Vanessa Nguyen

ID: 84566739

Aldol Condensation

Introduction

The purpose of this experiment is to use an unknown ketone as well as an unknown aldehyde in

order to obtain an aldol condensation product that will be analyzed using HNMR and melting

point to determine the identity of the unknown ketone and aldehyde.

Theory 1

In an aldol condensation, two carbonyl compounds are condensed to form a new carbon-carbon

double bond. A ketone or an aldehyde is deprotonated in order to form a small amount of

enolate. Another aldehyde or ketone has an electrophilic alpha carbon which is attacked by the

enolate to form the new carbon-carbon bond. The aldol reaction can be self-aldol where the two

components are the same or crossed-aldol where the two components are different. The

aldehydes in this reaction, however, do not have α-hydrogens to form the enolate so the enolate

will be formed from the ketone. The main difference in distinguishing the aldehyde and the

ketone is the presence of that α-hydrogen. The aldehyde is less sterically hindered and more

electronegative, so the enolate from the ketone will attack it. The condensation part of this

reaction occurs when –OH acts as a leaving group, even though it is normally a bad leaving

group. This occurs because the driving force is the formation of an energetically favorable α,β

conjugated system and conjugated aromatic ring. Since two equivalents of the aldehyde were

used, a double condensation will occur, allowing the removal of α-hydrogens from two different

α-carbons. NMR analysis of the product will help determine the starting materials because

structures of different compounds will have different NMR spectra. By looking at the different

chemical shifts, the product can be identified, and can then be traced back to the starting

materials. Also, melting points are taken in order to help determine the starting materials as

well.

Theory 2

Results

Vial G+H

Starting materials: 4-Methylcyclohexanone and cinnamaldehyde

Product:

Melting Point Theoretical

Yield

Actual Yield Percent Yield

Product145-150 °C

0.754 g 0.281 g 37.3%

Calculations

Theoretical yield: 0.20 mL 4-methylcyclohexanone x 0.933 g/mL x 1mol/84.12g x

340.0g/mol = 0.754 g

% yield: 0.281g / 0.754g x 100 = 37.3%

*NMR spectra attached in the appendix

Discussion 1

The unknowns used in lab were G + H. IR spectroscopy was taken and showed that G was the

aldehyde and H was the ketone. Based on the NMR spectra of the product, it was determined

that the starting materials were 4-methylcyclohexanone and cinnamaldehyde. Based on the

textbook H-NMR for cinnamaldehyde (spectrum attached in appendix), there are three distinct

peaks in the region between 7.0 ppm to 8.0 ppm, which can be seen in the HNMR given by the

stock room for the aldol condensation using G and H. Looking at the textbook HNMR for 4-

methylcyclohexanone (spectrum attached in appendix), there are four distinct peaks around 1.0

to 3.0 ppm, which can also be seen in the stock room HNMR of the product as well. Since all

the peaks shown in the stock room HNMR for the product contain all the same peaks in the

HNMR for cinnamaldehyde and 4-methylcyclohexanone, the identities of the starting materials

are confirmed. Taking the melting point of the product also confirmed this. The handout

suggests that the product should be 163 °C. Results showed that the actual melting range was

145-150 °C, which is not quite as high, but is most likely due to the fact that the product was

slightly impure. The percent yield was 37.3%, which is quite low, but can be attributed to a huge

loss of product during the lab when we tried to dry the product using a stream of air, but

accidentally turned the air up too high, which scattered our product in the fume hood.

Discussion 2

As mentioned earlier, the product was still impure. Therefore, in order to improve the

experiment to confirm the starting materials more accurately, recrystallization can be done to

obtain a more pure product, which would result in a more accurate melting point. The product

was also not fully dry, so the experiment could be redone so that the product is vacuum filtrated

longer. A lot of product was lost due to mechanical errors as mentioned in the previous

discussion, so more care can be taken when drying the product as well as transferring it over to a

weighing paper to be weighed. The results showed that the technique used to form the aldol

product is not the most viable technique. However, to confirm this, multiple experiments should

be done and analyzed, because there were a few sources of error in this experiment.

Conclusion

Looking at the HNMR of the product was the most accurate way to determine the starting

materials. Taking the melting range of the product proved to be less accurate because the

product was not pure enough, so the experiment could be redone using recrystallization to form a

purer product with a more accurate melting point. A future experiment could also be done using

a less bulky aldehyde like formaldehyde, along with acetone. In addition, a higher equivalence

of the base could be used in order to deprotonate the acetone more even after two additions. This

could possibly result in the aldehyde being attacked six times.

Appendix

Textbook HNMR for cinnamaldehyde:

Textbook HNMR for 4-methylcyclohexanone:

Stock room HNMR for products:

Product structure:

References

Smith, J. (2014). Organic chemistry (4th ed.). New York, NY: McGraw-Hill.