Many esters have "fruity" odors. Methyl salicylate, which is commonly referred to as oil of wintergreen and smells minty, is a compound having an ester function. It was first isolated in 1843 by extraction from a plant (Gaultheria spp.) which is in the mint family. And, it was soon discovered that methyl salicylate has analgesic (eases pain) and antipyretic (lowers fever) properties that are almost identical to salicylic acid when taken orally. The similar medicinal effect probably results from hydroylsis to salicylic acid under the alkaline conditions found in the lower intestine. Methyl salicylate is lipophilic and can be absorbed through the skin. It is in a number of topically applied preparations. Methyl salicylate's pleasant odor results in it being used in flavoring foods and confections.
Under either acidic or basic aqueous conditions, esters can be hydrolyzed
into their respective carboxylic acid and alcohol that they could have
been prepared from. In this experiment, methyl salicylate, will be reacted
with aqueous
sodium
hydroxide
under reflux. Since this hydrolytic method results in a basic solution
at the reaction's completion, the resulting carboxylic acid and phenolic
(-OH) groups, which are acidic, are ionized at this stage and the compound
exists as the sodium salt of salicylic
acid, sodium salicylate, which is the conjugate base of salicylic acid.
Therefore, the reaction mixture must be acidifed in order to recover the
salicylic acid. The salicylic acid is then purified by crystallization
from water.
Procedure: (Budget 1.5 lab periods)
Complete the following table and place in your lab notebook under the Physical Constants Data section for this experiment.
Structure |
Molecular Formula |
Molar mass(g/mol) |
m.p oC. |
b.p. oC |
d g/mL |
Safety |
|
| methyl salicylate | -8 |
223 |
1.17 |
||||
| salicylic acid | 159-161 |
||||||
| sodium hydroxide (6M) | |||||||
| sulfuric acid (3M) |
Your synthetic target is to prepare 2.00g of pure salicylic acid. Calculate the amount of methyl salicylate in both grams and mL that you will need to start with as part of your prelab. Clearly show your calculation of how many millimoles, grams, milliliters of methyl salicylate you plan to use to Dr. R. before going any farther. Then complete the calculations using the following experimental instructions, afterwards show your notebook again to Dr. R. for initialing.
Use
7 molar
equivalents
of base for each molar equivalent of methyl salicylate (MS). One molar equivalent
is needed to neutralize the acidic phenolic hydrogen, another will neutralize
the newly formed carboxylic acid. The rest of the base is in excess and must
be neutralized with acid as well. The sodium hydroxide
solution that is used is a 6M solution (6mol/L or 6mmol/mL).
Example calculation: If 1.52 g of MS were started with, which is 10 mmol
of MS, 70 mmol of base (sodium hydroxide) is required. ?
mL 6M NaOH =
70mmol NaOH x mL/ 6mmol = 11.6mL, which can
be rounded to 12mL.
Add the aqueous sodium hydroxide to the methyl salicylate in an appropriate
size single neck round bottom flask (rbf). [Specify the size in your prelab.]
A white precipitate may form but it will dissolve upon heating. Add a boiling
chip,
and a very lightly
greased
condenser
(to
prevent
the
standard
tapers
from "freezing" together due to the base attacking the glass).
Reflux for 30 minutes using a heating mantle. When the solution has cooled,
transfer it to a 100 mL beaker. The disodium salt of salicylic acid is in
the basic solution. Rinse the rbf with only a few mL of water and transfer
this wash to the same beaker.
Neutralize the basic solution and acidify the disodium salt of the
product by adding 3M sulfuric acid. Calculate how much sulfuric acid is
needed to neutralize the amount of NaOH you actually used. [Specify the
volume in your prelab.] A slight
excess [2-3%] is needed so that the solution is definitely acidic in order
to precipitate the product, salicylic acid.
Check the acidity with pH indicator paper, add an additional
5-10% of the sulfuric acid to completely precipitate the SA.
Based on the example above 70 mmol of
base that was used, which is 70 milliequivalents of base, you would need
a minimum of 70 milliequivalents of acid, but
remember sulfuric acid is diprotic and there are 2 equivalents per mole
of acid.
Cool the solution to 0oC. Filter the crude salicylic acid using a Buchner
funnel. Check the filtrate to be certain it is acidic.
If not, add additional sulfuric acid to see if anymorel SA precipitates.
The crude SA will contain water even after it is air dried for several
minutes so weighing the crude product provides only an approximate, maximum
yield.
Recrystallize the crude SA from very hot water (approx. 20 mL/g) using an Erlenmeyer flask. Use a minimum amount of boiling water by adding it slowly and observing how much compound goes into solution. Use a wooden stick to provide ebullation and prevent bumping. It may be necessary for you to gravity filter this solution. If so, this is one of the most difficult parts of the experiment since the solution and the glassware must be kept warm during the filtration otherwise the SA starts to precipitate too early. If necessary, gravity filter the hot solution using a piece of fluted filter paper in your powder funnel. Keep all solutions warm or else the SA will precipitate in the funnel rather than passing through into the receiver. If this occurs, it will be necessary to use a heat gun or heated filtrate to redissolve the solid.
The hot solution is cooled in an ice-water bath until precipitation appears
to have ceased. The recrystallized salicylic acid is vacuum filtered,
air dried for a few minutes, and then left in an open container to dry
until
the next lab period. At that time, you can weigh your dry sample and
calculate your % yield. Record the melting point .
Post lab questions: Answer the following 3 questions plus the on-line
questions (a-e).
1. What happened to the methanol produced in the reaction?
2. If you calculated that you needed 10 mL of 3M sulfuric acid but
you discover that the only acid available is 6M hydrochloric acid,
how many
mL would you
use?
3. If the entire salicylic acid
sample that you obtained were reacted to produce a 100% yield of aspirin,
how much aspirin (mg) would be produced.