1.
Place the indicated volume of acetic anhydride in a large
dry
test tube, add 3 drops
of concentrated sulfuric acid and mix thoroughly. Obtain 2.0 ml of one of the
alcohols in a
dry
small test tube.
2.
Hold the large test tube containing the anhydride in cold water (to cool the
reaction), and add the alcohol to it in several increments; mix between additions.
Without cooling, the strongly exothermic reaction could become too vigorous.
Then place the tube in hot water at about 70º C for about 5 minutes to complete the
reaction.
3.
After 5 minutes, add eight drops of water (pasteur pipet). Mix briefly after each
drop. The purpose is to hydrolyze excess acetic acid remaining after the
esterification is complete.
4.
Cool the reaction mixture, and add 6 ml of half-saturated sodium chloride solution
(3 ml of saturated NaCl and 3 ml of distilled water) to the test tube.
Mix
thoroughly and vigorously
, then set the tube aside until upper and lower phases
form. The salt helps to break emulsions; if water alone were used, two phases
would not separate easily.
The upper phase is the ester
, which is a volatile
liquid. In the propyl acetate and isoamyl acetate syntheses, the phases separate
cleanly within a minute or two. The octyl acetate and benzyl acetate produce
turbid, milky emulsions which should be given five minutes to separate; for these
two esters, both phases remain turbid even after separation.
5.
The lower phase is a water solution of sulfuric and acetic acids, and should be
removed and discarded. Use a pasteur pipet to remove it, but save it temporarily in
another container before throwing it away. The last portion of the lower phase can
be removed by placing the tip of the pasteur pipet (square end, not splintered)
against the rounded bottom of the tube, and watching carefully as you slowly
6
withdraw the liquid. Do not be concerned if a few drops remain behind.
Do not
remove the upper phase (the ester) from the test tube.
6.
Add 6.0 ml of saturated sodium bicarbonate to the upper phase in the test tube, mix
vigorously, and allow up to 5 minutes for the phases to separate. The base
neutralizes remaining acids, and helps to remove traces of acetic acid. Remove
and discard the lower phase. Retain the upper phase.
7. Again wash the upper phase with 6.0 ml of half-saturated NaCl. Mix vigorously,
and allow up to 5 minutes for the phases to separate. Remove and discard the
lower phase, and retain the upper phase.
8. Wash the upper phase with 6.0 ml of
saturated
sodium chloride. Mix thoroughly,
and allow the phases to separate. The upper phase (the ester) should not be turbid.
Again remove and discard the lower phase, but this time try to remove every drop
of it (without, of course, removing a significant amount of the ester). Because of
its high osmotic pressure, the concentrated salt solution removes dissolved water
from the upper phase (i.e. it dries the ester).
9.
Transfer the upper phase (pasteur pipet) to a small test tube. Allow any water
droplets to settle, and remove them with a pasteur pipet. Add about 1/3 volume of
anhydrous magnesium sulfate (MgSO
4
) to the small test tube, and stir occasionally
with a spatula for about 10 minutes. Allow the powder to settle.
10.
Weigh a small vial and cap, and transfer the liquid (not the powder) to it with a
pasteur pipet. To recover as much liquid as possible, place the (square, not
broken) tip of the pipet against the bottom of the test tube. Almost no MgSO
4
should be carried over to the vial. Cap the vial and weigh it. Add about 1/4
volume of MgSO
4
, cap it tightly, and store it in your locker.
Check out the odor before storing the ester. Sniff gently; putting your nose nearly into
the vial will overwhelm your ofactory machinery. Instead, place a drop or two on a piece of
paper and gently waft the delightful fragrance in the direction of your nose. Try not to be too
critical. When you smell propyl acetate, for instance, think