Transcript salicylic acid Salicylic acid

Exp 9B: From Oil of Wintergreen to
Salicylic Acid
Introduction
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In a chemical reaction bonds between atoms are
broken and formed resulting in the formation of new
chemical compounds
A rearrangement of atoms and bonds results in a
compound (the “product”) with different properties
than the original compound (the “reactant”)
Difference are sometimes large and easy to detect
The reactant in this reaction, methyl salicylate, is a
liquid; the product, salicylic acid, is a solid at room
temperature
The compounds in this reaction are organic
compounds, in which carbon and carbon-carbon
bonds are the main element
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Exp 9B: From Oil of Wintergreen to Salicylic
Acid
Purpose
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Study the conversion of methyl salicylate (“Oil
of Wintergreen”) to salicylic acid
Salicylic acid is the basic compound to prepare
aspirin, acetylsalicylic acid
Like aspirin, salicylic acid has pain-relieving
properties
Aspirin is easier on the stomach than salicylic
acid
The actual yield (“How much do you get?”) of
salicylic acid in this experiment will be compared
to the theoretical yield (“How much could you
have gotten, if…?”)
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Exp 9B: From Oil of Wintergreen to Salicylic
Acid
Reaction
+ H2O 
+ CH3OH
(methanol)
“hydrolysis”
+ H2O 
+ CH3OH
(methanol)
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Exp 9B: From Oil of Wintergreen to Salicylic
Acid
Reaction
• The reaction will go faster when concentrated
sodium hydroxide, NaOH, is used instead of
H2O
• Excess NaOH will be removed with sulfuric
acid, H2SO4 (“neutralization”)
• Oil of Wintergreen is soluble in water, but
salicylic acid is not soluble in water  it will
precipitate (“crystallize out”)
• Recover the precipitate by suction filtration
• Purify by recrystallization from boiling water
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Exp 9B: From Oil of Wintergreen to Salicylic Acid
Recrystallization
Purification technique for solids
• Solids are more soluble in solvent at higher
temperature
• Dissolution of solids in appropriate heated solvent
• Reformation/precipitation of crystals upon cooling
– If impurities remain in solution  pure crystals
– If impurities do not dissolve  filtration before cooling
• Determine purity, e.g., by melting point, or other
analytical techniques
• Recrystallization may have to be repeated to get
pure crystals
• Working range:
– Upper limit = boiling point of solvent
– Lower limit = depends on experiment; often ice-water
mix (0oC) or ice-salt (-20oC)
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Exp 9B: From Oil of Wintergreen to Salicylic Acid
A. Preparation of reaction
• Add 60 mL of distilled water to a 250-mL
beaker.
• Mark the level of the water with a Sharpie or
wax pencil
• Pour out the water and dry the beaker
• Put a flask with DI H2O on ice to cool down
(you will need this later in the experiment!)
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Exp 9B: From Oil of Wintergreen to Salicylic Acid
B. Reaction
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Measure 4.0 mL methyl salicylate with a pipet and pour in a
10-mL graduated cylinder
Measure the total mass on an electronic balance and record
in your notebook (How many decimals?)
Pour as much as possible of the methyl salicylate in the dry
250 mL beaker.
Measure the mass of the graduated cylinder.
• Calculate the amount of methyl salicylate that you used
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Add 40 mL of 6 M NaOH (sodium hydroxide) to the beaker
(use a graduated cylinder) (Observation: what happens?)
Stir the mixture with a glass stirring rod.
Leave the rod in the beaker!
Place the beaker on wire gauze and bring to boil.
Boil gently for 15 minutes
after the precipitate has dissolved.
Stir the solution regularly
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Exp 9B: From Oil of Wintergreen to Salicylic Acid
B. Reaction (continued)
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Rinse solids from the wall of the beaker with DI water from
a water bottle. Make sure the volume does not exceed
60 mL (the mark on the beaker).
After boiling for 15 min let the beaker cool for a few
minutes, then put in ice
Add 50 mL DI water to the beaker
Cautiously add 50 mL 8 M H2SO4 (sulfuric acid) to the
beaker while stirring
Cool the beaker in ice
Make sure that you have 50-100 mL of DI water in an
Erlenmeyer flask in an ice bath!!
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Exp 9B: From Oil of Wintergreen to Salicylic Acid
C. Filtration
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Pour the cold mixture on a paper filter in a
Büchner funnel
Filter the mixture by vacuum filtration over a
Büchner funnel
http://orgchem.colorado.edu/hndbksupport/filt/filtration.html
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Wash the crude extract with ice-cold water
Continue the suction for several minutes to dry
the precipitate
(Wash and dry the 250-mL beaker during this
time)
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clamp a side arm flask
securely to a ring stand
place the rubber adaptor in
the side arm flask
get a piece of filter paper …
. . . and put it in the
Buchner funnel
place the Buchner funnel
on the adaptor
connect the side arm flask
to a vacuum source
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Wet the paper with a small amount
of the solvent to be used in the
filtration, turn on the vacuum source
Rinse the flask with a little
fresh solvent and pour it into
the filter funnel
Filter the solution
Vacuum pulls the solvent through
the filter and into the filter flask
Disconnect the vacuum AT THE FLASK and before
turning off the water aspirator. This prevents water
from being sucked into the vacuum flask
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Carefully remove the filter paper
and solid from the Buchner funnel
Set the filter cake onto a
watch glass to air dry
Crude filtrate needs to be further
purified by recrystallization
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Exp 9B: From Oil of Wintergreen to Salicylic Acid
D. Recrystallization
http://orgchem.colorado.edu/hndbksupport/cryst/crystproc.html
1. Selection of appropriate solvent (in this case: diH2O)
2. Dissolve solid in boiling solvent
3. Formation of crystals by cooling for about 5 min at room
temperature
• Record: what do you observe, what type of crystals do form
4. Cool further in an ice-water bath
5. Make sure that you have enough ice-cold DI water (50 mL
dH2O) in an Erlenmeyer flask
6. Isolate purified crystals by suction filtration as done before
 Weigh the dry filterpaper before you do the
filtration step!
7. Wash the crystals with ice-cold water. Keep suctioning!
8. Keep suctioning for about 10-15 minutes after washing!
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After a while, crystals should
appear in the flask
You can now place the flask in
an ice bath to finish the
crystallization process
The crystals are now
isolated from the solution
by vacuum filtration (see
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before)
Filter paper with crystals on it
Carefully scrape the crystals onto
the watch glass
Let the crystal finish drying on
the watch glass
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Exp 9B: From Oil of Wintergreen to Salicylic Acid
Recrystallization
E. Drying the Crystals
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Determine the mass of a 250-ml beaker
Carefully transfer crystals with the filterpaper to the beaker
Cover the beaker with a watch glass
Write your name on the beaker and let the crystals dry in
the air
Next Monday:
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3.
Measure and record the mass of the beaker and the
contents without the watch glass
Determine yield: calculate the yield compared to the
theoretical yield (% recovery)
Determine melting point: this is a property that will
indicate how pure the reaction product is
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Exp 9B: From Oil of Wintergreen to Salicylic
Acid
Chemical name
Methyl salicylate
2-Hydroxybenzoic acid, Salicylic acid
Chemical formula
C8H8O3
C7H6O3
Molecular mass
152.1494 g/mol
138.12 g/mol
Melting point
8.6 °C
159 °C
Boiling point
220 - 224 °C
211 °C (at 20 mmHg)
Density
1.174 g/cm
Solubility
Appearance
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<0.1 g/100 mL (at 20oC)
Colorless, yellowish to
reddish oily liquid
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1.44 g/cm (at 20 °C)
0.2 g/100 mL (at 20oC)
Whitish crystals, light-sensitive
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Exp 9B: From Oil of Wintergreen to Salicylic
Acid
Sulfuric acid
Formula
H2SO4
Mole mass
98.0734
Melting point
3 oC
Boiling point
280 oC
Density
1.84 g/cm3
Solubility
Soluble.
Miscible/Reactive
Appearance
Colorless (pure) to
dark brown, oily, dense liquid
with sharp, acrid odor.
Comments
Corrosive
Sodium hydroxide
Formula
Mole mass
Melting point
Boiling point
Density
Solubility
Appearance
Comments
NaOH
39.99707
318 oC
1390 oC
2.13 g/cm3
50 g/100 mL.
Highly soluble
Colorless,
odorless solid
Corrosive
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Determination of Melting Point
INTRODUCTION
• The melting point of a pure compound is an
intensive property, like density and boiling
point. Intensive properties are independent of
the amount of substance present.
• The melting point of a compound is the
temperature at which it changes from a solid
to a liquid.
• Experimentally, melting point is actually
recorded as the range of temperatures in
which the first crystal starts to melt until the
temperature at which the last crystal just
disappears.
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Determination of Melting Point
Reasons for determining melting points:
1. The melting point indicates the level of purity
of a sample. An impure compound melts
over a wider range of temperatures, usually
greater than 2 degrees.
2. The melting point helps to identify unknown
samples, narrowing the number of
possibilities, because a pure solid melts
reproducibly over a narrow range of
temperatures.
3. The melting point helps to characterize new
compounds.
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Filling the Capillary and Packing the
Sample
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Loading
Sample
into
Apparatus
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Onset point
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Meniscus Point
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Clear point
(U.S. - based melting point)
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Side–by-side comparison