all original workvideos and questions attached belowassignemt is no longer than 1 hourvideos:https://drive.google.com/file/d/1zCalOEL7BWTcI2mDk…https://drive.google.com/file/d/1aa5M_wC_geFjm4eUl…https://drive.google.com/file/d/1AgYc_IgFNgdREmZhI…M05_LEHM3752_02_SE_C04.QXD
Experiment 4
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Synthesis of Salicylic Acid from Wintergreen Oil
Heating Under Reflux
Synthesis of Salicylic Acid
from Wintergreen Oil
61
EXPERIMENT
4
Preparation and Purification of Solids.
Operations
OP-2
OP-4
OP-7
OP-16
OP-26
OP-28
OP-33
Using Specialized Glassware
Weighing
Heating
Vacuum Filtration
Drying Solids
Recrystallization
Melting Point
Before You Begin
1. Read the experiment and operation OP-2. Read “Smooth Boiling
Devices” and “Heating Under Reflux” in OP-7, and review the other
operations as necessary.
2. Read Appendix IV, “Calculations for Organic Synthesis.” Then calculate
the mass and volume of 10.0 mmol (SS) or 2.00 mmol (µS) of methyl salicylate, the theoretical yield of salicylic acid from that much methyl salicylate, and the minimum volume of water needed to recrystallize that much
salicylic acid.
3. Complete the experimental plan that follows the “Reactions and Properties” section by specifying all sizes and quantities indicated by asterisks in the Chemicals and Supplies list. Note that reaction flasks (pear
shaped and round bottom) come in 10-, 25-, 50-, 100-, 250-, and 500-mL
sizes; conical vials in 3- and 5-mL sizes; Erlenmeyer flasks in 10-, 25-, 50-,
125-, and 250-mL sizes; and beakers in 10-, 20-, 30-, 50-, 150-, 250-, and
400-mL sizes.
Scenario
The new-age pharmaceutical company Natural Nostrums manufactures
drugs from “natural” starting materials. For example, the company manufactures a painkilling drug it advertises as “organic aspirin” starting with
methyl salicylate, which occurs naturally in wintergreen oil. Most commercially marketed aspirin is manufactured starting with benzene, a product of petroleum refining. An intermediate in both of these syntheses is
salicylic acid.
Natural Nostrums claims that its aspirin, which is supposedly more natural than aspirin made from benzene, has fewer side effects than ordinary aspirin. Critics have accused the company of false and misleading advertising,
Be sure to distinguish millimoles (mmol)
from moles (mol); most prelab assignments will specify amounts of chemicals
in millimoles.
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Mastering the Operations
OH
OH
COOH
salicylic
acid
OH
COOCH3
phenol
OH
COOH
methyl
salicylate
OCOCH3
benzene
COOH
OCOCH3
COOH
salicylic
acid
aspirin
aspirin
synthesis of aspirin from
methyl salicylate
synthesis of aspirin
from benzene
asserting that salicylic acid made from methyl salicylate is no different than
salicylic acid made from benzene, and that the resulting aspirin is therefore
no better than any other aspirin.
Les Payne, director of operations for the Association for Safe Pharmaceuticals (ASP), is investigating the company. He just shipped your supervisor a sample of salicylic acid manufactured from benzene and a bottle of
methyl salicylate that one of his agents obtained from the chemical stockroom at Natural Nostrums. Your assignment is to prepare salicylic acid from
this methyl salicylate and find out whether or not it differs from salicylic
acid made from benzene.
Applying Scientific Methodology
As in the previous experiments, you need to state the problem as a question, formulate a working hypothesis, follow the course of action described
in the Directions, gather and evaluate evidence, test your hypothesis, arrive
at a conclusion, and report your findings.
Wintergreen Oil
wintergreen plant
For many years, commercial methyl salicylate was obtained from wintergreen oil, an aromatic liquid distilled from the leaves of the wintergreen
plant (Gaultheria procumbens) or from the bark of sweet birch trees
(Betula lenta). When cheap raw materials became available from petroleum, a more economical commercial process was developed for synthesizing methyl salicylate from salicylic acid and methanol (CH3OH, methyl
alcohol).
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Synthesis of Salicylic Acid from Wintergreen Oil
OH
63
OH
COOH
COOCH3
CH3OH
salicylic
acid (from
benzene)
methyl
salicylate
Regardless of its source, methyl salicylate is methyl salicylate. There is no
difference between its pure natural and synthetic forms because both are
composed of the same kind of molecules and must therefore have the same
properties. Any differences between wintergreen oil obtained from natural
sources and synthetic methyl salicylate are due mainly to impurities in the
natural oil.
OH
OH
COOCH3
natural
methyl
salicylate
COOCH3
synthetic
methyl
salicylate
Methyl salicylate, both natural and synthetic, has been used for many
years as a flavoring agent because of its pleasant penetrating odor and flavor. According to Euell Gibbons, author of Stalking the Healthful Herbs and
other books about edible and medicinal wild plants, you can prepare a good
wintergreen tea by pouring boiling water into a jar filled with freshly picked
wintergreen leaves and letting it steep overnight or longer. The distinctive
flavor of wintergreen is found in a variety of commercial products, including candy, chewing gum, root beer, and toothpaste. Medicinally, wintergreen
oil has some of the painkilling properties of other salicylates, a group of
related organic compounds that includes salicylic acid and aspirin. When
absorbed through the skin, it produces an astringent but soothing sensation,
and it is frequently used in preparations such as Bengay to alleviate muscular aches and arthritis.
Understanding the Experiment
In this experiment, you will be carrying out an organic synthesis, the preparation of salicylic acid from methyl salicylate. An organic synthesis can be
described as the preparation of a desired organic compound by chemically
modifying the molecules of another organic compound, which is called the
starting material. Until 1874, all commercial salicylic acid was synthesized
from natural wintergreen oil. You will reproduce this nineteenth-century
synthesis of salicylic acid starting with methyl salicylate, the major constituent of wintergreen oil.
A natural substance is one obtained
directly from plant, animal, or mineral
sources. A synthetic substance is one
prepared by chemically altering other
substances, which may themselves be
either natural or synthetic.
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Part I
Mastering the Operations
Organic reactions are slower than most inorganic reactions. For example, when aqueous solutions of silver nitrate and sodium chloride are mixed,
the resulting reaction is almost instantaneous, taking place as rapidly as
silver and chloride ions can come together to form silver chloride.
Ag+ + Cl- ¡ AgCl
Key Concept: A mechanism is a stepby-step description of a chemical reaction, showing what bonds are broken
and formed as reactant molecules are
converted to product molecules.
In an organic reaction, the reacting molecules may come together as frequently as the ions in a precipitation reaction, but they may not collide with
enough energy or in the right orientation to react. An ion-combination reaction has such a low activation energy that ions need only “stick together”
to form a product. Covalent molecules must ordinarily undergo a process of
bond breaking and bond formation to be converted to product molecules,
and such processes have relatively high activation energies.
Some of the bond-breaking and bond-forming steps involved in this
experiment’s synthesis are shown in the following mechanism for the conversion of methyl salicylate to salicylic acid.
:O–
O
C
O–
–
:OH
OCH3
C
O–
O
OH
OCH3
O–
C OH
–
:OCH3
Mechanism for the reaction. Note that the ¬ OH group of methyl
salicylate is ionized at the high pH of the reaction mixture.
Completion implies that as many reactant molecules as possible have been
converted to product molecules under
the conditions of the reaction.
See Appendix IV for a discussion of
limiting reactants.
According to this mechanism, the formation of a bond between a hydroxide ion and a carbonyl (C “ O) carbon atom is accompanied by the
breaking of a pi bond in the C “ O group. That pi bond is then re-formed
while a bond to an ¬ OCH3 group is broken.
The reaction, then, is the crucial step in an organic synthesis—it may be
over in a few minutes or require a few weeks, but enough time should be
allowed to bring the reaction as close to completion as possible before
proceeding to the next steps. These steps have already been described in
Experiments 2 and 3; the desired product must be separated from the reaction mixture, purified to remove residual contaminants, and analyzed to
verify its identity and purity.
Methyl salicylate is the limiting reactant in this synthesis; sodium hydroxide is used in excess to ensure a reasonably fast, complete reaction. It is
best to measure limiting reactants that are liquids by mass, because the
mass of a liquid can be measured more accurately than its volume. To avoid
waste and minimize spillage at the balance, you should dispense the estimated volume of liquid from the stock bottle before you take it to the balance to weigh it. This is why you were asked (in “Before You Begin”) to
calculate both the mass and volume of methyl salicylate required. If the
mass of the methyl salicylate is not within about 5% of your calculated
value, you can add or remove liquid with a Pasteur pipet.
You will carry out the reaction by boiling a mixture of methyl salicylate and sodium hydroxide in either a round-bottom flask or a conical
vial equipped with a condenser. A heating mantle or other flameless heat
source capable of boiling water (not a steam bath) is needed for the standard scale reaction; an aluminum block or sand bath is suitable for the
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65
microscale reaction. The purpose of the condenser is to convert water and
methyl salicylate vapors back to liquids and return them to the reaction
flask so that they don’t boil away.
The product of the initial reaction will be a salt of salicylic acid—disodium
salicylate—and not salicylic acid itself. Adding 3 M sulfuric acid will then
precipitate salicylic acid. When you recover the product by vacuum filtration, remember that the solvent in the reaction mixture is water, so use cold
water (not 3 M sulfuric acid) to wash the product on the filter. The solubility
of salicylic acid in water is about 0.18 g兾100 mL at 20°C and 6.7 g兾100 mL at
the boiling point, so it can be purified by recrystallization from water. The
melting point of your product should give you a good indication of its purity;
the melting point of pure salicylic acid is reported to be 159°C. You will also
carry out a mixture melting point (see OP-33) with the benzene-derived
salicylic acid provided by your instructor to see whether it is identical to or
different from your own.
This is a relatively green synthesis because the reactants and products
(in the concentrations used) are not highly toxic, the only solvent used is
water, and the overall reaction has a high atom economy. Both sodium
hydroxide and sulfuric acid may be harmful to aqueous life, and sulfuric
acid is classified as a hazardous air pollutant, but small-scale release of
either chemical should not be particularly harmful.
To help you organize your time efficiently, an experimental plan for the
preparation of salicylic acid is provided. You will need to complete the plan
as directed in “Before You Begin.” For subsequent experiments, you should
write your own experimental plans after reading Appendix V. Note that the
30-minute reaction period should be used to collect the supplies and assemble the apparatus needed for upcoming operations.
Reactions and Properties
ONa
OH
COONa
COOCH3
+ 2NaOH
methyl salicylate
+ CH3OH + H2O
disodium salicylate
OH
ONa
COOH
COONa
+ H2SO4
+ Na2SO4
salicylic acid
Table 4.1 Physical properties
methyl salicylate
salicylic acid
mol wt
mp
bp
d
152.1
138.1
-8
159
223
1.174
Note: Melting points and boiling points are in °C; density (d) is in g/mL.
Stop and Think: Why? (Consider
the pH of the reaction mixture.)
A Greener Way: You can use natural
wintergreen oil as the starting material,
in place of synthetic methyl salicylate.
The natural oil can be obtained from
suppliers of essential oils, such as natural food stores, as well as certain chemical suppliers.
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Mastering the Operations
Experimental Plan
Chemicals and Supplies Needed
[An asterisk (*) indicates a number (size or quantity) to be filled in. Standard
scale and microscale alternatives are designated by (SS) and (µS), respectively.]
Heating under reflux [OP-7]
* g (* mL) of methyl salicylate (avoid contact, inhalation)
* mL of 6 M NaOH (wear gloves, avoid contact)
*-mL round-bottom flask (SS) or conical vial (µS)
heat source (specify type), condenser (specify type), boiling chip,
ring stand, clamps
Addition of sulfuric acid
* mL of 3 M H2SO4
* -mL beaker
pH paper, stirring rod, Pasteur pipet
Vacuum filtration [OP-16]
* mL of cold water for washing
Buchner funnel (SS) or Hirsch funnel (µS), filter flask, filter paper,
filter trap, spatula, watch glass
Recrystallization [OP-28]
* mL of water for recrystallization
* mL of cold water for washing [OP-26a]
two *-mL Erlenmeyer flasks (SS) or *-cm test tubes (µS),
*-mL graduated cylinder
Buchner funnel (SS) or Hirsch funnel (µS), filter flask, filter paper,
filter trap, spatula, watch glass
Drying [OP-26b] and weighing [OP-4]
drying container, desiccator (or oven), tared vial
Melting points [OP-33]
mp tubes, flat-bottomed stirring rod or spatula, watch glass
Lab Checklist
•
•
•
•
•
•
•
•
•
•
•
•
•
Collect and clean supplies for reflux.
Obtain heat source.
Assemble reflux apparatus.
Measure methyl salicylate and 6 M NaOH solution.
Add reactants and boiling chips to boiling flask, and turn on condenser water.
Heat reactants under reflux for 30 minutes.
Collect supplies for H2SO4 addition (during reflux).
Collect vacuum filtration supplies and assemble apparatus (during reflux).
Collect supplies for recrystallization (during reflux).
Measure 3 M H2SO4 solution (during reflux).
Precipitate product with 3 M H2SO4.
Filter and wash product; air-dry on filter.
Measure and boil water for recrystallization.
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Experiment 4
•
•
•
•
•
•
•
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67
Recrystallize product from boiling water.
Filter and wash product; air-dry on filter.
Dry product.
Weigh product.
Measure melting point.
Turn in product.
Clean up.
DIRECTIONS
Concentrated sodium hydroxide solutions are very corrosive and can
cause severe damage to skin and eyes. Wear protective gloves and goggles,
and avoid contact with the 6 M NaOH. (Always wear your safety goggles in
the organic chemistry lab!)
Methyl salicylate can irritate the eyes and skin; avoid contact and inhalation.
Safety Notes
Fire
Reactivity
0
4
1
1
sodium hydroxide
Contact
Vapor
toxicity
2
0
1
methyl
salicylate
Standard Scale
Reaction. Obtain an appropriate heat source and position it correctly.
Assemble an apparatus for heating under reflux [OP-7], using standardtaper glassware [OP-2] if available. Make sure the apparatus is clamped securely to a ring stand. Measure 10.0 mmol of methyl salicylate into the
boiling flask, followed by 15 mL of 6 M sodium hydroxide and a few boiling
chips. Do not add both reactants through the same funnel; if you do, a solid
may plug up the funnel stem. Start water flowing in the condenser jacket,
then have the instructor check your apparatus before you begin heating.
Heat the reaction mixture under reflux for 30 minutes, measuring from the
time the solution starts to boil. If, after 30 minutes under reflux, the reaction
mixture is cloudy or contains an oily upper layer, continue heating under
reflux until the cloudiness (or oil) disappears and the reaction mixture no
longer smells like wintergreen.
When the reaction is complete, remove the condenser as soon as it is
cool enough to handle; otherwise, the ground joints might freeze. Remove
the boiling chips, and transfer the reaction mixture to a beaker large enough
to contain it and the sulfuric acid to be added. Slowly add 16 mL of aqueous
3 M sulfuric acid while stirring, and test the pH of the supernatant liquid
(liquid near the surface) using your stirring rod and a strip of pH paper. If the
pH is above 2, add enough additional sulfuric acid—drop by drop—to bring it
down to 2. Cool the mixture in an ice/water bath for about 10 minutes.
Separation. Collect the salicylic acid by vacuum filtration [OP-16] using
a small Buchner funnel, and wash it on the filter with ice-cold water.
Take Care! Wear gloves, and avoid
contact with the NaOH.
Observe and Note: What did you
observe when you combined methyl
salicylate with 6 M NaOH? When
you heated the reaction mixture to
boiling?
Take Care! Wear gloves.
Observe and Note: What happened?
Waste Disposal: Unless your instructor directs otherwise, wash the
filtrate down the drain.
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Part I
Observe and Note: How much
water was required?
Purification and Analysis. Purify the salicylic acid by recrystallization
[OP-28] from boiling water.There should be no need to filter the hot solution.
Collect the product by vacuum filtration, and wash it [OP-26a] with a little
cold water. Dry [OP-26b] the product to constant mass and measure its
mass [OP-4]. Measure the melting points [OP-33] of the dry product and of
a 1:1 mixture of the product with salicylic acid synthesized from benzene. In
your report, include calculations of the theoretical yield of product (based on
the actual mass of your reactant) and the percent yield of your preparation,
and try to account for any significant material losses (see Exercise 4).
Waste Disposal: Unless your instructor directs otherwise, wash the
filtrate down the drain.
Stop and Think: Do the results support your initial hypothesis?
Mastering the Operations
Microscale
Alternatively, you can use a stirrer and
stir bar and omit the boiling chips. See
OP-10.
Take Care! Wear gloves, and avoid
contact with the NaOH.
Observe and Note: What did you
observe when you combined methyl
salicylate with 6 M NaOH? When
you heated the reaction mixture to
boiling?
Take Care! Wear gloves.
Observe and Note: What happened?
Waste Disposal: Unless your instructor directs otherwise, wash the
filtrate down the drain.
Observe and Note: How much
water was required?
Waste Disposal: Unless your instructor directs otherwise, wash the
filtrate down the drain.
Stop and Think: Do the results support your initial hypothesis?
Reaction. Obtain an appropriate heat source and assemble [OP-2] an
apparatus for heating under reflux [OP-7] consisting of an air condenser
attached to a conical reaction vial of appropriate size. Measure 2.00 mmol of
methyl salicylate into the reaction vial; then add 2.5 mL of aqueous 6 M sodium
hydroxide and a boiling chip or two. Heat the reaction mixture under gentle
reflux for 30 minutes, measuring from the time the solution starts to boil. Be
sure that the reflux ring of condensing vapor remains at least 2 cm below the
top of the condenser. If its level goes higher than this, reduce the heating rate.
If, after 30 minutes under reflux, the reaction mixture is cloudy or contains an
oily upper layer, continue heating under reflux until the cloudiness (or oil)
disappears and the reaction mixture no longer smells like wintergreen.
When the reaction is complete, remove the condenser as soon as it is cool
enough to handle; otherwise, the joints might freeze. Remove the boiling
chip(s), and transfer the reaction mixture to a beaker large enough to contain
it and the sulfuric acid to be added. Slowly add 2.7 mL of aqueous 3 M sulfuric acid while stirring, and test the pH of the supernatant liquid (liquid near
the surface) using your stirring rod and a strip of pH paper. If the pH is above
2, add enough additional sulfuric acid—drop by drop—to bring it down to 2.
Cool the mixture in an ice/water bath for five minutes or more.
Separation. Collect the salicylic acid by vacuum filtration [OP-16], and
wash it on the filter [OP-26a] with ice-cold water.
Purification and Analysis. Purify the salicylic acid by recrystallization
[OP-28] from boiling water. There should be no need to filter the hot solution. Collect the product by vacuum filtration, and wash it [OP-26a] with a
little ice-cold water. Dry [OP-26b] the product to constant mass and measure
its mass [OP-4]. Measure the melting points [OP-33] of the dry product and
of a 1:1 mixture of the product with salicylic acid synthesized from benzene.
In your report, include calculations of the theoretical yield of product (based
on the actual mass of your reactant) and the percent yield of your preparation,
and try to account for any significant material losses (see Exercise 4).
Exercises
1. (a) Calculate the volume of 6 M NaOH required to react completely with
the amount of methyl salicylate you used. How much of the 6 M NaOH
that you used was in excess of the theoretical amount? (b) What volume
of 3 M H2SO4 is needed to neutralize all of the disodium salicylate and
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Experiment 4
2.
3.
4.
5.
6.
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Synthesis of Salicylic Acid from Wintergreen Oil
the excess NaOH present after the initial reaction? How much sulfuric
acid was in excess?
Refer to the reaction equations in the “Reactions and Properties” section as you answer the following questions. (a) Which two hydrogen
atoms of salicylic acid are most likely to be acidic? Which hydrogen
atom(s) of methyl salicylate would you expect to be acidic? (b) Based
on your answer to (a), draw the structure of the white solid that forms
immediately after NaOH and methyl salicylate are combined, and write
an equation for its formation. (Hint: Acid-base reactions are much
faster than most organic reactions.)
(a) Write an equation for the overall reaction in this experiment, and
use it to calculate the atom economy of the synthesis and your reaction
efficiency. (b) Describe some green features of your synthesis, and any
that aren’t so green.
Based on the amount of water you used during the recrystallization,
estimate the amount of salicylic acid that was lost as a result of being
dissolved in the filtrate. Assume that the recrystallization solution was
cooled to 10°C; the solubility of salicylic acid at that temperature is
0.14 g per 100 mL of water.
Describe and explain the possible effect on your results of the following experimental errors or variations. (a) You added only enough 3 M
H2SO4 to bring the pH down to 4. (b) Your reaction mixture had an oily
layer on top when you added the 3 M sulfuric acid. (c) Because of a
label-reading error by a lab assistant, the bottle labeled “salicylic acid
from benzene” actually contained acetylsalicylic acid (aspirin).
From the equations in the “Reactions and Properties” section, you can
see that methanol and sodium sulfate are by-products of the synthesis of
salicylic acid. During which step of the synthesis would these compounds
have been separated from the final product? Explain your answer, based
on any relevant properties of salicylic acid and the by-products. (If necessary, you can look up the properties in a reference book.)
Other Things You Can Do
(Starred items require your instructor’s permission.)
*1. Develop and test a hypothesis based on observations you make while
performing Minilab 4. You can also test your product from this experiment with ferric chloride as described in Minilab 4.
*2. Convert your salicylic acid to aspirin by following the procedure in
Experiment 34. (You can omit the spectral analysis.)
3. Find some examples of organic compounds besides methyl salicylate
that are available commercially in both natural and synthetic forms.
You might start searching at a local drugstore or health food store.
Look up the compounds in The Merck Index and give their chemical
structures.
4. Write a research paper about salicylic acid and the salicylates using
references from the Bibliography.
69
Heating Under Reflux: Synthesis of Salicylic Acid (from Lehman text, 2nd edition)
* Note: Read the entire lab experiment provided in the Week 14 Module on Canvas (pages 6169) to obtain a more complete understanding of the experiment. If you performed the experiment
in person, please use your own data and results as much as possible to answer the following
questions. Any results that were not obtained in person should be obtained from the videos
provided in the Week 14 Module.
Lab Report Questions:
1. a) Determine the theoretical yield of the product (salicylic acid) and show all of your
calculations. (Note: This is done just like you learned in general chemistry starting with the mass
of the limiting reagent and converting to mass of the product).
b) Provide the final mass of product after recrystallization (show calculation).
c) Determine the percent yield of product.
2. Provide the melting point range of the product.
3. What is the purpose of using a reflux condenser in organic reactions?
4. Provide answers to the following Exercises from page 69 of the lab experiment handout.
Exercise 2a)
Exercise 2b) (Note: You will need to draw structures for the organic compounds involved
in the reaction)
Exercise 5a)
Exercise 5b)
Exercise 6)
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