FISCHER ESTERIFICATION OF BENZOIC ACID (03/11/02)
Carboxylic acids can be converted into esters by a number of methods. The most commonly
used of these esterification methods is the acid-catalyzed reaction between carboxylic acids and
alcohols. This is a reversible reaction with an equilibrium constant, Keq, approximately equal to
4 for most carboxylic acid - alcohol pairs. Thus when a carboxylic acid and an alcohol react in a
1:1 molar ratio, the ester is formed in ~67% yield. The yield of the ester can be increased by
using a large excess of the alcohol to shift the equilibrium to the right (Le Chatelier's principle).
A four-fold molar excess of the alcohol will produce the ester in ~95% yield.
RCO 2H
+
R'OH
H
RCO 2R'
H2O
+
Esterification using a large excess of an alcohol is known as the Fischer esterification procedure.
Although esters may be formed in high yield (>95%) by Fischer esterifications, the esters are
often isolated in appreciably lower yields (~75%). These lower isolated yields of esters are the
result of the aqueous work-up normally used with Fischer esterifications to remove the excess
alcohol and the acid catalyst. The aqueous alcohol solution formed when the reaction mixture is
poured into water will dissolve an appreciable amount of the ester.
In this experiment methyl benzoate will be prepared by a Fischer esterification of benzoic acid
with methanol.
CO2H
+
CH3OH
H2SO4
CO2CH3
+
H2O
The methyl benzoate will be used in the subsequent experiment for the synthesis of a tertiary
alcohol via a Grignard reaction.
Procedure:
Place 8.00g (0.0656mol) of benzoic acid and 25mL (0.617mol) of methanol in a 100mL roundbottomed flask, and carefully pour 3.0mL of concentrated sulfuric acid down the walls of the
flask. Swirl to mix the reactants, add 2-3 boiling stones, attach a condenser, and heat the mixture
at reflux with a thermowell for 30 minutes.
Cool the reaction mixture and pour it into 75mL of water contained in a separatory funnel, taking
care to keep the boiling stone from entering the separatory funnel. Rinse the reaction flask with
35mL of ether, pouring it into the separatory funnel. Stopper, shake, and drain off the aqueous
layer which contains the sulfuric acid and most of the methanol. Wash the ether layer with
another 15mL of water, drain, and then wash with two 25mL portions of 5% sodium bicarbonate.
Cautiously shake, venting often to release pressure, until no further reaction is apparent. Wash
with saturated sodium chloride solution (15-20mL) and dry the ether layer with anhydrous
sodium sulfate.
Decant the ether layer into an appropriate size, tared round-bottomed flask (solution should no
more than half fill the flask) and evaporate the ether on a rotary evaporator. Calculate a percent
yield. Dissolve some of your product in chloroform as directed by your instructor. Fill an IR
solution cell with this solution and obtain an IR spectrum. Save the methyl benzoate in a tightly
stoppered container for use in a Grignard synthesis scheduled for the next lab period. Note: Do
not use your 100mL round-bottomed flask for this purpose because it will be needed for the
preparation of the Grignard reagent during the next lab period.
% Concentrated sulfuric acid is very corrosive!
% Keep ether contained or in a hood at all times!
% Vent the separatory funnel often after the addition of sodium bicarbonate: CO2(g) is
generated from the reaction of H2SO4 + NaHCO3!
% None of the reagents in this experiment need to be discarded in the halogenated waste. The
water-soluble/aqueous reagents may be discarded in the sink w/ running water.
· To save time, obtain the solutions and set up for the washing steps while the reaction mixture
undergoes reflux.
POSTLAB EXERCISE:
FISCHER ESTERIFICATION OF BENZOIC ACID (03/11/02)
Ø Due in the lab following the completion of the experiment (usually one week). No papers
accepted after the Friday of the lab week it’s due (absence or no absence!). Please answer
questions on this form. (30pts TOTAL)
1. PRODUCT INFORMATION (5pts)
%Yield (5pts):
2. (12pts) Assuming Keq = 4.00 for the esterification of benzoic acid:
(a) (6pts) What is the maximum percent yield of methyl benzoate that would be produced
under the reaction conditions used in this experiment
(b) (6pts) How many moles of methanol would have to be used to produce methyl benzoate
in 99% yield?
3. (5pts) If one wanted to force the acid-catalyzed esterification toward completion without
using a large excess of methanol, how might this be accomplished?
4. (8pts) Compare the IR spectrum of methyl benzoate with the IR spectrum of benzoic acid
(http://www.chem.mtu.edu/kmroth/Organic/Manuals/Ester/Fischer.htm) Please make
functional group assignments and comparisons directly on the spectrum.