Properties of Carboxylic Acids and Derivatives
Rannilo D. Rocas Jr.
Department of Biochemistry, College of Humanities and Sciences,
De La Salle Medical and Health Sciences Institute, City of Dasmariňas, Cavite
27 July 2020
ABSTRACT
Carboxylic acids are the compounds containing the carboxyl functional group attached to the hydrocarbon. This type of
organic acid is largely distributed in nature and are intermediates in the degradation pathways of amino acids, fats, and
carbohydrates. The presence of both hydroxyl and the carbonyl groups in the molecule enabled the carboxylic acids to
exhibit hydrogen bonding with water and themselves, which leads to increased stabilization of the compounds and elevated
boiling points. This functional group allowed for the synthesis of numerous derivatives of the acid through different chemical
reactions. In this experiment, the properties of carboxylic acids and its derivatives were analyzed through solubility test,
reaction with base which led to carboxylic acid salt formation, hydrolysis of carboxylic acid salt, and formation of ester through
Fischer esterification. The solubility test showed that carboxylic acids (having ≤ 5 carbon atoms) were highly soluble in water.
The reaction of benzoic acid with a base produced sodium benzoate, which was observed to be soluble in water. Hydrolysis
of sodium acetate produced a slightly alkaline solution with a pH of 9. Three different types of esters were produced through
Fischer esterification, the heated solution of acetic acid and benzyl alcohol with sulfuric acid produced benzyl acetate, which
had a sweet and fruity odor. Isoamyl acetate, with a banana-like odor was produced from the reaction of acetic acid and
isoamyl alcohol. The third ester was produced from the reaction of salicylic acid and methanol, an ester call methyl salicylate,
which had a minty odor.
Keywords: Carboxylic acids, Carboxylic acid derivatives, Hydrolysis, Salt formation, Fischer esterification.
Introduction:
Carboxylic acids are organic acids characterized by a
carbonyl group and a hydroxyl group (COOH). This type of
organic acids is common in nature, often combined with
other functional groups. Simple alkyl carboxylic acids,
composed of four to ten carbon atoms, are liquids or low
melting solids having very unpleasant odors. Carboxylic
acids are typically weak acids. When a carboxylic acid is
dissolved in a basic solution, carboxylate salt is formed
which can undergo hydrolysis.
Figure 1: Carboxylic acid salt formation
The dipoles present in carboxylic acids allow these
compounds to have hydrogen bonding with like molecules
and water. The hydrogen bonding accounts for the water
solubility of the carboxylic acid (Ouellette, 2014). The
hydroxyl group of a carboxylic acid can be replaced by
another nucleophilic group through nucleophilic
substitution.
Ester is one of the derivatives of a carboxylic acid, in
which the hydroxyl group is replaced by an OR group. They
possess a pleasant scent that is used as flavoring
(Bonifacio, 2020).
Methodology:
A. Solubility of Carboxylic Acids and Derivatives
Figure 2: Fischer Esterification
Fischer esterification is a process wherein an ester
is made by heating a mixture of the carboxylic acid and
alcohol together with a strong acid (often sulfuric) as a
catalyst. The OR oxygen atom of the ester comes from the
alcohol and not from the carboxylic acid (Reusch, 2013).
On a clean and separate test tubes, two drops of
acetic acid, propionic acid, acetonitrile, ethyl acetate, and
acetanilide were transferred. Three drops of deionized
water were added to each test tube. The test tubes were
shaken up and the formation layers were observed.
Figure 3,4 and 5: methanol, isoamyl alcohol, and
benzyl alcohol.
The figures above are the types of alcohol used in
this experiment. The reaction of an alcohol with the
carboxylic acid is an equilibrium, it is driven to the right by
using excess alcohol or removing the water as it is formed
(Smith, 2020).
Figure 6: Set up for solubility test
B. Formation of a Carboxylic Acid Salt
A crystal of benzoic acid is added to two small test
tubes. On one test tube, two drops of cold deionized water
were added, and on the other test tube, two drops of 10%
NaOH solution were added. The test tubes were shaken up
repeatedly and the solubility of the benzoic acid was
observed.
C. Hydrolysis of Carboxylic Acid Salt
Figure 6 and 7: Acetic acid and salicylic acid
A crystal of sodium acetate was dissolved in five drops of
deionized water. The pH of the sodium acetate solution and
deionized water was obtained by using a pH paper.
The structure of the carboxylic acids used for Fischer
esterification. Esterification of carboxylic acid only occurs in
the presence of acid, not in the presence of a base, this is
due to the reaction of the base with the hydrogen atom of
the hydroxyl group forming an electron-rich carboxylate
anion. (Smith, 2020).
Objectives:
1. Understand the properties of carboxylic acids and their
derivatives
2. Predict the product of a Fisher esterification reaction.
Figure 7: pH reading
Acetonitrile
D. Fischer Esterification
Three test tubes were labeled with 1, 2 and 3. Two
drops of acetic acid were transferred to test tube 1 and 2
respectively. On test tube 3, two crystals of salicylic acid
were added. Two drops of benzyl alcohol were added to
test tube 1, two drops of isoamyl alcohol were added to test
tube 2 and two drops of methanol were added to test tube
3. A small drop of sulfuric acid was added to each test
tube. The tubes were shaken up and then covered. On a
60 degrees Celsius water bath, the tubes were heated for
15 minutes. The tubes were cooled to a room temperature
and the odor of the products were noted.
Figure 8: Part D set up
Ethyl acetate
Sparingly Soluble
Acetanilide
Sparingly Soluble
In the water solubility test, acetic
acid
and
propionic acid was observed to be soluble, this was due to
the hydrogen bonding capability of the carboxylic acids.
The large dipole-dipole attraction between acetonitrile and
water accounts for the observed solubility. Ethyl acetate
was sparingly soluble, although esters can have hydrogen
bonding with water through the oxygen atom, it lacks
hydrogen for additional hydrogen bonding. The solubility of
ethyl acetate at ambient temperature is roughly 8.5g per
100 ml of water. Acetanilide is also observed to be
sparingly soluble, almost insoluble in observation. The
large and nonpolar aromatic ring cannot form hydrogen
bonding with water. Although at high temperature, the
solubility of acetanilide increases to about 5.5 g/100 mL, at
room temperature, the water solubility drops to about 0.53
g/100 mL.
Table 2. Formation of Carboxylic Acid Salt
Solvent
Figure 9: Water bath set up
Results and Discussions:
Soluble
Observation
Deionized water
Insoluble
10% sodium hydroxide
Soluble
The benzoic acid was observed to be insoluble. This
agrees with the characteristic of the acid having a bulky
non-polar benzene ring. On the other hand, the reaction of
the benzoic acid with NaOH produced sodium benzoate,
which is highly soluble in water with about 60 g or more per
100 mL of water. The positive end of the water is drawn to
the negatively charged (COO-) group and the positively
charged Na+ draws the oxygen atom of the water. This
was observed in the test tube containing the benzoic acid
and NaOH solution.
Table 1. Solubility of Carboxylic Acids and Derivatives
Compound
Observation
Acetic acid
Soluble
Propionic acid
Soluble
Table 3. Hydrolysis of Carboxylic Acid Salt
Solution
Deionized water
pH
7
Sodium Acetate solution
9
The mixture of sodium acetate with water produced a
vinegary odor, this indicated that acetic acid is formed from
the reaction. The pH of the sodium acetate solution was
identified and was found to be basic compared to the
neutral pH of the deionized water. The alkalinity of the
solution was due to the hydrolysis of the acetate anion that
produced acetic acid and hydroxide ion.
Figure 10: Hydrolysis of Acetate anion
Table 4. Fischer Esterification
Test tube
In test tube 3, a minty odor was produced from the
reaction of methanol and salicylic acid, which indicated that
another type of ester was produced. This ester is methyl
salicylate, a type of compound used to treat minor aches
and pains of the muscles/joints (such as arthritis,
backache, sprains). Together with menthol, this compound
makes the skin feel cold and then warm, distracting a
person from feeling the aches/pain coming from the bone
or the muscle.
Observation
1
Sweet, fruity
2
Aroma of banana
3
Figure 12: Formation of Isoamyl acetate
Minty
In test tube 1, the solution of benzyl alcohol and
acetic acid produced a sweet and fruity odor. This indicated
that the two compounds reacted and produced an ester,
the odor agrees with the unique characteristic of esters,
having aromas which are commonly used in food
industries, cosmetics, and perfumes. The two compounds
in test tube one produced an ester called benzyl acetate.
This type of ester had been well known to have a sweet
and fruity odor and attract bees in nature.
Figure 11: Formation of Benzyl acetate
The solution of isoamyl alcohol and acetic acid in test
tube 2 produced a “banana-like” odor, this indicated that an
ester was produced in the reaction between the two
compounds. The ester synthesized was isoamyl acetate,
this compound has been commonly used to confer banana
flavor in foods, oils, and lacquers.
Figure 13: Formation of Methyl salicylate
Conclusion:
Carboxylic acids are compounds occurring naturally
in different stages of biological processes in life (living
organism-Krebs cycle; fermentation processes, and
geological processes) or can be produced in the
laboratories or at large scale. The vast array of the
applications of this acid is seen in the field of medicine,
agriculture, pharmaceuticals, food, and other industries.
The results in this experiment were in accordance
with the established knowledge about the properties of
carboxylic acids and its derivatives. The chemical structure
of this organic acid reflected its observed solubility in water,
and reaction with base showed a formation of negatively
charged (COO-) group, which accounts for the solubility of
benzoic acids in base. A simple demonstration of Fischer
esterification was attained, the reaction of the carboxylic
acids and alcohol produced esters that are widely used in
different industries.
It is highly advised that the glass apparatus in this
experiment is free from contaminants, to avoid altered
results. A hot water bath at an ideal temperature must be
applied to have better and faster results.
References:
Badea, G., & Radu, G. (2018, June 13). Introductory Chapter: Carboxylic Acids - Key Role in Life Sciences. Retrieved
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Ouellette, R., & Rawn, J. (2014, June 13). Carboxylic Acids. Retrieved July 21, 2020, from
https://www.sciencedirect.com/science/article/pii/B9780128007808000206
Reusch, W. (2013, May 5). Carboxylic Acids. Retrieved July 21, 2020, from
https://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/crbacid1.htm
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