CHEM 322: SYNTHESIS OF AN ESTER
Unmodified carboxyl groups of organic acids are not sufficiently reactive to undergo nucleophilic attack
by most nucleophiles, and hence must be activated in some manner. The three most common ways to
activate carboxyl groups of organic acids are the following:
1. Convert the acid to the acid chloride.
2. Convert the acid to its anhydride.
3. For ester synthesis, activate by strong mineral acid and keep free water in short
supply.
The reaction we wish to carry out is synthesis of a fragrant ester via acid-catalyzed esterification. For
example, the reaction for butyl butyrate is:
O
O
H+
OH +
HO
+ H2O
O
The purpose of this experiment is to illustrate one way to make an ester from a carboxylic acid and an
alcohol. Activation by acid has been chosen for three reasons:
1. While an acid chloride would work, its synthesis (an additional step) requires thionyl chloride, a
reactive compound that is dangerous and difficult to handle with large groups (it reacts
explosively with water, for example).
2. We could use an acid anhydride, but only a few kinds are readily available.
3. The reaction shown above is an equilibrium reaction, and the present protocol will illustrate how
the reaction can be “pushed” to completion by adding excess of one reagent to the reaction
mixture and "pulled" to completion by removal of one product as it is formed.
This table lists some combinations of acid and alcohol and the scent of the ester that results. Choose the
one you prefer plus one backup (in case necessary chemicals aren’t available).
Alcohol
Acid
Ester
Scent
n-pentanol
isopentanol
n-pentanol
n-butanol
n-propanol
isopentanol
pentanoic acid
pentanoic acid
n-butyric acid
n-butyric acid
isobutyric acid
acetic acid
pentyl pentanoate
isopentyl pentanoate
pentyl butyrate
butyl butyrate
n-propyl isobutyrate
isopentyl acetate
Apple
Synthetic apple
Apricot
Pineapple
Rum
Banana
Experimental Procedure: (Students will work in Pairs)
Prelab: You must do your calculations of volumes to use BEFORE lab. Use published densities for the
acid and alcohol you chose to compute necessary volumes. Before you begin your experiment, have the
instructor or teaching assistant approve your computations. (Pre-lab Q #1)
Add to a 100 mL flask a boiling chip and (in the following sequence and with good mixing after each
addition):
Carboxylic acid (0.11 mole)
0.5 – 1.0 mL concentrated H2SO4
Alcohol (0.10 mole)
After adding reactants, equipment should be
assembled as shown using special glassware
provided. It should be mounted at about the
angle shown so that the side arm points
generally downward. The flask should be in a
heating mantle. Assure that the heating mantle
is pressed firmly against the flask (for efficient
heat transfer). The side arm may contain a
glass rod (to minimize its internal volume) and
should have a rubber hose attached, closed
with a pinch clamp. Alternatively, a side arm
with a stopcock can be used. Check to see that
the side arm and the boiling flask are clean so
that good seals can be made.
Insulate the apparatus, covering from the flask as far as the bottom of the side arm. Heat the reaction
mixture strongly enough to cause rapid reflux (Variac at 75% of max setting). Continue heating as long as
the condensate returning from the condenser contains water (watch for droplets falling down through the
liquid inside the arm). The arm will fill up first with organic liquid. Do not drain water until the interface
between water (lower layer) and organic liquid (upper layer) nearly overflows back into the boiling flask.
Then drain only very small volumes at a time (just enough to prevent water from returning to the reaction
flask). Check each time to be sure there are no leaks.
When no more water appears in the condensate (interface height stops rising), the reaction is finished.
Allow the pot to cool, then turn the apparatus so that any liquid in the side arm runs back into the flask.
Pour the flask contents into a separatory funnel and rinse the flask once with about 10 mL of water. Pour
this into the sep funnel as well. Then add 40-50 mL of tap water to the funnel and shake vigorously.
If an emulsion forms at this point or later, pour everything into a beaker and add a little NaCl. Swirl
to dissolve – salt claims water as it dissolves and increases the density of the aqueous layer. This
helps organic and aqueous layers separate better. Return liquids to the sep funnel – avoid solids.
Discard the lower (aqueous) layer. The black polymer that forms in this reaction may make it difficult to
see the interface – observe carefully. Neutralize any remaining acids by washing the organic layer with 25
mL portions of 5% aqueous NaHCO3 solution (vigorous shaking) until no further CO2 is evolved. Discard
aqueous layer before adding fresh bicarbonate to the organic layer. Check the pH of the final LOWER
layer with a piece of litmus paper. It should stay (or become) blue. If the litmus is red, wash the organic
layer with more 5% NaHCO3 solution until the LOWER layer gives a blue litmus test. Dry the organic
layer well over anhydrous CaCl2. Decant into the smallest dry boiling flask that will hold your product
(25 or 50 mL) and distill through dry, insulated glassware with the variac setting at 60% of max scale.
Run an IR spectrum (using the ATR accessory) of the product, as well as each of your main starting
materials (the alcohol and carboxylic acid you used, specifically).
WEEK 4 ASSIGNMENT: Type and staple the following results to the front of your notebook pages.
1. Reactions (2) for the general acid-catalyzed esterification reaction (using R groups), and your
specific acid-catalyzed esterification reaction
2. Hard copy of the IR spectra of your ester product and the alcohol and carboxylic acid starting
reagents, all properly labeled (obtain all 3 in the lab)
3. Boiling range of product, listed
4. Mass of product, listed
5. Calculations for:
a. Determination of limiting reagent (consider your carboxylic acid and your alcohol)
b. Theoretical yield (in grams) from limiting reagent
c. % yield
STUDY QUESTIONS for Discussion Quiz #4 on 3/8/2016
1. Review the theory of esterification:
a. Specific reaction used in this experiment, and the general reaction / transformation that
occurs
b. Identify the functional group in the final product’s structure compared with starting
materials
c. Equilibrium reaction effects and how product formation was driven in this specific
experiment
2. What is the order and purpose of major procedural steps in the esterification experiment?
3. What was compound most likely to contaminate your product? What step(s) did you take in the
procedure to minimize the likelihood of this happening?
4. Aside from glassware transfers / spills / etc., during what steps may product have been lost due to
chemistry reasons? Explain the chemistry of each.
5. Boiling Range:
a. Why would the presence of trace contamination in the final liquid product cause boiling
point elevation to occur?
b. What error made during distillation setup might cause the observed boiling range to be
lower than expected? Why?
6. What compounds would make strong IR comparisons with your experimental product, and what
notable peak changes (presence/absence) would indicate the correct product had formed?
PRE-LAB QUESTIONS: (complete these in your notebook before coming to lab)
1. You must do your calculations of volumes to use BEFORE lab. Use published densities for the
acid and alcohol you chose to compute necessary volumes. Before you begin your experiment,
have the instructor or teaching assistant approve your computations.
2. What is more detrimental to the yield of your product: draining off too much water from the
sidearm during reflux and potentially losing some organic layer as well, or not draining off
enough water from the sidearm during reflux and potentially allowing water back into the
reaction flask?
3. What visual observation will indicate that there is no more acid contaminating your product?
4. What does a red litmus test result tell you about the acidity of the aqueous layer? Why does the
procedure ask you to test the aqueous layer for this and not the organic layer?