Illustrated Guide to Home Chemistry Experiments

376 DIY Science: Illustrated Guide to Home Chemistry Experiments

POCEDURER
This lab is broken into three parts. In Part I, we’ll synthesize
methyl salicylate. In Part II, we’ll isolate and purify the product.
In Part III, we’ll determine the density and freezing point of
our product.

Before you begin this lab (or, indeed, any organic synthesis), you
should calculate the theoretical yield for the reaction. We’ll react
13.0 g of aspirin with excess methanol to form methyl salicylate.
Enter the actual mass of aspirin you use to 0.01 g on line A of
Table 21-1. The formula weight of aspirin is 180.160 g/mol. Based
on the actual mass of your aspirin sample, calculate the number
of moles of aspirin to be reacted and enter that value on line B
of Table 21-1. In the presence of an acid catalyst, one mole of
salicylic acid reacts with one mole of methanol to produce one
mole of methyl salicylate. (One mole of aspirin reacts with two
moles of methanol to produce one mole of methyl acetate in
addition to one mole of methyl salicylate.) The formula weight
of methyl salicylate is 152.1494 g/mol. Calculate the theoretical
yield of methyl salicylate in grams, and enter that value on line C of
Table 21-1. The density of methyl salicylate is 1.1825 g/mL. Using
this density and the theoretical yield in grams, calculate the
theoretical yield of methyl salicylate in mL and enter that value
on line D of Table 21-1.

PRTI: A SyTHESIN zE mETHyL SALICyLATE
Perform this part of the lab under an exhaust hood or in a well-
ventilated area. The reaction produces a strong odor of methyl
salicylate and methanol vapor. Make certain that there are no
open flames or other ignition sources nearby.

1. If you have not already done so, put on your splash
   goggles, gloves, and protective clothing.


2. Transfer about 60 mL of methanol to the 125 mL
   Erlenmeyer flask. Add 13.00 g of aspirin to the flask
   and swirl or stir the contents until the aspirin tablets
   dissolve. (Aspirin tablets contain binders and other
   inactive ingredients that may not dissolve in methanol,
   which is no cause for concern.)


3. Add about 10.0 mL of concentrated sulfuric acid to the
   reaction vessel and swirl to mix the solutions.


4. Clamp the flask to a ring stand and partially immerse it
   in a hot water bath at about 60°C.


5. Allow the reaction to proceed, stirring the reaction
   mixture occasionally, for 60 minutes. As the methyl
   salicylate forms, you’ll notice its distinct wintergreen
   odor. Keep the reaction mixture at about 60°C, adding
   water to the bath if necessary to keep its level up. The
   level of the liquid in the reaction vessel will decrease as
   methanol evaporates from the flask. Add more methanol
   as needed to keep the liquid in the reaction vessel near
   its original volume.


6. After 60 minutes, stop adding methanol to the flask.
   Increase heat slightly to bring the liquid in the reaction

flask to a gentle boil. Boil the solution long enough

to vaporize most of the remaining methanol.

7. When enough methanol has boiled off to reduce the
   volume of liquid in the reaction vessel to about half its
   original volume, remove the flask from the water bath
   and set it aside to cool.

PRTII: A ATEISoL ANd pURIfy THE pRodUCT

dR. mARy CHERvENAk CommENTS:

Generally, sep funnels need to be vented after vigorous

shaking. Even if gas isn’t generated, it’s good practice. The

sodium bicarbonate wash in step 6 will definitely have to

be vented after shaking. Enough gas can be generated to

blow the stopper off the back of the sep funnel.

The brown liquid in the reaction vessel is a complex solution

that contains methanol, crude methyl salicylate, sulfuric acid,

unreacted aspirin, and other impurities. Methyl salicylate is

freely soluble in methanol, but only very slightly soluble in water.

We’ll take advantage of that differential solubility to extract most

of the water-soluble impurities from the crude product.

1. If you have not already done so, put on your splash
   goggles, gloves, and protective clothing.


2. Pour the contents of the reaction flask into the sep funnel.


3. Add about 50 mL of ice-cold tap water to the sep funnel,
   cap the funnel, and agitate the contents vigorously for
   30 seconds.


4. Allow the contents of the sep funnel to separate into
   two layers. The aqueous layer contains nearly all of
   the sulfuric acid and most of any other water-soluble
   impurities. (Make sure you know which layer is which.)
   Separate the two layers, and transfer the aqueous layer
   to the 250 mL beaker.


5. Do a second washing by repeating steps 3 and 4.


6. Add about 50 mL of sodium bicarbonate solution to
   the sep funnel, cap the funnel, and agitate the contents
   vigorously for 30 seconds. (See previous note on venting.)


7. Allow the contents of the sep funnel to separate into
   two layers. The aqueous layer contains an excess of
   sodium bicarbonate and a small amount of sodium
   sulfate produced by the neutralization of any sulfuric
   acid that remained in the organic layer. Separate the
   two layers, and transfer the aqueous layer to the
   250 mL beaker.


8. Neutralize the sulfuric acid solution in the 250 mL
   beaker with sodium carbonate or sodium bicarbonate
   and flush the neutralized solution down the drain with
   plenty of water. Retain the organic layer in the sep
   funnel, which contains the crude methyl salicylate.