EXPERIMENT 4
ALDEHYDES AND KETONES: PREPARATION AND QUALITATIVE
ANALYSIS1
Materials Needed
20 mL 70% 2-propanol (isopropyl alcohol)
100 mL acidic dichromate (K2Cr2O7/H2SO4) solution
Distillation apparatus including thermometer
2 mL propanal (propionaldehyde)
2 mL cyclohexanone
4 mL 5% AgNO3(aq)
12 mL 5% NaOH(aq)
5% NH3(aq) or conc. NH4OH(aq)
6 mL copper(II) citrate solution (Benedict's reagent)
4-hydroxy-3-methoxybenzaldehyde
Additional Reading Assignment
McMurry, chap 14.5 (pp 375-377), chap 16
Introduction
I.
Structure and Preparation of Aldehydes and Ketones
Aldehydes and ketones both contain the C=O or carbonyl group. Aldehydes have at least one hydrogen
bonded directly to the C=O whereas ketones always have two alkyl groups attached to the C=O.
O
O
C
C
R
H
an aldehyde
R
R'
a ketone
Aldehydes may be prepared by oxidation of 1 alcohols; potassium dichromate (K2Cr2O7) in acidic solution
(H2SO4(aq)) can sometimes be used as the oxidizing agent (eq 1).
R-CH2-OH + K2Cr2O7
(1 alcohol)
H+
---
H
|
R-C=O
(aldehyde)
+
Cr3+
(1)
However it is difficult to prevent further oxidation of the aldehyde product to a carboxylic acid (see
McMurry p 375-376 and Part III of this Introduction).
1
Adapted from E. Boschmann & Norman Wells, Chemistry in Action: A Laboratory Manual for General,
Organic, and Biological Chemistry, 4th Edition, page 309.
1
Ketones may be prepared by oxidation of 2 alcohols. Again, acidic K2Cr2O7 may be used (eq 2).
R'
|
R-CH-OH +
(2alcohol)
K2Cr2O7
H+
---
R'
|
R-C=O
(ketone)
+
Cr3+
(2)
Unlike with aldehydes (see above) further oxidation of the ketone product is not a problem (see McMurry p
376 and part III of this Introduction).
The common and important compound acetone (IUPAC name, 2-propanone) is the
simplest ketone) Acetone is a commercial solvent and is used in paint thinners and nail polish
removers. Acetone is easily prepared by the oxidation of 2-propanol with acidic dichromate, a
reaction that you will carry out in this lab. The acetone product will be purified using a distillation. CH3
O
C
CH3
acetone
II.
Physical Properties (see McMurry chapter 16.3)
Because they contain the polar carbonyl group, aldehydes and ketones are polar compounds. However, they
cannot form hydrogen bonds one to another, as do alcohols. Therefore, the boiling points of aldehydes and ketones
are less than those of alcohols of similar molecular weight, but greater than those of hydrocarbons of similar
molecular weight (see McMurry Table 16.2, p 421). The solubility of aldehydes and ketones in H 2O is significant if
they contain less than five carbons. This is because hydrogen bonds to the water molecules are formed (see p 422).
Acetaldehyde (ethanal, CH3CHO) and acetone are miscible with water in all proportions.
III.
Chemical Properties (see McMurry chapter 16.5)
Aldehydes are easily oxidized a fact due to the presence of the hydrogen attached to the carbonyl group
(this is not present in ketones, which are less easily oxidized). Oxidation of aldehydes yields carboxylic acids. Even
air will oxidize an aldehyde (eq 3).
H
OH
|
|
R-C=O
+
O2
---
R-C=O
(3)
(aldehyde)
(from air)
(carboxylic acid)
Other weak oxidizing agents can bring about this reaction. One of these is Tollens' reagent, a basic (OH-)
solution of the silver complex ion, Ag(NH3)+. The reaction produces metallic silver (Ag0), which often forms a shiny
"mirror" on the sides of the container (eq 4).
H
OH
|
|
R-C=O +
Ag(NH3)2+ ---
R-C=O
+
Ag0
(4)
(Tollens' reagent)
(silver mirror)
Tollens' reagent is used to detect the presence of aldehydes. A solution of Benedict's reagent can also
oxidize aldehydes. This solution consists of a basic (OH-) solution of copper(II) citrate (whose complex composition
cannot be represented by a simple formula):
H
OH
|
|
R-C=O + copper(II) citrate --- R-C=O
+
Cu2O
(5)
(Benedict's reagent)
(copper(I) oxide)
The conversion of the clear, blue copper(II) citrate to insoluble, reddish copper(I) oxide indicates a positive
test. The reaction occurs not only with simple aldehydes but also with "reducing sugars" such as glucose.
2
Procedure
SAFETY PRECAUTIONS
- WEAR YOUR SAFETY GLASSES AT ALL TIMES.
- IF YOU SPILL A SOLUTION ON YOUR SKIN, BE SURE TO FLUSH THE AREA PROMPTLY WITH LOTS
OF WATER.
- TAKE SPECIAL CARE WITH ACIDIC DICHROMATE SOLUTION, WHICH IS AN EXTREMELY
CORROSIVE AND DANGEROUS SUBSTANCE. IF YOU GET ANY ON YOUR SKIN OR CLOTHING,
RINSE IT WITH WATER IMMEDIATELY. DOING SO WILL AVOID SERIOUS INJURY.
- It is important that the directions be followed exactly. Carelessness can be DANGEROUS!
I.
Preparation of Acetone from 2-Propanol
Prepare an ice/water bath; this may be conveniently done in a large (i.e., > 500 mL) beaker. Place 20 mL of
70% 2-propanol in a 250-mL beaker, and add 20 mL of distilled H2O. Stir to mix, and cool the beaker in an ice bath
to about 10C.
With the solution still in the ice bath, add, all at once, 100 mL of "acidic dichromate" solution (CAUTION:
Corrosive!). In a few seconds, the mixture will turn dark, followed by a rather sudden rise in temperature to 5060C. Stir the mixture (still in the ice bath!) until its temperature has fallen to below 50C. NOTE: Do not use the
thermometer as a stirring rod.
Pour the mixture into a 250-mL (or larger) distilling flask using a funnel to prevent spilling any. Assemble a
distillation apparatus as demonstrated by your lab instructor. Use a graduated cylinder as the receiver.
Heat gently. After 10-15 minutes, the liquid should begin to boil and drops of acetone begin to collect in
the receiver. Record the temperature when the first drop appears. Continue the distillation until at least 5 mL of
acetone has collected. Record the temperature again, and then stop the distillation. Measure the volume of
acetone obtained.
Use the acetone you prepared for the qualitative analysis tests. Leave the distillation apparatus as is. It will
be disassembled and cleaned by stockroom personnel.
II.
Qualitative Analysis
1. Solubility Tests
Place 2 mL of distilled H2O in each of three test tubes.
- To the first, add 10 drops of acetone.
- To the second, add 10 drops of propanal.
- To the third, add 10 drops of cyclohexanone.
Gently shake each test tube, and note the degree of solubility on the data table. How do you account for
differences between the three compounds tested?
1. Chemical Tests: Oxidation
A. Tollens' Test: To prepare the reagent, clean a medium-size test tube thoroughly with soap and water and
rinse with distilled H2O. Place 4 mL of 5% AgNO3 in the test tube, and add 2 drops of 5% NaOH, mixing
thoroughly. A dark brown precipitate of silver oxide (Ag2O) will form. Now dissolve the precipitate by
adding 5% aqueous NH3 (conc. NH4OH) until the solid disappears and a clear colorless solution is
3
obtained.
Divide the Ag(NH3)2+ reagent you have just prepared approximately equally between three clean test tubes.
- To the first, add 3 drops of acetone.
- To the second, add 3 drops of propanal.
- To the third, add 3 drops of cyclohexanone.
If no precipitate forms and no mirror appear, heat the tubes in the water bath provided. Record your
observations on each tube on the data table.
CAUTION: Do not let the Tollens' reagent stand around, since it may form explosive substances.
Dispose of it in the appropriately labeled waste beaker in the hood.
B. Benedict's Test: Place 2 mL of Benedict's solution in each of three test tubes.
- To the first, add 20 drops of acetone.
- To the second, add 20 drops of propanal.
- To the third, add 20 drops of cyclohexanone.
Heat the test tubes in the water bath provided for 10 minutes (in the hood). Watch for the appearance of
reddish Cu2O. Record your observations on each tube on the data table.
III.
A Natural Product
A bottle of the material whose structure is shown below will be available. Fan your hand over the open
bottle and toward your nose. What does its odor remind you of?
Using a handbook (e.g., the CRC Handbook of Chemistry & Physics or Aldrich Chemical Co. Catalog),
find the common name for this naturally occurring compound.
O
H
C
O
CH3
OH
4-hydroxy-3-methoxybenzaldehyde
4
PRE-LABORATORY QUESTIONS
EXPERIMENT 4: ALDEHYDES AND KETONES: PREPARATION AND
QUALITATIVE ANALYSIS
Name _________________________________________________ Section ____________ Date ____________
1.
Give the structure of
a. Acetone
b. Propanal
c. Cyclohexanone
2.
(a) What is the result observed experimentally which indicates a positive test with the Tollens' reagent?
(b) What kind of compounds give a positive test?
5
IN-LAB OBSERVATIONS/DATA
EXPERIMENT 4: ALDEHYDES AND KETONES: PREPARATION AND
QUALITATIVE ANALYSIS
Name ________________________________________________ Section ____________ Date ____________
Partner _______________________________________________
I.
Preparation of Acetone
Observations on reaction of 2-propanol with K2Cr2O7/H2SO4.
Appearance of reactants___________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Appearance of solution after reaction_________________________________________________
Highest temperature reached by the reacting solution _________ C
Distillation results
1.
Temperature when first drop of distillate is obtained _________ C
2.
Temperature at which distillation is stopped
_________ C
3.
Handbook value for boiling point of acetone
_________ C
4.
Volume of distillate obtained (yield of acetone)
_________ C
5.
Observations on the distillate (appearance, odor, etc.)____________________________________
______________________________________________________________________________
______________________________________________________________________________
6
II.
Qualitative Analysis
As in experiments #2 and #3 you should carefully observe all stages of each part of the experiment and note your
observations regarding each test directly in the table.
Acetone:
solubility in water ______________________________________________________________________
reactivity with Tollens reagent_____________________________________________________________
reactivity with Benedict's reagent___________________________________________________________
Propanal
general observations ____________________________________________________________________
solubility in water ______________________________________________________________________
reactivity with Tollens reagent_____________________________________________________________
reactivity with Benedict's reagent___________________________________________________________
Cyclohexanone
general observations ____________________________________________________________________
solubility in water ______________________________________________________________________
reactivity with Tollens reagent_____________________________________________________________
reactivity with Benedict's reagent___________________________________________________________
III.
A Natural Product
Describe the odor and appearance of 4-hydroxy-3-methoxybenzaldehyde.
7
REPORT SHEET
EXPERIMENT 4: ALDEHYDES AND KETONES: PREPARATION AND
QUALITATIVE ANALYSIS
I. Preparation of Acetone Results
Yield _____________
bp range ________________ literature bp ________________
II. Qualitative Analysis Results Table
Compound
Structure
Appearance
H2O solubility
Tollens' test
Benedict's test
Acetone
Propanal
Cyclohexanone
III.
A Natural Product
Common name of 4-hydroxy-3-methoxybenzaldehyde ________________________________________
8
Questions
I. Preparation of Acetone
1.
How does the handbook value for the boiling point of acetone compare to the temperature range
over which your distillation proceeded? What distilling temperature would be expected for a
distillation of pure acetone?
II. Qualitative Analysis
1.
How do you account for the differences in water solubility between the three compounds tested?
Explain clearly and completely.
2.
Tollens' Test. Which compound(s) gave a positive test? Why?
3.
Benedict's Test. Which compound(s) gave a positive test? Why?
4.
Write chemical equations (not necessarily balanced) for the reaction of propanal with
a. Tollens' reagent
b. Benedict's reagent
9
5.
What results would be expected if the following tests were carried out on the natural product used
in part III of this lab? (Explain your answers.)
a. H2O solubility
b. Tollens' test
c. Benedict's test
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