Download CH 102 Laboratory 7 Ester Synthesis and Smells

example acid anhydrides
anhydride, shown below:
CH 102 Laboratory 7
Ester Synthesis and Smells
H3C C
-To use acid anhydrides to make esters.
O
Acetic anhydride
solvent extraction.
(Note: The acetic anhydride forms the
exact same ester as acetic acid, the
acetate ester, it just works a lot better.)
-To learn about drying agents
INTRODUCTION
Esters are the product formed in the
reaction of an acid with an alcohol. The
reaction is general and almost any acid
can be combined with any alcohol. Most
common examples involve organic acids
and alcohols. For example, the reaction of
acetic acid and ethyl alcohol produces
ethyl acetate.
O
O
OH
+
C
HO H2 CH3
O
H3C C
-To become familiar with two-phase
H3C
acetic
O
GOALS
C
like
H3C
C
O
C
H2
CH3
+
H2O
Ethyl acetate is the familiar solvent used
in fingernail polish remover and similar
applications.
Under ordinary conditions the equilibrium
described above is unfavorable and not
suitable for the preparation of esters. This
can usually be remedied by using a
reaction mixture that does not contain
water and by removing the water that is
produced by the reaction. Various
strategies have been developed to
perform this task.
For example,
concentrated sulfuric acid can be added to
the reaction. The sulfuric acid reacts
rapidly with water to form a hydrated
form of sulfuric acid which effectively
removes the water from consideration in
the equilibrium. Acids can also be supplied
in forms that do not contain water. For
It is not unusual for acetic anhydride to
react with in minutes with alcohols to form
the corresponding esters. We will take
advantage of this strategy in our
laboratory exercise.
Recovering the desired products from
reactions is often a very difficult task. In
the case of ester formation it is
advantageous to mix the reaction mixture
with strongly basic water after the
reaction
is
complete.
Under
these
conditions, the unreacted acid will be in
the anion form and soluble in the water. If
an organic solvent that does not mix with
water is added to the reaction mixture,
the ester will be selectively extracted into
the organic solvent. Pet. ether is often
used for this task because it has a very
low boiling point and can be rapidly
removed by evaporation leaving the ester
behind. Naturally this procedure will not
work if the ester also has a low boiling
point.
Since esters can be formed from a wide
variety of acids and alcohols it is not
surprising that there is a very large
number of possible esters. For example,
consider the reaction of the five simplest
alcohols with five simple acids. If all
possible combinations were prepared we
would have 25 different esters. This would
also require twenty five different reactions
and work ups which would require a
considerable amount of time and effort.
There are many more acids and alcohols
than those considered in this example and
the resultant number of possible esters is
tremendous.
hot liquid out of the test tube. It is
best to let the first few drops slide
down the wall of the test tube and
gently mix at the bottom.
The preparation and screening of large
numbers of compounds like esters is an
important question and has resulted in the
formation of a new area of chemistry
called combinatorial chemistry. It is not
surprising that the development of new
drugs can benefit tremendously from this
strategy. Naturally, the synthesis of drugs
is more complex than the preparation of
esters and libraries of compounds
numbering in the millions have been
produced.
Warm the tubes in 400 mL beaker filled
with 200 mL of water heated to 70°C.
Maintain the mixture at 70 °C for 15
minutes. Cool the reaction mixture in an
ice bath and very carefully add 1 mL of
cold water. Then add slowly while mixing
1 mL of saturated sodium bicarbonate
solution-IT WILL BUBBLE!.
Add
additional drops of saturated sodium
bicarbonate solution with mixing until no
additional bubbling occurs.
PROCEDURE
WEAR GLOVES WHEN HANDLING
ACETIC ANHYDRIDE AND SULFURIC
ACID AS THEY ARE VERY DANGEROUS
CHEMICALS
Prepare 4 large test tubes and label them
A, B, C, D. Place 10 drops of the acid
anhydride in each test tube and add 2
drops of concentrated sulfuric acid and
mix by tapping the tube with a gloved
finger. Cool the test tubes on ice and
carefully add 10 drops of the alcohols with
mixing after each drop.
Alcohol
1-propanol
3-methyl-1-butanol
1-octanol
benzyl alcohol
letter
A
B
C
D
Use Caution: Make sure that the test
tube is facing away from anyone
before adding the alcohols. In some
cases the reaction is very energetic
and can create enough heat to shoot
Add 1ml saturated NaCl solution, mix and
then add 1ml pet. Ether, mix again and let
two layers form.
Place a new glass
Pasteur pipet through the layers to the
bottom of the test tube and slowly draw
up the lower layer, remove the pipet
before it sucks up any of the top layer,
which is where our esters are. Put the
water layer you just removed into a
beaker for later disposal. Add 1 ml of the
saturated NaCl solution, mix gently and let
two layers separate. Again remove as
much of the bottom layer as possible and
place it in your waste beaker for later
disposal.
Add a small amount of
anhydrous MgSO4 and swirl to remove
any remaining H2O, then draw off most of
the pet ether solution from above the
MgSO4 salt at the bottom of the tube.
Transfer this to a clean (labeled) small
test tube, add one boiling chip and let the
pet ether boil off in the hot water bath in
the hood, provided by the instructor. Get
a breath of fresh air to clear your nasal
receptors and then try smelling each ester
in turn and describing their aroma, as
directed by your instructor, who may also
provide additional esters to evaluate.
QUESTIONS
1.
Some have described these esters as smelling like:
1) banana, 2) oranges, 3) pear, and 4) peach.
Which of your esters smells closest to these?
2.
What are the structures of these four esters, and any others
your instructor has provided?