Aldol Condensation
Uploaded by
ScribdTranslationsAldol Condensation
Uploaded by
ScribdTranslationsTable 1. Synthesis of Benzalacetone.
Theoretical Yield % of yield Melting point
experimental obtained
1.5698 1.0217 65.08 105
Table 1 presents the results obtained from the synthesis of Dibenzalacetone through
a crossed aldol condensation reaction between Benzaldehyde and Acetone, catalyzed
for sodium hydroxide and ethanol, sodium hydroxide was also used because, under
under these conditions, the ketones (in this case acetone) do not undergo self-condensation in a
appreciable and the dehydration occurred due to the ease of the conjugated double bond; both with
the carbonyl group like with the benzene ring, so the system was extended
conjugated.
Benzalacetone by Aldol Condensation
Figure No 1
As can be observed in Figure No. 1, two molecules of Benzaldehyde are necessary.
to form a single molecule of Dibenzalacetone; therefore, the explanation of its
training will be given in two stages; the first stage with the reaction between one molecule of
acetone and a molecule of benzaldehyde and the second stage with the reaction between the product
from the first stage and its reaction with the second molecule of benzaldehyde.
The first stage consists of the formation of the enolate. Taking into account the two reactants of
In stage one, it is easy to deduce that only Acetone can form an enolate due to the
presence of hydrogens α. Therefore, once the enolate is formed, it performs an attack
nucleophilic to the carbonyl of benzaldehyde, generating an alkoxide which is protonated with the
half forming an aldol.
First Stage - Figure No 2
As can be seen, the product formed in this first stage still retains one carbon.
α with its corresponding hydrogen α, which retains its acidic characteristics that
allow the formation of a second enolate, which nucleophilically attacks the carbonyl of
benzaldehyde to form a new alkoxide, which takes up a proton again generating
a second aldol, the OH- takes an alpha proton forming a new enolate that leads to
to the dehydration of the aldol, thus forming the final product.
First Stage: Formation of the second enolate
Figure No 3
Second Stage – Figure No 4
Thanks to the stability of the molecule of Dibenzalacetone, which is due to conformation.
conjugated with its links throughout the molecule, it is unlikely that this will continue
reacting since it would imply losing this type of stability, which,
energetically, it is unfavorable.
However, the explanation of why by-products are not formed, or at least not in a quantity
very reduced, it is thanks to the order in which the reactants and the solvent (catalyst) are added
in the reaction medium. The following order was used for this practice:
1ml of NaOH
10ml of ethanol( 3 2 )
1.5 ml Benzaldehyde
0.5 ml Acetone
As mentioned earlier, among these compounds, only acetone could perform a
enolization process; since benzaldehyde lacks α hydrogens; which means that
given that acetone is found in a basic medium, it will form its respective
enolate could react with another enolate molecule or even with another acetone, which
would provoke a chain of reactions that would ultimately form totally new products.
different from the enolate necessary for the production of Dibenzalacetone; that is why
add the last and slowly to the acetone, as the latter leads to it being more likely
that interacts with molecules of Benzaldehyde rather than with its own molecules, moreover, the
It makes the acetone quite diluted, increasing the chances that the
Aldol condensation is explained in the first and second stages.
On the other hand, the purpose of adding hydrochloric acid to the reaction mixture was to
neutralization since the medium was very alkaline due to the addition of NaOH.
The proper handling of the order of the reactants, as well as the preparation of the reaction medium,
it can be considered as a method for controlling the reaction and its effectiveness is reflected
in it % of performance obtained which was 65.08%. However, it would not be correct
discard the formation of by-products as a factor that reduces the percentage of
performance, as there is quite a number of products that can be obtained when it comes to
of aldolic condensations, however, the greatest loss of product is attributed to the
processes where human errors can occur, which involve the use
of laboratory instruments, such as in the measurement of reagents, the
recrystallization and filtration, mainly the latter, which had to be carried out in two
occasions since the obtained crystals were so fine, which caused the holes of the
Buchner funnels became clogged, preventing effective filtration.
As for the obtained melting point, it turned out to be 105 °C, in contrast to the value
bibliographically reported, which is at 111 °C, it can be said that the obtained compound
it has an acceptable purity, however this variation of 6 °C may be due to that
an excessive amount of sample was used during the determination of the melting point, since this
it is affected by the amount of sample used as well as the state of subdivision
on the same, on the other hand, this could also be affected by the speed of
heating and the accuracy and calibration of the thermometer.
Conclusion
The synthesis of Dibenzalacetone from Benzaldehyde and acetone is carried out
through a directed crossed aldol condensation reaction, which is one of the
characteristic reactions of the carbonyl group, this reaction is useful for increasing the
formation of C-C bonds, in this case because acetone has two α carbons, is
It is possible to form two new carbon-carbon bonds from this molecule. Mainly
this reaction is favorable due to the alpha hydrogens that acetone has and the absence
of the same in benzaldehyde, making its condensation possible, moreover this reaction
is accompanied by dehydration due to the conjugated system that was had for both
the carbonyl group as for the benzene ring which leads to the formation of the
dibenzalacetone.