SDSU The Structure of A Natural Product in Anise Oil Lab Report

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Experiment 38
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Determination of the Structure of a Natural Product in Anise Oil
Determination of
the Structure of a Natural
Product in Anise Oil
Reactions of Alkenylbenzenes. Preparation of Carboxylic Acids. Side-Chain
Oxidation. Structure Determination. Infrared Spectrometry.
Operations
OP-7
OP-10
OP-16
OP-26
OP-28
OP-33
OP-39
Heating
Mixing
Vacuum Filtration
Washing and Drying Solids
Recrystallization
Melting Point
Infrared Spectrometry
Before You Begin
Read the experiment, read or review the operations as necessary, and write
an experimental plan.
Scenario
Basil Wormwood, the new-age herbalist with a chemistry degree, has another
puzzle for you (see Experiment 18 for his previous puzzles). He obtained
some Chinese star anise from an oriental-foods wholesaler, steam-distilled
its essential oil, and isolated the major component of the oil. Not knowing
its identity, he tentatively named this compound anisene. He sent it off to
a chemical analyst for elemental analysis, and from the results found its
molecular formula to be C10H12O. He also carried out some experiments
(described next) showing that the compound contains a methoxyl group
and a three-carbon side chain on a benzene ring, but he doesn’t know the
identity of the side chain or where it is located with respect to the methoxyl
group. He has just shipped a sample of the compound to your supervisor,
hoping that your institute’s consulting chemists can solve this structure
puzzle. Your supervisor thinks that the position of the side chain can be
determined by oxidizing it to a COOH group, and that its structure can
be determined by infrared (IR) analysis. Your assignment is to determine
the complete structure of anisene.
Applying Scientific Methodology
You will have to carry out some experimental work before you can propose
a hypothesis about the structure of anisene.
333
EXPERIMENT
38
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Correlated Laboratory Experiments
The Structure Puzzle—Taking Molecules Apart
and Putting Them Back Together
star anise seed clusters
Key Concept: Chemists determine the
structures of organic molecules by
breaking them down into smaller fragments and identifying the fragments,
probing them with different kinds of
electromagnetic radiation and interpreting the resulting signals, or both.
Chinese star anise (Illicium verum) is a small evergreen tree of the magnolia family. When its dried, star-shaped seed clusters are ground up and
steam distilled, they yield an oily liquid with a strong odor of licorice.
Anise oil (from star anise and other spices) or its synthetic equivalent is
widely used as a flavoring for licorice, cough drops, chewing gum, and
liqueurs such as ouzo and anisette. In this experiment, you will use both
classical and modern methods of structural analysis to determine the complete structure of its major component, which we will call “anisene” (not its
real name).
Today, when a chemist can run an NMR spectrum or a mass spectrum
of an organic compound and often determine its structure in a matter of
minutes, it is hard to imagine how much time and effort were once required
to determine the structures of even the simpler natural products. In a classical structure determination, the molecular formula of a compound is first
obtained by elemental analysis and molecular-weight measurement. Then
the compound is degraded (broken down) into smaller structural units that
are isolated and, if possible, identified. Finding how the smaller units fit together to form the original molecule is an intellectual challenge that might
be compared to putting together a jigsaw puzzle with some pieces missing,
others that don’t belong, and still others that have been chewed up by the
family dog and are no longer recognizable. Finally, when enough information has been gathered to suggest a possible structure, that structure must
usually be proven by an independent synthesis in which the compound is
built up again, from known compounds, by reactions whose outcomes can be
reliably predicted.
In many cases, classical structure determinations involved the efforts of
dozens or even hundreds of chemists over many decades, and included the
generation of much irrelevant or misleading information and many synthetic
dead ends. The advent of modern spectrometric methods has simplified
the process enormously by providing detailed structural information that
wasn’t readily available to the chemists of earlier times.
Understanding the Experiment
In this experiment, you will attempt to determine the structure of the major
component of star anise oil, which has the molecular formula C10H12O.
Most open-chain saturated organic compounds (except those containing
nitrogen, phosphorus, or halogen atoms) have 2n + 2 hydrogen atoms for
every n carbon atoms. If anisene were such a compound, it would have
2(10) + 2 = 22 hydrogen atoms, but since it has only 12, it is said to be
“deficient” by 10 hydrogens. Every ring or pi bond in a molecule represents
a deficit of two hydrogens. That is, an open-chain compound must lose
two hydrogen atoms to form a ring, and a saturated compound must lose
two hydrogens to form a pi bond (or a pi-bond equivalent in the Kekulé
structure of an aromatic ring). Thus, its deficiency of 10 hydrogens indicates
that there must be a total of five rings and pi bonds (or pi-bond equivalents)
in an anisene molecule; this is called its index of hydrogen deficiency (IHD).
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The IHD of a compound that has n carbon atoms and x hydrogen atoms can
be calculated using the following formula:
(2n + 2) – x
IHD =
2
Catalytic hydrogenation of anisene under high pressure yields a saturated compound with the formula C10H20O. The gain of eight hydrogens
indicates that anisene has four pi bonds, so it must contain only one ring. A
high carbon/hydrogen ratio often indicates an aromatic structure, and we
can account for the ring and three pi bonds by assuming that anisene contains a benzene ring.
Heating anisene with hydriodic acid yields a phenol with the molecular
formula C9H9OH and a volatile compound identified as methyl iodide. This
reaction is used to test for certain ether functions. Methyl ethers yield
methyl iodide, and the formation of a phenol indicates that anisene is an aryl
methyl ether, whose formula we write as C9H9OCH3 in the following equation for the reaction:
C9H9OCH3 + HI ¡ C9 H9OH + CH3 I
At this point, we know that anisene contains a methoxyl 1 ¬ OCH32 group
and a benzene ring, which accounts for seven carbon atoms and three
pi bonds. That leaves three more carbons and one pi bond to be accounted
for.This remaining fragment could be a three-carbon unsaturated side chain,
whose formula can be determined by subtracting the fragments already
identified from the molecular formula of anisene:
molecular formula
disubstituted benzene ring
methoxyl group
side chain
C10 H12 O
-C6 H4
-CH3 O
C3 H5
Now we can write a partial structure for anisene, shown in the margin. All
that remains is to determine the structure of the unsaturated side chain and
its location on the benzene ring.
Potassium permanganate is capable of oxidizing most aliphatic side
chains all the way down to the benzylic carbon atom, leaving a COOH
group where the side chain was originally located. Oxidizing anisene should
yield one of three possible methoxybenzoic acids, whose melting points are
given in Table 38.3. By identifying the oxidation product as one of these
three, you will establish the position of anisene’s side chain.
Although aqueous potassium permanganate is a powerful oxidizing
agent, it reacts slowly with water-insoluble organic compounds because
KMnO4 is essentially insoluble in the organic phase. In 1974, Herriot and
Picker added a quaternary ammonium salt to a stirred heterogeneous mixture of aqueous KMnO4 and benzene, which caused permanganate ions to
dissolve in the organic layer and form “purple benzene.” The quaternary salt
acted as a phase-transfer catalyst, escorting the permanganate ions across
the phase boundary into the organic phase (see Experiment 24 for a discussion of phase-transfer catalysis). When an oxidizable organic compound is
dissolved in purple benzene, it reacts much more rapidly and under milder
conditions than it would with aqueous KMnO4.
Benzene is toxic and can cause leukemia in humans, so you will use
a simplified procedure in which anisene and a phase-transfer catalyst
Another possibility, that anisene has two
side chains, is explored in Exercise 3.
OCH3
C3H5
partial structure of anisene
OCH3
COOH
a methoxybenzoic acid
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Correlated Laboratory Experiments
(tricaprylmethylammonium chloride) are combined directly with aqueous
potassium permanganate.Thus, anisene itself will be the organic phase of the
two-phase system, and no organic solvent is needed. Because you require
only enough product for a melting point, you will start with only a few drops
of anisene. Excess potassium permanganate is used because some of it may
decompose during the reaction.As the reaction proceeds, permanganate ion
is reduced to manganese dioxide, which forms a fine brown precipitate that
is difficult to filter and wash. Fortunately, this precipitate can be dissolved
during the workup by acidifying the solution and adding sodium bisulfite,
which reduces manganese dioxide (and any unreacted permanganate ion) to
soluble manganese(II) sulfate.
Removal of manganese dioxide
MnO2 + NaHSO3 + H+ ¡ MnSO4 + H2O + Na+
The methoxybenzoic acid can then be separated by vacuum filtration and
purified by recrystallization from water.
Carbon–carbon double bonds give rise to characteristic “ C ¬ H outof-plane bending bands in the 1000 – 650 cm-1 region of an IR spectrum.
The wave numbers of these bands can reveal the number and location of
substituents on the carbon–carbon double bond, as shown in Table 38.1.
There are four possible structures for an unsaturated C3H5 side chain, corresponding to the four structure types in the table. From the wave number(s) of anisene’s “ C ¬ H bending band(s), you should be able to deduce
the structure of the side chain. First, you must locate the right absorption
bands, which is more easily said than done because aromatic C ¬ H bonds
give rise to strong bands in the same region (as shown in Table 38.2).
The carbon atom of a vinylic C ¬ H
bond is doubly bonded to an adjacent
carbon.
Table 38.1 Out-of-plane bending vibrations
of vinylic C ¬ H bonds
Structure type
Frequency range, cm-1
RCH “ CH2
RCH “ CHR (cis)
RCH “ CHR (trans)
R2 C “ CH2
995–985 and 915–905
730–665
980–960
895–885
Note: R = alkyl or aryl.
Table 38.2 Out-of-plane bending vibrations
of aromatic C ¬ H bonds
Ring substitution
Frequency range, cm-1
ortho
meta
para
770–735
810–750 and 710–690
840–810
Once you learn the position of the side chain on anisene’s benzene ring, you
should be able to locate any bands due to aromatic C ¬ H bonds in its
infrared spectrum, which will help you pick out one or more vinylic C ¬ H
bands from the remaining strong bands in the 1000 – 650 cm-1 region.
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This is a relatively green experiment in that no organic solvents are
used for the synthesis. Potassium permanganate and manganese dioxide are
harmful to aquatic life, but the by-product MnO2 and any excess KMnO4
are converted to manganese(II) sulfate, which is not known to be a serious
environmental contaminant.
Reactions and Properties
OCH3
OCH3
KMnO4, Q+
COO–K+
C3H5
OCH3
OCH3
+ HCl
+ KCl
COO–K+
COOH
+
Q+ ⫽ [CH3(CH2)7]3NCH3
Table 38.3 Physical properties
potassium permanganate
o-methoxybenzoic acid
m-methoxybenzoic acid
p-methoxybenzoic acid
toluene
mol wt
mp
bp
d
158.0
152.2
152.2
152.2
92.2
101
110
185
-95
111
0.867
Note: mp and bp are in °C; density is in g/mL.
DIRECTIONS
Potassium permanganate can react violently with oxidizable materials;
keep it away from other chemicals and combustibles.
Sodium bisulfite produces harmful vapors when it reacts with acids; do
not breathe them.
Safety Notes
0
2
Standard Scale and Microscale
Reaction. Obtain some anisene (or anise oil) from your instructor, or isolate anise oil from anise seeds as described in “Other Things You Can Do.”
Add 0.50 g of crystalline potassium permanganate and 2 drops of tricaprylmethylammonium chloride (Aliquat 336) to 10 mL of water in a 25-mL
OX
2
potassium
permanganate
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Part II
Take Care! Keep KMnO4 away from
oxidizable materials.
Erlenmeyer flask, and then drop in a stir bar. Heat [OP-7] the mixture in a
boiling-water bath, while stirring [OP-10], for 5 minutes or more to dissolve most of the KMnO4. Add 5 drops of anisene or anise oil using a
medicine dropper (not a Pasteur pipet), place a watch glass (convex side
down) over the mouth of the flask to prevent evaporation, and heat the
mixture in a boiling-water bath—with vigorous magnetic stirring—for
15 minutes or more. (Alternatively, the flask can be swirled and shaken
vigorously over a steam bath for 15 minutes.)
Take Care! Do not breathe the
vapors that may be produced.
Stop and Think: What vapors may
form, and how are they produced?
What are the brown and white
precipitates?
Waste Disposal: Place the filtrate
in a designated waste container.
Correlated Laboratory Experiments
Separation. Cool the reaction mixture to room temperature, and transfer it to a small beaker. Under the hood, add 1 mL of 6 M hydrochloric acid
and test the solution with blue litmus paper; if it is not acidic, add more HCl
until it is. Add just enough solid sodium bisulfite, in small portions while
stirring or swirling, to reduce any excess permanganate and remove the
brown manganese dioxide (0.5–1.0 g of NaHSO3 should be sufficient). Test
the solution with pH paper after each bisulfite addition, and add 6 M HCl as
needed to keep it acidic. When all of the brown precipitate has disappeared
and only a white precipitate remains, again test the solution with pH paper.
If the pH is higher than 2, add enough 6 M HCl to reduce it to 2. Collect the
product from the reaction mixture by vacuum filtration [OP-16], and wash
it on the filter [OP-26a] with ice-cold water.
Purification and Analysis. Recrystallize [OP-28] the product from boiling water, and dry [OP-26b] it to constant mass. Measure the melting point
[OP-33] of the methoxybenzoic acid. Record the IR spectrum [OP-39] of
anisene (not of the methoxybenzoic acid), or obtain its spectrum from your
instructor. Deduce the location and structure of the side chain, and draw
the structure of anisene. Turn in the IR spectrum with your report.
Exercises
1. Derive a systematic name for anisene and find its common name in The
Merck Index or another reference book.
2. (a) Write a balanced equation for the reaction of anisene with potassium permanganate, assuming that the products include manganese dioxide and the potassium salt of acetic acid. (Note: The reaction mixture is
alkaline.) (b) Assuming that 5 drops of anisene is about 1.0 mmol, calculate the mass of potassium permanganate required to oxidize that
much anisene and the percentage in excess that was actually used.
3. (a) The three carbon atoms of anisene’s side chain might have formed
two separate side chains rather than one. Give the structures of these
side chains. (b) Give the structures of all of the dicarboxylic acids that
could have resulted from complete side-chain oxidation of anisene had
it contained these two side chains.
4. (a) Using a balanced equation for the oxidation reaction (see Exercise 2a), calculate the atom economy and reaction efficiency of your
synthesis. (b) Describe some green features of your synthesis, and any
that aren’t so green.
5. Describe and explain the possible effect on your results of the following
experimental errors or variations. (a) You forgot to add the Aliquat 336.
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Experiment 38
6.
7.
8.
9.
10.
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Determination of the Structure of a Natural Product in Anise Oil
(b) The pH of the reaction mixture was 7 when you filtered it, and you
obtained a brown solid. (c) You recorded the IR spectrum of the oxidation product rather than that of anisene itself.
Describe the probable role of the phase-transfer catalyst in this reaction, giving equations for the relevant reactions.
Following the format in Appendix V, construct a flow diagram for the
synthesis of your methoxybenzoic acid.
(a) Draw the structure of the compound C10H20O that is obtained by
the catalytic hydrogenation of anisene. (b) Draw the structure of the
compound C9H9OH that is obtained when anisene is treated with
hydriodic acid.
You could confirm the structure of anisene by synthesizing it from
known starting materials. Outline a synthesis of anisene from benzene
and alcohols that have four carbon atoms or fewer.
The structure shown has been proposed for coniferyl alcohol, which
can be obtained by the hydrolysis of coniferin, a natural product found
in the sap of conifer trees. Assuming that the structure of coniferyl
alcohol had not been reported in the literature, describe how you
would go about proving its structure. Indicate what chemical tests and
degradations might be carried out, and describe the expected results
and conclusions. Summarize the information that could be derived
from infrared analysis. Then show how the alcohol could be synthesized
from readily available starting materials.
Other Things You Can Do
(Starred items require your instructor’s permission.)
*1. Isolate anise oil from anise seeds (or star anise) as follows. Weigh out
1g (µS) or 10g (SS) of fresh anise seeds and grind them finely, using a
spice grinder or a mortar and pestle. Isolate the anise oil by steam distillation and extraction of the distillate with dichloromethane, following the procedure for clove oil given in Experiment 10. (Do not extract
the dichloromethane layer with NaOH as described in the standard
scale procedure.) Dry the dichloromethane solution, and evaporate
the solvent completely. You can obtain a gas chromatogram of the oil
and estimate the percentage of anisene it contains.
*2. Use air as an oxidizing agent to convert fluorene to fluorenone as
described in Minilab 32.
3. Starting with sources listed in the Bibliography, write a research paper
on the use of chemical methods for structure determination of natural
products. Illustrate it with examples of actual structure determinations.
339
CH
CHCH2OH
OCH3
OH
proposed structure for
coniferyl alcohol
Experiment 38: Determination of the Structure of a Natural Product in Anise Oil (from
Lehman text, 2nd edition)
Read the entire lab experiment from the Lehman text (pages 333 – 339) to obtain a more
complete understanding of Experiment 38 (Note: The full experiment is provided on Canvas).
Data for Procedure:
Reagent masses used:
KMnO4: 0.50 g
Tricaprylmethylammonium chloride (Aliquat 336): 2 drops
D.I. H2O 10.0 mL
Anisene: 5 drops
Results:
Melting point of methoxybenzoic acid product: 184.4 -185.1 oC
The IR spectrum for anisene is provided on Canvas.
Lab Report Instructions:
1. Title Page (This section should contain the experiment title, your name, and chemistry course
number)
2. Analysis:
•
Provide the name and structure of the oxidation product that you produced in the
reaction.
•
Provide a table with important IR peaks for anisene. The table should have two columns
labeled bond type and wavenumber (cm-1). Refer to the IR spectrum for anisene that is
provided on Canvas.
•
Provide the structure of anisene.
•
Provide answers to Exercises 3, 5, and 8 from Experiment 38 in the Lehman text
*Note: Your report should be uploaded to Canvas in pdf or Microsoft Word format.