UNIVERSITI TUNKU ABDUL RAHMAN

Faculty of Science
Bachelor of Science (Hons) Chemistry
Year 1 Trimester 1
UDEC1224 – Chemistry Laboratory II

Name : Chang Ling Ying (1901683), Kirthinee Jegatheesan

(1902731), Ho Jingyou (1705750)

Experiment : Experiment 21

Title : Esterification

Date of Experiment : 7 August 2019

Date of Submission : 14 August 2019

Lecturer : Dr. Mohammod Aminuzzaman

Practical group : P (3)

Title:
Esterification of Acetylsalicylic Acid

Objective:
To synthesise acetylsalicylic acid by esterification.

Introduction:
Esters are a functional group commonly encountered in organic chemistry. They
are characterized by a carbon bound to three other atoms: a single bond to a carbon, a
double bond to an oxygen, and a single bond to an oxygen. The singly bound oxygen
is bound to another carbon. Ester names are derived from the parent alcohol and
parent acid and ends with the suffix “-oate.” For example, the ester formed by ethanol
and ethanoic acid is known as ethyl ethanoate; “ethanol” is reduced to “ethyl,” while
“ethanoic acid” is reduced to “ethanoate.” Esters are typically fragrant. Many are
responsible for the fragrance and flavor of flowers and fruits. For example, isopentyl
acetate is present in bananas and ethyl butyrate can be found in pineapples. Esters
with low enough molecular weights to be volatile are commonly used in synthetic
flavors, perfumes and cosmetics. Besides, esters are also used in the manufacture of
many other industrial products. For example, they are used as softening agents for
resins and plastics, or as solvents for lacquers, paints and varnishes (Rogers, 2015).
Esters can be synthesized through several ways: from carboxylic acids, acyl chloride
or acid anhydrides. Generally, an esterification occurs when an alcohol and a
carboxylic acid are reacted in the presence of a mineral acid catalyst, such as sulfuric
acid. In this experiment, we synthesized acetylsalicylic acid through the esterification
of salicylic acid via reaction with acetic anhydride. The use of acetic anhydride as a
reactant, instead of acetic acid, results in a rapid and irreversible conversion of
salicylic acid to acetylsalicylic acid.

an ester group
Apparatus and Materials:
Salicylic Acid, 100 mL Erlenmeyer Flask，Acetic Anhydride, 85% Phosphoric Acid,
hot plate, distilled water, ice bath, glass rod, 2 watch glasses, Ethanol, filter paper

Procedures:
1. A 400 ml beaker containing about 150 ml of water was heated to the boiling
point on a hot plate.
2. 2 g of salicylic acid was weighed out and placed in a 100 ml conical flask.
3. 5 ml of acetic anhydride was added into the conical flask.
4. 5 drops of 85% phosphoric acid, H3PO4, was added. The flask was swirled to
mix the reactants and then clamped in the boiling water bath. It was heated for
about 8 minutes.
5. The flask was carefully removed from the hot water bath and the hot plate was
turned off. Cautiously, 2 ml of distilled water was added to the flask. The
water reacted with any unreacted acetic anhydride which remained.
6. Once the reaction between acetic anhydride and water had subsided, 40 ml of
water was added to the flask.
7. The contents was allowed to cool to room temperature for 10 minutes and then
the flask was placed in ice.
8. The contents of the flask were stirred with a glass rod periodically during this
cooling period.
9. To recrystallize the acetylsalicylic acid, it was transferred to a 100 ml conical
flask and 10 ml of 95% ethanol was added.
10. The flask was heated slowly on a hot plate just until the aspirin completely
dissolved. It was then removed from the hot plate.
11. Water was added into the solution in 5 ml portions, swirling after each
addition, until a total of 25 ml of water had been added.
12. The flask was placed in an ice bath for 10 minutes to complete the
crystallization.
13. The purified aspirin was collected by vacuum filtration. The crystals was were
washed in the funnel with ice-cold water. The vacuum was left on for a few
minutes to air dry the purified aspirin.
14. The mass, yield and melting point of the dried aspirin were obtained.

Results and calculations:

Mass of salicylic acid (g) 2.0132
Volume of acetic anhydride (ml) 5.00
Mass of plastic (g) 0.5146
Mass of plastic + recrystallized product (g) 1.9155
Mass of product (g) 1.4009
Melting point of recrystallized product (°C) 139
Theoretical yield:
To determine the limiting reagent:
mass
Density of acetic anhydride, ρ =
volume
mass
1.082 g/ml =
5 ml
Mass = 5.41 g
Since the mass of salicylic acid is 2.0132 g which is less than the mass of acetic
anhydride, thus the limiting reagent is salicylic acid.

mass
Mole of Salicylic Acid =
molar mass
= (2.0132 g)/138.12 g/mol
= 0.01458 mol

1 mol of salicylic acid = 1 mol of acetylsalicylic acid

Therefore, 0.01458 mol of salicylic acid = 0.01458 mol of acetylsalicylic acid

The theoretical mass of ASA = molar mass x mole

= 180.16 g/mol x 0.01458 mol
= 2.6267 g
actual yield
Percentage yield = x 100 %
theoretical yield
= (1.4009/2.6267) x 100 %
= 53.33 %

Discussion:
The actual yield of aspirin synthesized was 1.4009 g and the theoretical yield was
2.6267 g , thus the percent yield was 53.33 % . It is difficult to obtain a percent yield
of 100% because products can react to produce the reactants. The percent yield is
relatively slightly low because of possible sources of error. One possible source of
error could result from loss of product due to exposure to atmospheric air causing
prolonged air drying. Another possible source of error could be due to insufficient
heating, which would also yield a loss of product. A lower yield could also result
from an incomplete conversion of reactants to products due to decomposition of acetic
acid in the solution. Esterification is the process of forming a carboxylate ester by
reacting a carboxyl group with a hydroxyl group or phenol group. The formation of
acetylsalicylic acid is an esterification reaction as well as an equilibrium process. Le
Chatelier’s principle is also known as the equilibrium law and allows one to
determine the effects on equilibrium due to pressure, temperature, and concentration.
As the reactants are used, the concentration of the reactions decreases and the
concentration of the products increases. Le Chatelier’s principle is used to favor
products, because excess acetic anhydride forces the equilibrium to shift towards the
desired product, aspirin. Le Chatelier’s principle also favors the reactants because
aspirin can be converted back to salicylic acid and acetic anhydride and oncte the
changes are adjusted, a new equilibrium will be established (3). Salicylic acid
contains two acidic functional groups; a phenol group and a carboxylic acid. The
phenol group on the salicylic acid causes stomach irritation. An ester is formed from
the phenol group and carboxylic acid on the acetic acid. It is desirable to do away
with one of these groups because the acids on the molecules are what cause irritation.
By replacing one of the acid groups, the acid strength is reduced making it easier to
digest

Above is the reaction occurring between salicylic acid, phosphoric acid, and acetic
anhydride. Phosphoric acid attacks the carbon oxygen bond (C=O) of the acetic
anhydride giving it a positive charge, thus acetic anhydride is more prone to
nucleophilic attacks. The nucleophile in the reaction is salicylic acid. It was formed
when the oxygen on the phenol group of salicylic acid attacked the partial positive
charge of a carbon from acetic anhydride, transferring electrons to oxygen. Salicylic
acid then attacks the acetic anhydride because of its positive charge. The oxygen from
the phenol group now has a positive charge and the carbonyl groups withhold a
negative charge. A tetrahedral intermediate is then formed. The electrophilic carbon is
attached to the –OH group. The –OH group protonates the hydrogen and donates an
electron to form a double bond between carbon and oxygen (C=O). The phosphoric
acid is then reformed again as it gains the loss of the proton. Acetylsalicylic acid is
thus formed
Safety precautions:
Some precaution steps were taken into care while conducting the experiment to
ensure a smooth running of the experiment. Firstly, gloves, lab coat and safety
goggles were worn throughout the experiment to avoid direct contact of the skin or
eyes with the chemical reagents. Besides, all the reacted reagents or unused chemicals
were discarded into the labelled waste bottles given.

Conclusion:
The amount of pure acetylsalicylic acid obtained was 1.4009 g with a yield of 53.33
%. The melting point obtained was 139 °C.

References:
Rogers, K., 2015. Ester. [online]
Available at: <https://www.britannica.com/science/ester-chemical-compound>
[Accessed 12 August 2018]