acetaldehyde
H OH
H OO
H-OH
H C C H + -OH
H C C H
H C C H
step 2
H CH2CH
H
H CH2CH step 3
O
O
carbanion
aldol
of acetaldehyde
Figure 6. Synthesis of aldol.
Steps 1-3 of figures 4 and 6 show the complete mechanism of an aldol
addition reaction. In Step 1, base abstracts an alpha hydrogen to make an enolate.
In Step 2, the enolate adds to the carbonyl of a second molecule, forming an
oxyanion, which abstracts a proton from water to make 3-hydroxybutanal. The
hydrogen atom that is abstracted in Step 3 replaces the H atom that was abstracted
in Step 1, so the product is a dimer of the starting compound. 3-Hydroxybutanal is
the first member of a family of compounds called aldols. Aldols are so named
because they contain an aldehyde and an alcohol, and the alcohol hydroxyl group
is bonded to a carbon atom that is two carbon atoms away from the carbonyl
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carbon atom. The structural feature in an aldol is a-hydroxycarbonyl. Any
aldehyde or ketone with a -hydroxyl group is an aldol. Figure 7 shows examples
of aldols.
O
O
OH
OH O
 
 
H


OH
Figure 7. Aldols.
Acetaldehyde is the simplest compound that has alpha hydrogens. Therefore,
the common name of the aldol made the self-aldol addition reaction of
acetaldehyde is called aldol. All other aldehydes or ketones that contain the hydroxycarbonyl structure are members of the aldol family and are called aldols.
Crossed-Aldol Addition Reaction
In a self-aldol addition reaction, both reactants come from the same
compound. When the two reactants in Step 2 come from different compounds, the
reaction is called a crossed-aldol addition reaction.
A crossed-aldol addition reaction generally involves one aldehyde or ketone
without alpha hydrogens and one with alpha hydrogens; therefore, only the
compound with alpha hydrogens can form an enolate in base. If both compounds
contained alpha hydrogens, then multiple products would be produced. The enolate
is the nucleophile that adds to the carbonyl group of the compound without alpha
hydrogens. Benzaldehyde contains no alpha hydrogens and acetone contains six
equivalent alpha hydrogens; therefore, these two compounds can undergo a
crossed-aldol addition reaction without forming multiple products. Figure 8 shows
the structures of benzaldehyde and acetone. You should know the structures of
these compounds.
O
 C
not 
H
benzaldehyde
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 O
CH3CCH3
acetone
5
Figure 8. Benzaldehyde and acetone.
By inspecting the structures in Figure 8, we see that benzaldehyde has no
alpha hydrogen atoms and acetone does. Thus, when these compounds are placed
in base, acetone will form an enolate by the mechanism shown in Figure 4. The
enolate serves as the nucleophile that adds to the carbonyl group of benzaldehyde,
which serves as the substrate in the addition reaction. Figure 9 shows the reaction.
O
CH
-OH
O
CH3CCH3
Step 1
O
-CH CCH
2
3
Step 2
OCH
HOH
CH2CCH3
Step 3
O
OH
CH
CH2CCH3
O
Figure 9. Synthesis of an aldol.
In Step 1, base removes an alpha hydrogen atom from acetone (the only
reactant with alpha hydrogens), forming the cabanion or enolate that serves as the
nucleophile. In Step 2, the negative carbanion adds to the positive carbonyl carbon
to form the oxyanion. In Step 3, the adduct from Step 2 abstracts a proton from
solvent to form the aldol (-hydroxyketone) product. Note that the aldol addition
and Grignard reactions are similar; they differ only in how the nucleophile is made.
The nucleophile in an aldol reaction is formed from an aldehyde or ketone with
alpha hydrogens; the nucleophile of a Grignard reagent is formed from an alkyl
halide.
Aldol Condensation Reaction
We learned first semester that alcohols can eliminate water to form an
alkene. Aldols are alcohols, and they can also eliminate water to form alkenes.
Thus, initially formed aldols eliminate water when they are heated, generally in
acid. Recall that a mineral acid protonates a hydroxyl oxygen atom, making a good
leaving group. Consider aldol itself. Figure 10 shows that two different alkenes are
possible when water is eliminated. In one alkene, the hydroxyl group and another
alpha hydrogen are eliminated. In the second alkene, the hydroxyl group and a
gamma hydrogen are eliminated.
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H

H C C
H OH H O
H  C C C C H

H H H
aldol
Step 4
H H

X
H
H C


H O
C C H

-unsaturated aldehyde
(conjugated, more stable)
H O
C
C
H
H
C H
-unsaturated aldehyde
(unconjugated, less stable)
Figure 10. Dehydration of aldol.
The dehydration of aldol gives only the -unsaturated aldehyde, because it
is more stable than the -unsaturated aldehyde. When an aldol is heated, it
dehydrates to give an -unsaturated aldehyde or ketone. Thus, an aldol reaction
may be stopped after Step 3 to give an aldol, or after Step 4 to give an unsaturated aldehyde or ketone. When the reaction sequence includes Step 4, the
overall reaction is an aldol-condensation reaction. Condensation implies the loss of
water (or another small molecule such as ethanol) from two organic reactants.
Figure 11 shows Step 4 that changes the aldol addition in Figure 9 into an aldolcondensation reaction.
OH
CH
CH2CCH3
O
- H2O
O
CH CHCCH3

aldol
-unsaturated ketone
Figure 11. Conversion of an aldol to an -unsaturated ketone.
The Experimental Reaction
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In this experiment, we conduct an interesting aldol condensation that
involves one ketone that has -hydrogen atoms (dibenzyl ketone) and one that
does not (benzil). Thus, dibenzyl ketone will serve as the nucleophile, because it
will donate a proton to a base to form an enolate. The overall reaction is shown in
Figure 12.
O
O O
C C
benzil
+
O
CH2 C CH2
dibenzyl ketone
KOH
- H2O
tetraphenylcyclopenatadienone
Figure 12. Double-crossed aldol condensation.
This reaction involves two different ketones, so the reaction is described as a
crossed-aldol condensation. Note that we have formed two -unsaturated
groupings, so the crossed aldol reaction occurs twice. Thus, the reaction can be
called a double-crossed aldol condensation. You can work out the mechanism for
the reaction by forming an enolate from dibenzyl ketone in base, allowing it to
attack on carbonyl group in benzil, and eliminating water from the adduct. Repeat
the process with the remaining -hydrogen atoms while closing the five-membered
ring. The final product is a purple solid.
Procedure
1. Set up a sand bath on your lab bench. [Metallic heating mantle, sand, and
rheostat] Adjust the setting on the rheostat to 70.
2. Add 0.1-g benzil, 0.1-g dibenzyl ketone, 1.5-mL absolute ethanol, and a tiny
boiling chip to a 15-mL round-bottomed flask. [Use a creased, glassine paper to
transfer the solids.]
3. Attach a condenser from a micro kit to the round-bottomed flask with the blue
adapter from the kit.
4. Affix the condenser-flask to a ring stand so that the round-bottom flask is
slightly into the sand of the sand bath.
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5. Heat the mixture on the sand bath until the solids dissolve but do not let the
ethanol boil (bp = 78 oC).
6. Slowly add seven drops of 5% (weight-volume) alcoholic potassium hydroxide
(KOH) through the condenser.
7. Bring the ethanol to a boil and allow the mixture to reflux for 10 min.
8. If a solid begins to form and cake on the round-bottomed flask, add a few drops
of absolute ethanol to keep the solid moist.
9. While the mixture is under reflux, prepare an ice-water bath in a beaker large
enough to hold the round-bottomed flask.
10. Remove the apparatus from the sand bath and allow the round-bottomed flask
to cool to room temperature.
11. Place the flask, still attached to the condenser, in the ice-water bath and allow
the flask to come to the temperature of the ice bath.
12. Remove the condenser and collect the crystals on a Büchner funnel.
13. Continue to apply suction until the crystals are as dry as possible.
14. Weigh the crystals and record the yield directly in your notebook.
15. Show the crystals to your instructor, who will tell you what to do with them.
16. Clean your work area. Replace all equipment to the correct storage location.
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Aldol Condensation Questions
Last name___________________________, First name________________________
1. Circle the structures below that represent compounds that can undergo self-aldol condensations.
(Get all correct for one point.) O
CH2O
CH3CHO
CH3CCH3
CH2O2
O
C
CHO
2. Write an equation for the enolization of acetaldehyde in an acidic medium.
3. Write an equation for the enolate formation of acetone in base (OH-).
4. Write an equation for the self-aldol addition (not condensation) of acetaldehyde.
5. Circle the following stuctures that have -hydrogen atoms.
O
CCH2
O
HCH
CHO
CH2CHO
Complete the following equations.
O
CH
6.
7.
O
C
(1)OH+
CH3CHO
(2) heat
(1)OH+
CH3CHO
(2) heat
8.
O
C
+
O
CH3CCH3
(1)OH(2) heat
(1)OH9.
O
(2) heat
10. Why was your product in lab a colored compound?
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