Synthesis of methyl salicylate by Fischer-Speiers esterification

1. Synthesis of Oil of Wintergreen (Methyl Salicylate) by the Fischer-Speiers
esterification method
Molly Rose Winterbottom
1. Introduction
This experiment consists of the production of an ester, so to start, lets clarify what an ester is.
An ester is formed from a carboxylic acid and an alcohol, in this case the carboxylic acid being
salicylic acid and the methanol as the alcohol.
Figure 1. Synthesis of Methyl Salicylate (highlighted Carboxylic acid group, Alcohol group,
Methyl group joining salicylic acid and water molecule)
Esters are commonly known for their aromatic properties which are used to give herbs and
plants their properties and distinctive smells. The ester formed in this esterification method is
methyl salicylate which has a well-known mint-like smell to it, found in everyday life like in
chewing gum, toothpaste and in most pain relief ointments such as “Deep Heat” and “vapour
rub” for congestion. Esters are useful in these substances as they’re easily absorbed into the
skin to help with the aches and pains. Methyl salicylate is extracted from the Wintergreen plant
species hence the name of this experiment “Oil of Wintergreen”, when removed from the plant
Gaultheria procumbens it gives a type of oil1
. Methyl salicylate is mainly produced through
organic synthesis as it tends to be the easier method than extracting it from the Wintergreen
plant, so one way to produce methyl salicylate (shown in figure 1) is to use excess of the alcohol
(the methanol) under reflux using concentrated sulphuric acid as the acid catalyst, it is fine to
add excess of methanol because it has a low boiling point so it will all evaporate. This then, in
accordance to Le Chatelir’s Principle allows the equilibrium reaction to go to completion and
a sample can be taken after 20-30 minutes under reflux and used on a TLC plate against
salicylic acid and a pure sample of methyl salicylate, although it is best to leave the reaction
under reflux for longer since larger esters tend to form more slowly2
, so most of the reactants
have reacted to form the methyl salicylate. The excess methanol will be removed by the rotary
evaporator at the next stage before taking the second TLC plate. Although there are a few
methods in producing methyl salicylate, for example from Aspirin, this lab report just focused
on the Fischer-Speier esterification method.
The mechanism for the reaction done can be seen below in figure 2.
OH OH
O H3C OH
OH O
O
CH3
O
H H
water
+
H
D
+
excess
salicylic acid methyl salicylate
The carboxylic acid group drops 1 of its H atoms and exchanges it for a methyl group,
leaving the H atom to bond to the alcohol group forming Water as a product.

2. Figure 2. Mechanism of the Fischer-Speier method for the formation of methyl salicylate
As you can see from this mechanism the concentrated sulphuric acid is used as an electron
pair acceptor from the oxygen on the salicylic acid molecule, this then allows the methanol
molecule to join onto salicylic acid on the carboxylic acid group which then released water as
a product along with methyl salicylate and allows the catalyst to regenerate.3
2. Results and Discussion
Calculating Yield collected:
Yield of methyl salicylate obtained = 6.46g
% 𝑦𝑖𝑒𝑙𝑑 =
!"#$!% '()%*
#+),-)#("!% '()%*
x 100
Theoretical yield: mass of salicylic acid used = 7.0000g
Mr of salicylic acid = 138
Mr of methyl salicylate = 152
moles = weight / Mol. Wt. (Mr)
moles = 7.0000 / 138 moles = 0.0507mol
weight = moles x Mr
weight = 0.0507mol / 152 weight = 7.7064g
OH O
OH
H OH O
OH
H
H3C
O
H
OH OH
O
H3C
H
O
H
OH OH
O
CH3
O
H
H
OH O
CH3
O
H +
H2O
-
catalyst
regenerated
catalyst
methyl salicylate

3. Theoretical Yield = 7.71g
% Yield = (6.46g / 7.71g) x 100
% Yield = 83.8 %
(Below are the IR spectrums of the molecules and compound, however I do have to clarify that these were
NOT produced in the lab by myself, but will give an analysis, therefore I do not have an IR spectrum for my
crude methyl salicylate produced)
Figure 3. The IR spectrum of salicylic acid
This is the IR spectrum of salicylic acid taken from the lab book which shows a broad peak
for the -OH functional group due to it having a carboxylic acid group at 3200-3400 cm-1
Figure 4. The IR spectrum of methyl salicylate
This IR of methyl salicylate shows the lack of the -OH functional group at 3200-3400 cm-1
as
seen in the IR of salicylic acid (figure 3) because the carboxylic acid group has been
converted into an ester. Instead it shows a strong and sharp peak at 1673 cm-1
showing
the >C=O ketone group bring formed.

4. TLC plates:
During the experiment 2 TLC (thin layer chromatography) plates were taken. These were
taken once the reaction mixture was obtained of both salicylic acid and methanol which had
undergone an equilibrium reaction forming some methyl salicylate but still with the presence
of some salicylic acid as you can see from TLC plate 1 (figure 5). Then the second TLC plate
once the mixture had finished in the rotary evaporator and all the methanol had evaporated
off, leaving the methyl salicylate alone which was compared with the pure sample of methyl
salicylate given as seen in TLC plate 2 on the right-hand side (figure 6)
Figure 5 (TLC plate 1) Figure 6 (TLC plate 2)
KEY:
SA = salicylic acid cMS = crude methyl salicylate (my product)
RM = reaction mixture pMS = pure methyl salicylate (sample given)
MS = methyl salicylate
SA RM MS SA cMS pMS

5. 3. Experimental Method
To start with, salicylic acid (7.00g, 0.507 mol) was added to a 100ml round bottomed flask
then added methanol (30ml, 0.75 mol), using a magnetic stirrer to swirl the mixture together
until the salicylic acid was dissolved. Proceeding to then weigh out the concentrated
sulphuric acid (8ml) which was added to the mixture whilst it was still stirring, it was added
drop by drop using a pipette. Then the round bottomed flask was connected to a water
condenser vertically (under reflux) and maintained a steady flow of water through the
condenser itself. To allow the reaction to occur the round bottomed flask was placed onto a
hot plate to allow the mixture to reflux for about 30 minutes after bubbling (boiling) was
observed. After the time was up, the water condenser was disconnected from the round
bottomed flask and allowed the mixture in the flask to cool in an ice bath, as the mixture
cooled the purity was tested using TLC in the TLC solvent [ethyl acetate: heptane 1:4]
against salicylic acid and a pure sample given of methyl salicylate (seen in figure 5 above).
The reaction mixture was then moved from the round bottomed flask into a separating funnel
using the same filter funnel, so no product was lost. The reaction mixture had
dichloromethane (75ml) added to it and shaken in the separating funnel making sure to invert
the funnel and release pressure building up by opening the tap to let the gas out to prevent
shattering of the funnel. This step was repeated when the organic layer was extracted into a
conical flask and placed back into the separating funnel once the aqueous layer was disposed
of, making sure to still shake and release pressure build-up of gas in the separating funnel.
Once both organic layers from these repeated steps are collected in the same conical flask it
was placed back into the separating funnel for the third time and washed with saturated
bicarbonate solution (75ml), shaken CAREFULLY since CO2 gas is released the tap will
need to be opened plenty of times till the hissing sound of the gas coming out decreases in its
volume.
Once the mixture is extracted again, it is then dried using anhydrous magnesium sulphate and
filtered into a weighed round bottomed flask which was placed onto a rotary evaporator
where the solvent was evaporated off. As mentioned above the second TLC plate was then
taken using the same TLC solvent [ethyl acetate: heptane 1:4] with the crude product
produced to check its purity against the pure sample of methyl salicylate given along with
salicylic acid (see figure 6 above).4
4. Summary
To summarise the Oil of Wintergreen Fisher-Spier esterification experiment, in order to add
excess of methanol, more time may be required whilst the reaction mixture is being heated
under reflux so the reaction will have more time to come to completion and produce the
product methyl salicylate and for more yield to be collected.

6. 1
Vogel's Textbook of Practical Organic Chemistry 5th Ed. London: Longman Science & Technical. pp. 695–697
& 699–704. ISBN 9780582462366.
2
http://www.chemguide.co.uk/organicprops/acids/esterification.html chapter on esterification – alcohols
and carboxylic acids (accessed 28/11/2020)
3
Organic chemistry, Clayden, Jonathan, Greeves, Nick, Warren, Stuart, Wothers, 2000
4
Lab book used for experimental method on the day (experiment done on 19/11/2020)