Survey of Chemistry I (CH 131)
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Properties of Carboxylic Acids and Esters
Background
Carboxylic acids are structurally like aldehydes and ketones in that they contain the carbonyl
group. However, an important difference is that carboxylic acids contain a hydroxyl group
attached to the carbonyl carbon.
—COOH The carboxylic acid group
This combination gives the group its most important characteristic; it behaves as an acid.
As a family, carboxylic acids are weak acids that ionize only slightly in water. As
aqueous solutions, typical carboxylic acids ionize to the extent of only one percent or less.
R—COOH + H2O → R—COO- + H3O+
At equilibrium, most of the acid is present as un-ionized molecules. Dissociation constants, Ka,
of carboxylic acids, where R is an alkyl group, are 10-5 or less. Water solubility depends to a
large extent on the size of the R-group. Only a few low-molecular-weight acids (up to four
carbons) are very soluble in water.
Although carboxylic acids are weak, they are capable of reacting with bases stronger than water.
Thus while benzoic acid shows limited water solubility, it reacts with sodium hydroxide to form
the soluble salt sodium benzoate. (Sodium benzoate is a preservative in soft drinks.)
C6H5-COOH + NaOH → C6H5-COO-Na+ + H2O
Benzoic acid
Sodium benzoate
Insoluble
Soluble
Sodium carbonate, Na2CO3, and sodium bicarbonate, NaHCO3 , solutions can neutralize
carboxylic acids also.
Survey of Chemistry I (CH 131)
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The combination of a carboxylic acid and an alcohol gives an ester; water is eliminated. Ester
formation is an equilibrium process, catalyzed by an acid catalyst.
CH3CH2CH2 COOH + CH3CH2OH → H2O + CH3CH2CH2 COOCH2CH3
Butyric acid
Ethyl alcohol
Ethyl butyrate (Ester)
Esterification →
←Hydrolysis
The reaction typically gives 60% to 70% of the maximum yield. The reaction is a reversible
process. An ester reacting with water, giving the carboxylic acid and alcohol, is called
hydrolysis; it is acid catalyzed. The base-promoted decomposition of esters yields an alcohol and
a salt of the carboxylic acid; this process is called saponification. Saponification means, “soap
making,” and the sodium salt of a fatty acid (e.g., sodium stearate) is a soap.
CH3CH2CH2COOCH2CH3 + NaOH → CH3CH2CH2COO-Na+ + CH3CH2OH
Saponification →
A distinctive difference between carboxylic acids and esters is in their characteristic odors.
Carboxylic acids are noted for their sour, disagreeable odors. On the other hand, esters have
sweet and pleasant odors often associated with fruits, and fruits smell the way they do because
they contain esters. These compounds are used in the food industry as fragrances and flavoring
agents. For example, the putrid odor of rancid butter is due to the presence of butyric acid, while
the odor of pineapple is due to the presence of the ester, ethyl butyrate. Only those carboxylic
acids of low molecular weight have odor at room temperature. Higher-molecular-weight
carboxylic acids form strong hydrogen bonds, are solid, and have a low vapor pressure. Thus few
molecules reach our noses. Esters, however, do not form hydrogen bonds among themselves;
they are liquid at room temperature, even when the molecular weight is high. Thus they have
high vapor pressure and many molecules can reach our noses, providing odor.
Objectives:
1. To study the physical and chemical properties of carboxylic acids: solubility, acidity,
and aroma.
2. To prepare a variety of esters and note their odors.
3. To demonstrate saponification.
Survey of Chemistry I (CH 131)
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Procedure
Carboxylic Acids and Their Salts
Characteristics of acetic acid
1. Place into a clean, dry small test tube (100 x13 mm) 2 mL of water and 10 drops of glacial
acetic acid. Note its odor by wafting (moving your hand quickly over the open end of the test
tube) the vapors toward your nose. Of what does it remind you?
2. Take a glass rod and dip it into the solution. Using wide-range indicator paper (pH 1—12),
test the pH of the solution by touching the pH paper with the wet glass rod. Determine the
value of the pH by comparing the color of the pH paper with the chart on the dispenser.
3. Now, add 2 mL of 2 M NaOH to the solution. Cork the test tube and sharply tap it with your
finger. Remove the cork and determine the pH of the solution as before; if not basic, continue
to add more base (dropwise) until the solution is basic. Note the odor and compare to the odor
of the solution before the addition of base.
4. By dropwise addition of 3 M HCl, carefully reacidify the solution from step no. 3 (above);
test the solution as before with pH paper until the solution tests acid. Does the original odor
return?
Characteristics of benzoic acid
1. You will weigh out 0.1 g of benzoic acid. Add the solid to a small test tube (100 x 13 mm)
along with 2 mL of water. Is there any odor? Mix the solution by sharply tapping the test tube
with your finger. How soluble is the benzoic acid?
2. Now add 1 mL of 2 M NaOH to the solution from step no. 1 (above), cork, and mix by
sharply tapping the test tube with your finger. What happens to the solid benzoic acid? Is
there any odor?
3. By drop-wise addition of 3M HCl, carefully reacidify the solution from step no. 2 (above);
test as before with pH paper until acidic. As the solution becomes acidic, what do you
observe?
Esterification
1. Into five clean, dry small test tubes (100 x 13 mm), add 10 drops of liquid carboxylic acid or
0.1 g of solid carboxylic acid and 10 drops of alcohol according to the scheme in table Note
the odor of each reactant.
2. Add 5 drops of concentrated H2SO4 to each test tube and mix the contents thoroughly by
sharply tapping the test tube with your finger.
Survey of Chemistry I (CH 131)
Acids and Alcohols
Test tube #
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Carboxylic acid
Alcohol
1
Formic
Isobutyl
2
3
4
5
Acetic
Acetic
Acetic
Salicylic
Benzyl
Isopentyl
Ethyl
Methyl
3. Place the test tubes in a warm water bath at 600C for 15 min. Remove the test tubes from the
water bath, cool, and add 2 mL or water to each. Note that there is a layer on top of the water
in each test tube. With a Pasteur pipet, take a few drops from this top layer and place on a
watch glass. Note the odor. Match the ester from each test tube with one of the following
odors: banana, peach, raspberry, nail polish remover, and wintergreen.
Saponification
This part of the experiment can be done while the esterification reactions are being heated.
1. Place into a large test tube (150 x 18 mm) 10 drops of methyl salicylate and 0.5 mL of
6 M NaOH. Heat the contents in a boiling water bath for 30 min. Record on the Data Sheet
what has happened to the ester layer (1).
2. Cool the test tube to room temperature by placing it in an ice water bath. Determine the odor
of the solution and record your observation on the Data Sheet (2).
3. Carefully add 6 M HC1 to the solution, 1 mL at a time, until the solution is acidic. After each
addition, mix the contents and test the solution with pH paper. When the solution is acidic,
what do you observe? What is the name of the compound formed? Answer these questions on
the Data Sheet (3)
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Esters
Data Sheet
NAME: ___________________________
DATE: ______________
PARTNER: ________________________
SECTION: ___________
Characteristics of Acetic Acid
Property
Water Solution
NaOH Solution
HCl Solution
Odor
Solubility
pH
Characteristics of Benzoic Acid
Property
Odor
Solubility
pH
Water Solution
NaOH Solution
HCl Solution
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Esterification
Test Tube
Acid
Odor
Alcohol
1
Formic
Isobutyl
2
Acetic
Benzyl
3
Acetic
Isopentyl
4
Acetic
Ethyl
5
Salicylic
Methyl
Odor
Ester
Odor
Saponification
1.a What has happened to the ester layer when you added NaOH?
1.b Write the chemical equation for the saponification of methyl salicylate.
2. What has happened to the odor of the ester when cooled?
3. What forms on reacidification of the solution of methyl salicylate with HCl?
Name that compound.
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Esters
Post-lab Questions
NAME: ___________________________
DATE: ______________
SECTION: ___________
1. How do carboxylic acids and esters differ in their characteristic odors?
2. Look in the Chapter Carboxilic acids and Esters in your textbook ( or consult library,
Internet ) and find:
A) Citric acid is present in _______ fruits, malic acid is present in __________, acetic acid is
present in __________ and lactic acid is present in sour __________.
Overuse of muscle tissue results in the buildup of __________ acid.
The sting of an ant is caused by _________ acid.
___________acid is a constituent of butterfat.
Spinach, cabbage and rhubarb contain __________ acid in addition to important
vitamins.
B) Which esters are responsible for the flavor and odor of:
-raspberry
-apricot
-peach
-pear
-apple
C) Aspirin, an ester of ______________ acid with ____________ acid, inhibits the synthesis
of a class of hormones called ___________________, molecules that cause ________, ________
and ___________________ when present in the bloodstream in ____________________ levels.
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Esters
Pre-lab Assignment
NAME: ___________________________
DATE: ______________
SECTION: ___________
1. Write the structures of the following carboxylic acids:
a. Acetic acid
b. Formic acid
c. Salicylic acid
2. Write the products from the reaction of benzoic acid and sodium hydroxide.
3. Octyl formate has the flavor of oranges. Name the alcohol and the carboxylic acid needed to
synthesize this ester.
4. What is a soap?