The Synthesis, Separation, and Analysis of Organic Fruit Volatiles
Nicole Bernstein
Department of Chemistry, The Pennsylvania State University, University Park, PA 16802
nwb5129@psu.edu
Abstract
Hexyl Acetate (1)—an organic volatile contained in apples—was synthesized from 1hexanol (2) and acetic acid (3) by running a Fischer Esterification reaction in a Dean-Stark
apparatus. The efficiency of the reaction was measured by running 1H NMR and IR analyses of
the product, and determining any impurities in the spectra. The technique of fractional
distillation was then explored by separating an unknown mixture of two fruit volatiles, which
were determined to be ethyl acetate (4) and 1-butanol (5). This type of distillation was found to
be an efficient separation technique of two compounds with similar boiling points.
Introduction
This module surrounds the study of organic volatiles—compounds which are partially
responsible for the smell and taste of fruit. Each different fruit has as many as a hundred
compounds working to produce its aroma. Yet, some single ester compounds can be used to
approximate the natural odor of the fruit, and are often used in food production as artificial
fragrances and flavor.1 An objective of this module is to synthesize one such compound—hexyl
acetate, which is responsible for the aroma of apples.
Hexyl acetate (1) is an ester which can be synthesized via a Fischer Esterification
reaction of 1-hexanol (2) and glacial acetic acid (3) (Scheme 1). An ion-exchange resin is used as
an acid catalyst for the reaction, as it is easily filtered out of the solution once the product has
been obtained. The resin protonates the carbonyl carbon of acetic acid, which then encourages
attack by the nucleophilic 1-hexanol.While the equilibrium constant of this reaction is not
extremely favorable, using a Dean-Stark Apparatus to remove water (the bi-product) from the
reaction mixture will push the synthesis to completion.
This module also examines the technique of fractional distillation, which is used to
separate compounds with relatively close boiling points. As a simple distillation will not achieve
accurate results unless the compounds have boiling points separated by at least 75oC, a
fractioning column must be used to induce multiple theoretical plates, which allows the
components of a mixture to be separated more efficiently. This technique is useful, because it
not only allows for efficient distillation of components with boiling points which may be close
together, but it also aids with the identification of unknowns by providing an accurate boiling
1
point. In this experiment, fractional distillation was used to separate a mixture of two unknown
fruit volatiles.
Scheme 1. Fischer Esterification Reaction of 1-Hexanol and Glacial Acetic Acid
OH
+
2
O
H+
OH
min.
O
+
O
3
H2O
1
Results and Discussion
The Synthesis of Hexyl Acetate
The Fischer Esterification of 1-hexanol (2) and glacial acetic acid (3) was carried out on a
microscale to produce hexyl acetate (1) and water. The acid catalyst—Dowex 50 x 2-100 ion
exchange resin—was filtered from the solution and the resulting product has a distinct smell of
sour apples, a sign that the reaction was successful.
The identity of the product was confirmed by both 1H NMR and IR spectral analyses. The
splitting pattern and the number of signals were consistent with what one would expect of hexyl
acetate. The key observations included a triplet at 4.06 ppm with an integration of 2, which is
indicative of hydrogen next to an electron withdrawing group such as oxygen, and a single with
integration 3 at 2.05 ppm, both distinct to hexyl acetate. A peak at 3.64 ppm indicates
contamination from unreacted 1-hexanol, which has a characteristic peak at this value.
Additional contamination from 1-hexanol affected the integration values of the peaks at 1.60
ppm, 1.32 ppm, and 0.89 ppm. An analysis of the contamination showed that the ratio of product
hexyl acetate to 1-hexanol was 2:1. Contamination from acetic acid also appeared as a singlet
peak at 2.07 ppm, in a ratio of hexyl acetate to acetic acid of 2:1. In the future, to assure that no
reactants impurified the product, the reaction would be allowed to run longer, as that would
ensure that it had run to completion. Additionally, a stirrer bar would be used instead of boiling
chips, as the boiling chips were not effective in preventing bumping, which therefore may have
caused errors in the reaction. The IR spectrum contained a C=O stretch at 1736.2 cm-1 and a C—
O stretch at 1236.2cm-1, which are consistent with the functionalities of hexyl acetate.
The Fractional Distillation of Two Unknown Fruit Volatiles
The fractional distillation of unknown mixture number 73 was an effective method for
separating the two unknown fruit volatiles. The first compound to separate was found to have a
boiling point range of 77oC to 79oC (Figure 1). The NMR spectrum consisted of a quartet at 4.12
ppm, a singlet at 2.03 ppm, and a triplet at 1.25 ppm, with integrations of 2, 3, and 3
respectively. This data is consistent with what one would expect of the compound ethyl acetate
(4). Ethyl Acetate had a boiling point of 77o, which agrees with the experimental value.1 The IR
2
spectrum displayed a C=O stretch at 1736.0cm-1 and a C—O stretch at 1233.7cm-1, also
consistent with ethyl acetate.
The second distillate was found to have a boiling point range of 107oC to 109oC (Figure
1). The 1H NMR spectrum consisted of a distinct triplet that integrated to three hydrogens at 0.93
ppm, indicative of a compound containing a terminal methyl group next to a secondary carbon. It
also contained two multuplets at 1.54 ppm and 1.38 ppm, a singlet at 3.23 ppm, and another
triplet at 3.60 ppm, all indicative of the compound 1-butanol. Contamination from ethyl acetate
(4) was seen as a quartet at 4.12 ppm, a singlet at 2.05 ppm, and a triplet at 1.26 ppm. Relative
integration values showed that the ratio of 1-butanol to ethyl acetate was 6:1. The IR spectrum
indicated an O—H stretch at 3327.8cm-1. These spectroscopic results override the boiling point
data, which was close enough to be consistent with 1-butanol’s true boiling point of 118 oC, and
are consistent with what one would expect of the compound 1-butanol (5).
As can be seen in Figure One, Fractional distillation allowed for an effective method of
separation for the two unknowns. Plateaus at 77oC and 109oC indicate the separate boiling points
of ethyl acetate (4) and 1-butanol (5). A sharp spike in temperature after 53 drops of liquid
denotes the change in distillate from the first unknown to the second unknown. The clarity of this
curve denotes the effectiveness of fractional distillation.
O
O
OH
5
4
Figure One. Distillation Curve for Fractional Distillation
120
Temperature oC
110
100
90
80
70
60
0
10
20
30
40
50
60
70
80
Number of Drops
3
Conclusion
The synthesis of hexyl acetate was successful, as spectroscopic data indicated successful
formation of the product with little contamination from the reactants. The Dean-Stark apparatus
which was used to push the chemical equilibrium to the products was instrumental in obtaining a
good percent yield of hexyl acetate.
The fractional distillation of the two component solution of fruit esters was successful, as
a combination of boiling point data and spectroscopic analyses allowed for identification of the
unknown compounds. The two components were identified as ethyl acetate and 1-butanol.
Experimental
General Methods
All compounds were purchased by the course instructor for CHEM 213H, and were used
by the student without further purification. 1H NMR spectra were run on a 60 MHz spectrometer,
and then on a 400 MHz Bruker AVANCE spectrometer if further clarity was needed. IR spectra
were collected on a Thermo Nicolet 380 FT-IR. Melting point ranges were found using a
thermometer during the fractional distillation.
Hexyl Acetate (1)
Dowex 50 x 2-100 ion exchange resin (0.200g) was washed with distilled water and
filtered in a vacuum filtration apparatus. Glacial acetic acid (0.6mL, 10.48 mmol) and 1-hexanol
(1.3mL, 10.35 mmol) were added to the resin, and the reaction mixture was inserted into a sand
bath and hooked up to a Dean-Stark apparatus. The Varistat was set to 50, and the reaction was
allowed to heat to reflux for 40 minutes. The resulting mixture was clear in color, with the solid
orange resin. The product was filtered to remove the resin, producing a clear liquid that smelled
strongly of sour apples. FT-IR 2928.9, 1736.2, and 1236.2 cm-1; 1H NMR (CDCl3, 400 MHz)
4.07 (t, J =6.8 Hz, 1H), 3.62 (t, J = 6.7 Hz, 1H), 2.03 (s, 3H), 1.60 (m, 2H), 1.38 (m, 6H), 1.1
(t, J = 6.5 Hz, 3H).
Acknowledgements
The author wishes to thank Katherine Masters, the course administrator for CHEM 213H, as well
as Zachary Reitz who assisted with the editing of the lab guide. David Juang is also owed credit
for the development of this agricultural sciences module. The author also wishes to thank Jerry
Feng, the teaching assistant for her laboratory section.
References
1. Masters, K. M. Chem 213H Lab Guide, Spring 2013 Edition
Supporting Information
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Annotated Spectral Data for Hexyl Acetate
10
Annotated Spectral Data of Ethyl Acetate
11
Annotated Spectral Data for 1-butanol
12