Using the Aldol Reaction for the Synthesis of Dibenzalacetone,
a UV-absorbing Ingredient in Sunscreens
Benjamin Dharsi
Department of Chemistry, University of Warwick
Introduction
• The depletion of the ozone layer has lead to more
ultraviolet (UV) radiation passing (figure 1), increasing the
prevalence of sun burns and skin cancer.1
Yellow precipitate formed
collected using Büchner
filtration (figure 3)
Solid dissolved in ethyl
acetate, then dried using
magnesium sulphate
Figure 3. Büchner filtration
Figure 7. 1H NMR spectrum of DBA
Figure 1. Ozone layer protection against UV light
• Sunscreens protect the skin against UV light.
• Dibenzalacetone (DBA) (figure 2) strongly absorbs UV-A
radiation due to its π-conjugation, making it a useful
ingredient in sun screens.2
Solution filtered by gravity
and then put through rotary
evaporation (figure 4)
The product was
recrystallized using ethanol
and collected via Büchner
filtration, giving yellow
crystals (figure 5)
Figure 4. Rotary evaporation
Table 2. 1H NMR environment
assignments
Chemical Shift
Assignment
(ppm)
7.78
2H, d, J 0.5, O=CCH
7.65
4H, s, ortho-Ph or
meta-Ph
7.45
6H, m, para-Ph and
ortho-Ph or metaPh
7.12
2H, d, J 0.5, O=CCH=CH
• The IR and NMR data strongly suggests that the product
made is DBA.
• The coupling constants of the two alkene hydrogen
environments indicate that the trans-trans isomer of DBA
was the main product.
• A yield of 74% was attained.
• In addition to base catalysts, catalysts such as enzymes,
metal ions and enamines can also be used for the aldol
reaction.3-5 The different yields and purities obtained from
using different catalysts could in future be studied and
compared.
Conclusions
Figure 2. Trans-trans DBA isomer
• The aldol reaction is a carbon-carbon linking reaction
between two carbonyls and can thus be used to synthesise
DBA.
• The aim of this study was to evaluate the base-catalysed
aldol reaction as a pathway for the synthesis of DBA.
Infrared (IR) and Nuclear
magnetic resonance (NMR)
data of the product was
collected
Results & Discussion
Experimental
Benzaldehyde and acetone
(2:1) were reacted in a basic
solution of ethanol and water
Figure 6. IR spectrum of DBA
Scheme 1. Aldol reaction mechanism between
benzaldehyde and acetone (2:1) in basic conditions
Figure 5. DBA crystals
• The findings of this study suggest that the base-catalysed
aldol reaction is a viable synthetic route for making DBA,
providing a high yield.
• The trans-trans isomer of DBA was the main product,
however the other two isomers, their properties, uses and
differing syntheses could be explored in future.
• The efficacy of the aldol reaction using different catalysts
could be investigated in future studies.
Table 1. Key peak assignments of DBA IR
spectrum
References
Wavenumber (cm-1)
Assignment
3000 (weak)
C-H stretch (alkene)
1650 (strong)
C=O stretch (ketone)
985 (strong)
C-H bend (alkene)
1. J.C. Van Der Leun, Photodermatol., Photoimmunol.
Photomed., 2004, 20, 159-162.
2. L. A. Huck and W. J. Leigh, J. Chem. Educ., 2010, 87,
1384-1387.
3. N. Iranpoor and F. Kazemi, Tetrahedron, 1998, 54,
9475-9480.
4. H. Tsuji, F. Yagi, H. Hattori and H. Kita, J. Catal., 1994,
148, 759-770.
5. A. B. Northrup and D. W. C. MacMillan, J. Am.
Chem. Soc., 2002, 124, 6798-6799.