CHEM 232 Grossmont College Directive Effects in the Bromination of Vanillin Lab Report

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Experiment 35
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Directive Effects in the Bromination of Vanillin
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Directive Effects in the
Bromination of Vanillin
EXPERIMENT
Reactions of Aromatic Compounds. Preparation of Aryl Bromides. Electrophilic
Aromatic Substitution. Directive Effects.
Operations
OP-10
OP-16
OP-26
OP-28
OP-33
Mixing
Vacuum Filtration
Washing and Drying Solids
Recrystallization
Melting Point
Before You Begin
1. Read the experiment, read or review the operations as necessary, and write
an experimental plan.
2. Calculate the mass of 10.0 mmol (SS) or 2.00 mmol (␮S) of vanillin, and
the theoretical yield of bromovanillin from this amount of vanillin.
Scenario
Pulpchem Inc., a subsidiary of a large paper company, produces useful
chemicals from lignin and other by-products of paper production. Chemical
treatment of lignin can yield vanillin, a white solid that is responsible for the
characteristic aroma of vanilla extract. The product development manager
of Pulpchem, Woody Aspin, wants to develop new products from vanillin
that might be marketed commercially. He has heard of a new way to brominate aromatic compounds on the benzene ring using hydrobromic acid and
potassium bromate rather than hazardous liquid bromine, but he doesn’t
know what products to expect from the reaction. If the ortho-para directing
methoxyl group of vanillin controls product formation, the product could
be 2-bromovanillin, 6-bromovanillin, or a mixture of the two. But, if the
hydroxyl group and the meta-directing aldehyde group win out, the product
should be 5-bromovanillin.
CHO
CHO
CHO
CHO
Br
Br
OCH3
OH
vanillin
OCH3
OH
2-bromovanillin
Br
OCH3
OCH3
OH
5-bromovanillin
OH
6-bromovanillin
Your assignment, and that of your coworkers, is to carry out the bromination of vanillin and identify the product.
35
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Part II
Correlated Laboratory Experiments
Applying Scientific Methodology
Your working hypothesis should involve a prediction of the product (or
products) you think the bromination of vanillin should yield. You will test
your hypothesis by measuring the melting point of the product.
The Flavor of Vanilla
CHO
OCH2CH3
OH
bourbonal
The essay in Experiment 29 describes
several ways of synthesizing vanillin.
Most good cooks have a bottle labeled “pure vanilla extract” in the cupboard. It contains a brown liquid with the delightful scent and flavor that
we associate with vanilla ice cream, vanilla pudding, cream soda, and
many baked goods. Although cheaper forms of vanilla may consist mainly
of vanillin synthesized from guaiacol or wood pulp, the real thing comes
from the seedpod of a tropical orchid. One of these pods—often called a
vanilla bean—looks much like a long (12–15 cm) green bean and has virtually no odor. The characteristic vanilla odor develops during a curing
process in which sugar derivatives called glycosides are broken down into
vanillin and other flavorful substances. Cured vanilla beans typically contain more than 400 such components. Vanilla extract is made by shredding
the beans into small pieces, typically in a machine that works like a giant
blender, and soaking the pieces in successive quantities of hot 65–70%
ethanol.
Vanilla orchids were probably first discovered and used in southeastern
Mexico more than 1000 years ago. By the time Spanish conquistadores
discovered Mexico’s Aztec Empire in the 1500s, Aztecs were flavoring a
cocoa-containing beverage called xocolatl (chocolatl in Spanish) with honey
and vanilla. Vanilla has long had a reputation as an aphrodisiac; the Aztec
emperor Montezuma drank xocolatl from a golden goblet before visiting his
wives, and in Europe newly married men were once advised to drink beverages flavored with vanilla.There is some scientific support for such practices;
recent tests at the Institute for Smell and Taste in Chicago showed that the
aroma of vanilla is a powerful stimulant to men.
Today, about half of the world’s vanilla is grown in the tropical forests
of Madagascar, with smaller amounts from Indonesia, Mexico, Tahiti, and
the Comoro Islands. Bourbon vanilla, which is usually considered to be the
world’s finest, is grown in Madagascar, the Comoros, and the nearby island
of Reunion, which was once ruled by the Bourbon kings of France. In addition to vanillin, Bourbon vanilla contains the flavor ingredient bourbonal,
an ethyl homolog of vanillin.
Although vanilla’s dominant flavor ingredient is vanillin, its other components contribute to the rich, complex flavor of a natural vanilla extract.
Many so-called vanilla extracts are actually blends of natural vanilla extract
and synthetic vanillin. Because natural vanilla extract costs more than $500
a kilogram, the cheaper products are mostly synthetic vanillin, and the
blend labeled “imitation vanilla” is made entirely from synthetic substances. Carbon-13 nuclear magnetic resonance (see OP-40b) can be used
to determine the origin of vanillin by measuring the intensities of the 13C
signals for each of its eight carbon atoms and calculating their 13C/12C
ratios. This method can reveal any attempt to fraudulently substitute cheap
synthetic vanillin for the natural substance.
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Directive Effects in the Bromination of Vanillin
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Understanding the Experiment
Aromatic compounds can be brominated on the ring by elemental bromine
(Br2) in the presence of a Lewis acid catalyst such as iron(III) bromide,
FeBr3. But liquid bromine, with its toxic red fumes, is dangerous to inhale
and can cause severe burns. Paul F. Schatz of the University of Wisconsin–
Madison discovered that a mixture of potassium bromate and hydrobromic
acid in acetic acid is an efficient reagent for aromatic bromination. This combination generates bromine in the reaction mixture rather than requiring its
direct addition.
CHO
6
2
5
5HBr + KBrO3 + CH3COOH : 3Br2 + CH3COOK + 3H2O
OCH3
OH
The bromine generated could cause bromination at either the 2, 5, or 6
position of vanillin’s benzene ring. Both the methoxyl (¬ OCH3) and
hydroxyl (¬ OH) substituents are ortho-para directing, while the aldehyde
functional group (¬ CHO) is meta directing. Thus, the —OCH3 group can
direct electrophiles to both the #2 position, which is ortho to it, and the
#6 position, which is para to it, but electrophiles tend to attack less crowded
ring sites. As a result, one of these positions is more likely to be attacked than
the other. (Stop and Think: Which position is it?) The OH group would
tend to direct incoming electrophiles to the #5 position, which is the only
open position that is ortho or para to it. The meta-directing ¬ CHO also
directs to the #5 position, but meta-directors are more weakly directing than
ortho-para directors, so having two substituents directing to the same position doesn’t necessarily mean the electrophile will end up in that position—
a strong ortho-para director could overpower both.
The procedure isn’t difficult and involves only operations that you
should have performed before. Because your conclusion will depend on the
melting point of your product, it is important to make sure that the product
is dry and to measure it melting point accurately.
This is a relatively green experiment because it requires no organic solvents but acetic acid, which occurs naturally in living organisms and readily
breaks down to carbon dioxide and water in the environment. Little information is available about the environmental fate and toxicity of hydrobromic
acid and potassium bromate.
Key Concept: Electron-donating substituents such as ¬OCH3 tend to make
the ortho and para positions of a benzene ring more inviting to incoming
electrophiles than the meta position.
Electron-withdrawing substituents such
as ¬ CHO tend to make the ortho and
para positions less inviting than the meta
position.
Reactions and Properties
CHO
CHO
3
+ CH3COOH + 2HBr + KBrO3
3 Br
+ CH3COOK + 3H2O
OCH3
OCH3
OH
vanillin
OH
?-bromovanillin
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Correlated Laboratory Experiments
Table 35.1 Physical properties
mol wt
mp
152.15
231.0
231.0
231.5
60.05
167.0
80.9
81–83
155
166
178
vanillin
2-bromovanillin
5-bromovanillin
6-bromovanillin
acetic acid
potassium bromate
48% hydrobromic acid
d
1.049
3.27
1.490
‘ 350
Note: mp is in °C; density is in g/mL.
DIRECTIONS
Safety Notes
Acetic acid causes chemical burns that can seriously damage skin and
eyes; its vapors are highly irritating to the eyes and respiratory tract.
Wear gloves, dispense under a hood, avoid contact, and do not breathe its
vapors.
Hydrobromic acid is toxic and very corrosive, and it can cause very serious
damage to the skin, eyes, and respiratory tract. Wear gloves, and dispense
under a hood. Avoid contact with the acid, and do not inhale its vapors.
Potassium bromate is toxic; avoid ingestion.
2
2
0
1
3
2
acetic acid
0
0
hydrobromic acid
OX
1
0
potassium bromate
Standard Scale
Take Care! Wear gloves, avoid
contact with hydrobromic acid, and
do not breathe its vapors.
Reaction. Under the hood, dissolve 10.0 mmol of vanillin in 20 mL of
acetic acid in a 50-mL Erlenmeyer flask. Add 0.75 g of potassium bromate,
followed by 2.0 mL of 48% hydrobromic acid. (Observe and Note: What
happens?) Stir [OP-10] the reaction mixture at room temperature for
45 minutes. Pour the mixture into an Erlenmeyer flask containing 150 mL of
ice-cold water, and continue to stir for 15 to 20 minutes. If the liquid is an
orange color, add 30% sodium thiosulfate solution, drop by drop with stirring, until it turns yellow.
Separation. Collect the product by vacuum filtration [OP-16], washing it
on the filter [OP-26a] with ice-cold water.
Waste Disposal: Unless your instructor directs otherwise, all filtrates can
be washed down the drain.
Purification and Analysis. Recrystallize [OP-28] the product from 50%
ethanol/water. Dry [OP-26b] the product thoroughly in a desiccator until the
next lab period. Measure its mass and melting point [OP-33], and name it.
Microscale
Reaction. Under the hood, dissolve 2.00 mmol of vanillin in 4.0 mL of
acetic acid in a 10-mL Erlenmeyer flask. Add 0.15 g of potassium bromate,
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Directive Effects in the Bromination of Vanillin
followed by 0.40 mL of 48% hydrobromic acid. (Observe and Note: What
happens?) Stir [OP-10] the reaction mixture at room temperature for
45 minutes. Pour the mixture into an Erlenmeyer flask containing 30 mL of
ice-cold water, and continue to stir for 15 to 20 minutes. If the liquid is an
orange color, add 30% sodium thiosulfate solution, drop by drop with
stirring, until it turns yellow.
315
Take Care! Wear gloves, avoid
contact with hydrobromic acid, and
do not breathe its vapors
Separation. Collect the product by vacuum filtration [OP-16], washing it
on the filter [OP-26a] with ice-cold water. Use a small Buchner funnel, if
one is available.
Purification and Analysis. Recrystallize [OP-28] the product from 50%
ethanol/water. Dry [OP-26b] the product thoroughly in a desiccator until the
next lab period. Measure its mass and melting point [OP-33], and name it.
Waste Disposal: Unless your instructor directs otherwise, all filtrates
can be washed down the drain.
Exercises
1. Explain why the major product of the reaction was the one you obtained,
rather than either of the other two possible products.
2. What did you observe immediately after you added hydrobromic acid to
the reaction mixture? Write a chemical equation that explains this result.
3. Describe and explain the possible effect on your results of the following experimental errors or variations. (a) The lab assistant set out a
bottle of potassium bromide rather than potassium bromate. (b) You
used anisaldehyde (4-methoxybenzaldehyde) rather than vanillin.
4. (a) 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. Following the format in Appendix V, construct a flow diagram for the
experiment.
6. Write a complete mechanism for the reaction you carried out.
7. Show how vanillin is synthesized commercially from guaiacol.
Other Things You Can Do
(Starred items require your instructor’s permission.)
*1. Use a different kind of bromination reaction to determine the relative
stabilities of free radicals in Minilab 21.
2. Record a 13C NMR spectrum of vanillin, and identify as many of the
signals as you can.
3. Starting with sources listed in the Bibliography, write a research paper
about artificial flavorings and their constituents.
OH
OCH3
guaiacol
Experiment 35: Directive Effects in the Bromination of Vanillin (from Lehman text, 2nd
edition)
Read the entire lab experiment from the Lehman text (pages 311 – 315) to obtain a more
complete understanding of Experiment 35 (Note: The full experiment is provided on Canvas).
The experimental data and results below are based on the Standard Scale procedure.
Data and Observations for Procedure:
Reagent masses used:
Vanillin = 1.52 g
Acetic acid = 20.0 mL
Potassium bromate = 0.75 g
48% hydrobromic acid = 2.00 mL
30% sodium thiosulfate = dropwise as needed
Observations:
The reaction mixture turned a red-brown color after the addition of hydrobromic acid.
The reaction mixture was slightly orange at the end of the reaction period, so a few drops of
sodium thiosulfate were added until the mixture turned yellow.
Results:
Final mass of the product: 1.95 g
Melting point: 164.2 – 165.6 oC
Lab Report Instructions:
1. Title Page (This section should contain the experiment title, your name, and chemistry course
number)
2. Analysis:
•
Calculate the theoretical yield and % yield of the product using the data and results given
above (Show calculations).
•
Provide answers to Exercises 1, 2, 3b and 6 from Experiment 35 in the Lehman text.
*Note: Your report should be uploaded to Canvas in pdf or Microsoft Word format.