Chemistry 108 lab
Name_________________________
Lab #6: CARBOXYLIC ACIDS LAB
PART I: Preparation of Carboxylic Acids
(a) Oxidation of an Aldehyde by Oxygen from the Air:
Benzaldehyde is an aromatic aldehyde with a familiar odor. On a clean, dry watch glass, place 3
or 4 drops of the pure liquid benzaldehyde (this is 100% benzaldehyde, it is not a solution). Spread
the benzaldehyde out on the glass so as to expose more of it to the oxygen in the air and allow it to
remain for most of the lab period. Near the end of the period, examine the material on the watch
glass. Record your observations describing fully the starting material and any changes that occur.
Write a balanced equation for the reaction of benzaldehyde and oxygen gas to produce the
carboxylic acid product:
Move on to part (b) and the rest of the experiment, then come back to this question:
Based your observation of the product, how do you know a chemical reaction occurred?
______________________________________________________________________________
______________________________________________________________________________
(b) Oxidation of an Aldehyde by Potassium Permanganate Solution:
Place about 2 drops of benzaldehyde in a small test tube and add about 4 drops of 6 M sodium
hydroxide solution and about 12 drops of 0.1 M potassium permanganate (KMnO4) solution. MIX
contents of the test tube. Allow the mixture to react for 10 minutes. (You can do a model from Part
3 while you are waiting.)
Describe your observations:
Note: The oxidizing agent is the permanganate ion (MnO4-). MnO4- is reduced to MnO2, a brown
precipitate, in the reaction.
Chemistry 108
Carboxylic Acids Lab
Does the acid-form (benzoic acid) or the base-form
(benzoate ion) dominate the equilibrium in this tube?
Since the solution is very basic, the pH is much
greater than 7 and the pKa of benzoic acid is about 5,
you should know if the acid- or base-form dominates
based on what we learned in lecture (chapter 9 and 10,
comparing pH and pKa). Draw the line bond
structure of that predominant form in the box to
the right.
Obtain a piece of filter paper from the supplied for the experiment. Folding it properly, first, place
it in your funnel and then moisten the filter paper with DI water. Put your funnel in a clean large
test tube and pour the brown suspension from the small test tube into the filter/funnel. Wait until
at least a few drops of liquid pass through the funnel and into the large test tube. Then, REMOVE
THE FUNNEL, and add 4 drops of 12 M HCl to the filtrate (liquid that went through the filter
paper and is now in the large test tube). The HCl will make the solution acidic. Describe what you
see. If you do not see a change occur when you add the HCl, see the instructor.
Observations:
Think about what you observed based on the solubility of carboxylic acids vs. carboxylate ions!
Write a balanced chemical equation for the reaction that occurred when you added the HCl.
• By adding the acid, you are converting your product formed (you drew it in the box in the
upper right hand corner of this page) in the oxidation reaction to its acid form (reacting with
the HCl, HINT: see the prelab neutralization equations).
Equation:
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Chemistry 108
Carboxylic Acids Lab
Part II: Solubility of a Carboxylic Acid and Its Salt in Water:
Neutralization of a Carboxylic Acid:
The solubility of carboxylic acids in water is very low when they contain more than 5 or 6
carbon atoms. Knowing this, predict the water solubility of benzoic acid and explain your
answer.
Circle one:
predicted to be soluble
or
predicted to be insoluble
Explain_______________________________________________________________________
Now test your prediction by adding benzoic acid crystals (0.01g to 0.02 g) to 2 ml of water in a
large test tube. Shake well and record your observations (what observation tells you if it was
soluble or not):
Test the pH of the mixture in your test tube by touching the tip of your stirring rod to the suspension
in the test tube and then touching a spot of liquid from the tip of the stirring rod onto a piece of red
litmus paper, and then place another spot of the liquid onto blue litmus paper. The spot on the
red litmus paper should look red (indicating that the solution is acidic, pH<7). The spot on the blue
litmus paper should look also look red (indicating that the solution is acidic, pH<7).
Record the spot color on the red litmus paper: __________________________________
Record the spot color on the blue litmus paper: _________________________________
Note that although carboxylic acids are “weak acids”, they do produce enough H3O+ to make the
solution acid.
Write a balanced chemical equation for the acid reacting with H2O (see prelab):
•
Is the acid form or the base form predominant at pH < 5? __________________.
Now add, one drop at a time, 1 M NaOH solution, stirring the contents of the test tube after each
addition of the NaOH and testing the pH of the solution using the red litmus paper.
Continue adding NaOH solution in this manner until the solution pH causes red litmus paper to turn
blue, indicating that the pH >7.
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Chemistry 108
Carboxylic Acids Lab
Check the contents of the test tube and record your observations (is the solid still present?):
Write the chemical equation describing this reaction (see prelab the reaction of a carboxylic acid and
a base):
Explain why you no longer see a solid present.
What kind of reaction is this (see prelab)? _____________________________ .
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Chemistry 108
Carboxylic Acids Lab
Part III: Models
Examine the numbered models set out by the instructor in the balance room. Draw condensed
structural formulas for the compounds represented. Write the organic family name of the molecule
(for example: alkane, alkene, amine, carboxylic acid etc.….) Also write the systematic name for each
molecule. Atoms Colors: Black = carbon, White = hydrogen, Red = oxygen, Blue = nitrogen
MODEL #
CONDENSED STRUCTURAL FORMULA
FAMILY
SYSTEMATIC NAME
1
2
3
4
For this model, do not draw the condensed form
here; instead, draw a side view. See you Chapter 4
Lecture Notes if you do not recall side-view
representations.
5
6
7
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Chemistry 108
Carboxylic Acids Lab
Post-Lab Questions: Draw the products for the following reactions:
Hint: Check your pre- lab
CH3CH2CH2COOH + NaOH
CH3CH2CH2COO- + HCl
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