NUCLEOPHILIC ACYL SUBSTITUTION: THE SYNTHESIS OF ESTERS Anna Sophia M. Ebuen Department of of Mining, Metallurgical Metallurgical and Materials Engineering, Engineering, College College of Engineering Engineering University of the Philippines, Diliman, Quezon City 1101 Philippines Date Performed: Performed: February February 12, 2013; 2013; Date Submitted: Submitted: February February 19, 2013
The objective of this experiment is to observe the synthesis of an ester, also known as the process of esterification. It can be synthesize from the reaction of an alcohol and a carboxylic acid with an acid catalyst for it to proceed. The main mechanism of the reaction is a nucleophilic acyl substitution wherein the carboxylic acid is attacked by a nucleophile which results to the departure of the leaving group. The experiment yielded 4.1 grams of propyl ethanoate with an 80 °C boiling point which should emit a fragrance of a pear. With a 44.61 percent recovery, there is an ample amount of deviation from the experimental value with respect to the theoretical because of an imperfect set-up and other various factors that contributed to inaccuracy of the data.
INTRODUCTION
Esters are compounds which consist of a carbonyl (C=O) (C=O) together with an ether linkage. They are widespread in nature because of their capability to occur naturally in plants and animals.
alcohol solvent to yield an ester and water as end [1] products . This reaction is reversible and the position of the equilibrium can be shifted by the addition of the excess that favors the formation of the ester.
Several esters have a strong sweet smell that causes the pleasant fragrance emitted by fruits and flowers. They can also be in food, beverages and perfumes to enhance the flavour by using synthetically synthetically made esters. esters.
Here, an acetic acid and 1-propanol was used as the carboxylic acid and the alcohol while a sulfuric acid was used as an acid catalyst to yield a propyl ethanoate or more commonly known as propyl acetate which emits a pear-like odor thus achieves the objective of the experiment which is to synthesize an ester.
Esters are formed though the process called esterification. It can be formed through several ways from acyl chlorides and alcohols, anhydrides and alcohols, and also from the reaction of carboxylic acids and alcohols which this experiment focuses about. In this experiment, Fischer Esterification reaction was used. This involves heating the carboxylic acid with an acid catalyst in an
EXPERIMENTAL DETAILS
Pear was the assigned flavour that was done; thus, 6.74 ± 0.3 ml of 0.09 mol of 1 propanol and a 5.15 ml of 0.09 mol of acetic acid was added in a 25-ml round bottom flask.
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Afterwards, 2-3 drops of concentrated sulphuric acid (H2SO4) was carefully added and gently swirled. Then, the round bottom flask was attached to a reflux condenser in order for it to reflux for 45-60 minutes (shown in Figure 1).
Figure2. Simple Distillation Set-up
Figure1. Reflux Setup
It was then cooled at a room temperature using the tap water and later poured in a 30-ml separatory funnel. The reaction flask was then rinsed with cold distilled water and the washings were added in the funnel. This process was repeated until the aqueous layer is already twice the volume of the organic layer. Then, it was shook and let the layers to stratify. In the separatory funnel, a saturated sodium chloride (NaCl) solution was added to hasten the separation of the layers. Also, solid sodium bicarbonate (NaHCO3) was also added until no effervescence can be seen which indicates that the acid is completely removed. Afterwards, the organic layer of the solution (which is located at the top part) was collected and dried by adding an anhydrous Na 2SO4. Then, the ester was placed in a round bottom flask and was distilled using glasswares dried thoroughly inside the fume hood (shown in Figure 2).
The product was then placed in a vial and then weighed in order to calculate the percentage yield.
RESULTS AND DISCUSSION
The main objective of the experiment is to synthesize an ester from the reaction of a carboxylic acid and an alcohol.
Figure3. Reaction Equation
The general mechanism for the Fischer esterification reaction involves heating the carboxylic acid with an acid catalyst in an [1] alcohol solvent is the Nucleophilic Acyl Substitution. In this mechanism, the carboxylic acid derivative reacts with the nucleophile wherein the leaving group bonded to the acyl carbon can leave as a stable anion which results to the formation of a new carbonyl compound.
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The next step involves the carbocation in the acetic acid being attacked by one of the lone pairs of oxygen in propanol producing a tetrahedral intermediate with the electrons moving towards the oxonium ion. Figure4. General Mechanism of Nucleophilic Acyl Substitution
The more electron-poor the carbonyl carbon is, the more readily the compound reacts with a nucleophile. While with respect to the leaving group, the weaker the base, the better the leaving group. However, if the reaction components are less reactive then the reaction will be very slow unless promoted. The reaction rate of compounds with poor leaving groups (shown in Figure 5) can be accelerated by protonation in acidic media.
Figure5. Order of Reactivity of the Leaving Groups
Figure7 . Attraction of the alcohol to the carbocation intermediate in the carbonyl group
Afterwards, the proton in the oxygen on the propanol which attached to acetic acid is deprotonated. This results for the conversion of the – OH group into a good leaving group by protonation from another propanol molecule.
In this experiment, the acetic acid having a hydroxide ion which acts as the leaving group is a strong base resulting as a poor leaving group. In order for the reaction to proceed, protonation is required to make it more reactive. The first step of the mechanism is the protonation or the taking of a proton by the acetic acid from the propanol to have a more electrophilic carbocation.
Figure8. Transfer protons from one oxygen atom to another
Finally, the leaving group is pushed out of the compound with the help of the electrons in the adjacent oxygen. Also, deprotonation of the oxonium ion produces the carbonyl in the propyl ethanoate with water as products. Figure6 . Protonation of a lone pair of oxygen in the carbonyl group
The proton is now attached to one of the lone pairs of oxygen double bonded to a carbon and is now positively charged. It is in resonance with the structure wherein the double bond becomes a single bond; thus, forms another lone pair and makes the carbon positively charged being a carbocation intermediate.
Figure9. Formation of ester
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In the experiment proper, in order for an ester with a pear flavour to be produced, a specific quantity of 0.09 mol acetic acid (5.15 ml) and 0.09 mol of 1-propanol (6.74 ml) were added.
because there is a reaction that can be observed (effervescence) while adding to the organic layer.
C2H4O2 + C3H8O ---> C5H10O2 + H2O Figure10. Reaction Equation of Propyl Ethanoate
Le Chatelier’s Principle applies to this wherein the conditions were chosen to favour the desired product as much as possible. According to this principle, “if a change is imposed on a system at equilibrium, the equilibrium position will shift in a direction that tends to reduce that change in condition” [2]; thus, this implies that at the equilibrium point, the state in which the concentration of all reactants and products remain constant with time [2], the addition of an excess alcohol (1-propanol) was used for the reaction to proceed forward causing an increase in yield. 1-propanol was added having 100.48mmol while acetic acid is the limiting reagent having 90.03mmol. Another method to increase the yield is removing the water from the reaction as it is formed due to the sulfuric acid being added Reflux is a distillation technique involving the condensation of vapors and the return of portions of the condensate to the system where it came from [3]. The flask is heated for 45 minutes and the purpose of this procedure is to thermally accelerate the reaction. After doing so, it was cooled to room temperature and transferred into a separatory funnel. Two layers formed in which the aqueous layer located below was discarded to prevent hydrolysis (reaction with water) while the organic layer was kept It was then added by sodium chloride (NaCl) to hasten the separation and also sodium carbonate (NaHCO3) to remove the acid. Effervescence was observed upon addition of the solid, due to the formation of carbon dioxide gas upon neutralization. When none of it was observed, it signifies the complete removal of the acid. NaHCO3 was preferred that NaOH
Figure11. Reaction Equation of Sodium Bicarbonate with Sulfuric Acid
Figure11. Reaction Equation of Sodium Hydroxide with Sulfuric Acid
After the addition of such compounds, a crude ester is formed. In order to further remove the impurities, distillation is needed. In this process, distillation is necessary to purify substances and can also eliminate the excess 1 propanol present. Through simple distillation, the boiling point of the ester may be determined by reading the temperature where it remained constant. Esters can also be prepared using different alcohols. However, when using an ethanol it is necessary to use the absolute one since it is more pure and has a lesser water content than the regular one. As mentioned previously, the lesser the water content results to a greater yield. Following the procedures thoroughly and also keeping in mind the important notes, the outcome of the experiment has a percent yield of 44.61% and a boiling point of 80 °C producing a pure propyl ethanoate or propyl acetate which has a pear smell.
CONCLUSION
With the completion of the experiment, the objective which is to synthesize an ester was achieved with a percentage yield of 44.61% and 4
a boiling point of 80°C producing a propyl ethanoate or more commonly called as propyl acetate having a pear smell. The mechanism of the reaction is by nucleophilic acyl substitution of acetic acid and 1-propanol using sulphuric acid as a catalyst. 4.1 grams of propyl ethanoate was obtained while the theoretical value of it is 9.19 grams. Even though the set-up and the process are not completely ideal that it may incur various errors, they are really inevitable because they are a part of experimentation. However, it is best to reduce these errors for the improvement of the experiment. This can be done by properly cleaning the apparatus, handling the reagents, and following the procedures carefully in order to increase the percentage yield of the experiment.
REFERENCES
[1] McMurry, J. Fundamentals of Organic Chemistry, 7th ed.; Brooks/Cole Cengage Learning: USA, 2011; pp 330, 339-340 [2] Zumhdal, S.S. and Zumdahl, S.A. Chemisty, th 8 ed; Brooks/Cole Cengage Learning: Belmont, CA, 2010; p 230 [3] Reflux (distillation) http://en.citizendium.org/wiki/Reflux_(distillatio n) [4] Silava, N.D. Nucleophilic Acyl Substitution: Synthesis of Esters. 2011. Retrieved from http://www.scribd.com/doc/50922950/Nucleoph ilic-Acyl-Substitution-The-Synthesis-of-Esters
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APPENDIX
A.
Calculations
= 9.19 g (
Weight of 1-propanol = (0.896
g ml
) (6.74 ml)
1mol 60.1 g
)(
1000mmol 1mol
= )
0.888 ml 1 g
)
=8.16 ml Percent yield =
= 6.039 g Mmol of 1-propanol = (6.039 g) (
= 9.19 g Volume of propyl ethanoate
actual
x100 theoretical 4.1 g 9.19 g
x 100
= 44.61 %
= 100.48 mmol
Weight of acetic acid = (1.049
g ml
B.
) (5.15 ml)
= 5.402 g
Mmol of acetic acid = (5.402 g) (
1mol 60 g
)(
1000mmol 1mol
)
= 90.03 mmol
1ml 0.888 g
)
= 4.617 ml Mmol of propyl ethanoate = 90.03 mmol acetic acid (
propyl _ ethanoate acetic _ acid
Volume of propyl ethanoate = (4.1 g) (
Guide Questions used How did you use Le Chatelier’s Principle to ensure a reasonable yield of ester? Which starting material is used as the limiting reagent? Which reagent was used in excess? Suggest another method that will favour the formation of esters? Give the equation to explain the relevance of using NaHCO 3 over NaOH in neutralizing H2SO4.
1mmol
1mmol
)
= 90.03 mmol Theoretical yield of propyl ethanoate = 90.03 mmol propyl ethanoate (
1mol 1000mmol
)(
102.13 g 1mol
)
C.
How was the excess alcohol removed from the crude ester after the reaction was completed? Write the mechanism for the acid catalysed esterification of the carboxylic acid and alcohol used. If you were preparing an ester with ethanol, why is it necessary to use absolute ethanol?
Data Sheet (attached)
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