Experiment 13
Qualitative Tests for Carbonyls; Unknown Carbonyl
This experiment is similar to Experiments 6 and 9, the identification of alcohols
and hydrocarbons, in that we will use simple chemical tests to identify ketones and
aldehydes and to identify an unknown carbonyl. As before you will do the tests first with
a series of known compounds in order to practice doing them and then you will use them
to determine the identity of an unknown carbonyl. This is a long experiment and usually
requires two full lab periods.
Part I Chemical Tests
Test 1: 2,4-Dinitrophenylhydrazone Test
Most aldehydes and ketones will react with dinitrophenylhydrazine within a few
minutes to give a brightly colored orange-yellow precipitate. The reaction and
mechanism are given in Figure 13.1.
Figure 13.1 2,4-Dinitrophenylhydrazone Formation
Overall Reaction
NO 2
O
H 2N N NO2 H2 SO4 R
C + H C N N NO2 + H2 O
R R' H2 O H
R'
NO 2
2,4-dinitrophenylhydrazone
2,4-dinitrophenylhydrazine
orange-yellow solid
Mechanism H3 O+
O OH H H
H 2N N NO2 O H H
C + H R C N N NO2
R R' R C N N NO2
NO 2 R'
R' H NO 2
attack by the amine NO 2
H2 O + H2 O
on the carbonyl H
NO 2 OH 2
H H
R -H 2O
O H H
H2 O + C N N NO2
H R C N N NO2
R'
R'
Final loss of water to give the NO 2
imine product
Procedure:
The stock-room will prepare the 2,4-dinitrophenylhydrazine test reagent for you.
It is prepared by dissolving 1.0 g of 2,4-dinitrophenylhydrazine in 5.0 mL of concentrated
sulfuric acid and then slowly adding this solution with stirring to a solution of 7.0 mL
water in 25 mL 95% ethanol.
You will perform this test on six different sample compounds: cyclohexanol,
cyclohexanone, acetone, benzaldehyde, tetrahydrofuran and ethyl acetate. Prepare six
small test tubes. Be sure there is no acetone present. If you use acetone to clean the
test tubes, make sure you rinse the tubes with water before using them. They do not
need to be dry.
Dissolve 2-3 drops of your test compound in 2 mL 95% ethanol in a small test
tube and mix this solution with 2 mL of the 2,4-dinitrophenylhydrazine reagent. Look for
the formation of an orange-yellow precipitate to indicate the presence of an aldehyde or
ketone. Record your results in your notebook. The color of the crystals, if any, is not
indicative of an aldehyde or ketone but only that such a group is present.
Test 2: Tollen’s Test
This test is selective for aldehydes. It is based on the fact that aldehydes are
much easier to oxidize than ketones. In the Tollen’s test, the aldehyde reduces Ag+,
complexed with ammonia, to Ag0 (metallic silver) while it is oxidized to a carboxylic acid.
Ketones do not undergo this reaction. When the metallic Ag0 is produced it forms a
silver mirror on the inner side of the test tube. In order for the mirror to form, the test
tube must be very clean and without many scratches inside. The test tubes must be
thoroughly rinsed with deionized water and soap. The reaction is shown in Figure 13.2.
Figure 13.2 Silver(I) Oxidation of Aldehyes to Carboxylic Acids
O O
C H2 O C -
R H + 2 Ag(NH3 )2 + - OH R O NH4 + + 2 Ag0 + NH4 + -
OH
aldehyde carboxylic acid solid
(ammonium salt)
Procedure:
We will do the Tollen’s test only twice, using a ketone for one test (either
cyclohexanone or acetone) and an aldehyde for the other (use benzaldehyde).
First prepare the Tollen’s reagent by adding 5 mL of 5% silver nitrate to 2 freshly
cleaned medium sized test tubes. Add 3 drops of 10% sodium hydroxide to each test
tube. Mix the solutions thoroughly. A brown-gray precipitate should form. To each tube
add 10% ammonium hydroxide solution drop-wise, shaking after the addition of each
drop, until the precipitate just dissolves.
Add 2-3 drops of ketone to one test tube and 2-3 drops of aldehyde to the other
one. Shake the tubes to mix them and allow them to stand without shaking for 10
minutes. Look for the formation of the silver mirror. Record your results in your
notebook.
Clean-up: To clean your tests tubes from the Tollen’s test you need to dissolve the
silver in 20% nitric acid. This forms water-soluble silver nitrate. Add a few mL of 20%
nitric acid solution to each test tube and using your test tube brush, dissolve all the solid
material.
Test 3: Benedict’s Test
This test is similar to the Tollen’s test but it uses cupric salts (Cu 2+) as the
oxidizing reagent. These are reduced to copper I salts (Cu+) as the aldehyde is oxidized
to a carboxylic acid. Again, ketones do not react. Benedict’s solution is made by
dissolving copper sulfate, sodium citrate, and sodium carbonate together in water. The
Cu2+ cation is solubilized by the citrate. When the copper is reduced to form Cu+, it
precipitates as brick-red cuprous oxide.
Procedure:
Set-up six medium test tubes. We will do the Benedict’s test on cyclohexanol,
cyclohexanone, acetone, benzaldehyde, tetrahydrofuran and ethyl acetate. Label the
test tubes accordingly.
Add 2 mL Benedict’s solution to each of the test tubes and then add 2-3 drops of
test compound to each test tube. Heat the test tubes in boiling water for 10-15 minutes.
Look for the formation first of a pale green color followed by the formation of the reddish
precipitate of cuprous oxide. Record the results in your notebook.
Figure 13.3 Copper (II) Oxidation of Aldehydes to Carboxylic Acids
O O OH O
O
C + Cu2+ -
O C CH2 C CH2 C ONa sodium carbonate, H2 O
R H C - + + Cu2O
O C R O Na
aldehyde carboxylic acid red solid
O- (sodium salt)
blue copper-citrate complex
Test 4: Chromic Acid Test (also called Bordwell-Wellman Test)
This test is similar to the Tollen’s test and the Benedict’s test in that it
distinguishes aldehydes from ketones on the basis of their ease of oxidation. In the
chromic acid test we use chromic acid to oxidize aldehydes to carboxylic acids; ketones
do not react with chromic acid. Chromic acid is prepared by mixing chromium trioxide
with dilute sulfuric acid. The orange Cr6+ cation is reduced to the blue-green Cr3+ion.
Figure 13.4 Chromium (VI) Oxidation of Aldehyde to Carboxylic Acid
O O
H2 SO4
C + H2 Cr2 O7 C + Cr2 (SO4 )3
R H orange R OH
aldehyde carboxylic acid blue-green
Cr6+
Cr3+
Procedure:
Set-up six small test tubes in your test tube rack. We will test the same six
compounds as in the 2,4-dinnitrophenylhydrazone and Benedict’s test (cyclohexanol,
cyclohexanone, acetone, benzaldehyde, tetrahydrofuran and ethyl acetate). Add 1 mL
of acetone to each tube and 2-3 drops of the test compound to the first test tube. Add 1
drop of the Chromic Acid reagent and shake vigorously using a small cork to stopper it.
Record how long it takes for a color change (if any) to appear. Looking down the length
of the upright test tube, resting on a white background, is the best way to judge color
changes. Repeat with all of the test compounds. Record the results in your notebook.
Test 5: Schiff’s Test
The Schiff’s test is also used to discriminate between aldehydes and ketones.
Aldehydes give a positive test; ketones do not. Schiff’s reagent is made by adding the
intensely colored triphenylmethane dye called fuschsin to a solution of sodium bisulfite
(NaHSO3). The bisulfite reacts with the dye to produce a colorless solution. This
colorless solution will then react with aldehydes – but not ketones – to produce a new
triphenylmethane dye that also has a brilliant purple color. This test is extremely
sensitive and may give false positives. DO NOT RETURN USED REAGENT TO THE
BOTTLE SINCE IT MAY BE CONTAMINATED AND DO NOT TOUCH THE MEDICINE
DROPPER OF THE REAGENT BOTTLE WITH YOUR FINGERS. The reactions
involved are shown in Figure 13.5.
O
Figure 13.5 Schiff's Test with Aldehydes
HN S OH
NH2
O H O
NaHSO 3 HO S N C S
H2 N+ C OH
H
fuschin colorless
bright purple
HN S OH
NH2
O O
C
R H O OH
HN S C R
O H
OH O H
R C S N C
H O H
bright purple O OH
HN S C R
O H
Procedure:
We will do this test on two compounds, a ketone (negative test) and an aldehyde
(positive test). Set up two test tubes and add 20 drops (~ 1 mL) of the Schiff’s Reagent
to each test tube. Add 1 drop of the test compound. Observe any color change.
Record your results in your notebook.
Test 6: Iodoform Test
This test identifies the presence of a methyl ketone functional group. The
reaction is shown in Figure 13.6. In the basic reaction conditions, the proton that is next
to the carbonyl (the α-proton) is removed. This makes the α-carbon into a nucleophile
that attacks a molecule of iodine in the second step. The presence of one iodine atom
on the α-carbon increases the acidity of the remaining hydrogens. Each one is also
removed by the base to make a nucleophile that attacks more iodine molecules until all
three hydrogens are replaced by iodine. The hydroxide ion attacks the carbonyl, which
has made more electron deficient by the -CI3 substituent. With the three electron
withdrawing iodine atoms attached, the –CI3 is a good leaving group. Finally, we
produce HCI3, iodoform, as a yellow solid that precipitates from solution.
Figure 13.6 Iodoform Reaction
-
OH
O O O H O I
R C CH2 R C CH 2 + H2 O R C CH + I - R C CH
H I I I
-
I I
OH
-
OH
O O- I O I O I O H
R C O R C C I R C C I R C C I R C C I + I-
H
H OH I I I
+ I I
-
OH
I
I C
I
O
HCI3
R C O- +
Iodoform -
yellow solid
Procedure:
Dissolve 4 drops of a methyl ketone in 1 mL of methanol in a large test tube.
Add 1.0 mL of a 10% NaOH solution. Then add KI/I2 solution drop-wise with shaking
until a slight excess of I2 remains as indicated by the dark brown color of iodine.
If 2 mL or more of the KI/I2 reagent was used, then fill the test tube with water
and allow it to stand for 15 minutes. A yellow precipitate indicates a positive test.
If less than 2 mL of the KI/I2 reagent was used, then it may be necessary to heat
the reaction for a short time (2-3 minutes) at 60 °C in a water bath. If all of the color
disappears on heating, then add a few more drops of the KI/I2 reagent until the color
persists. Fill the test tube with water and allow it to stand for 15 minutes. A yellow
precipitate indicates a positive test for a methyl ketone but a positive test is also
produced by acetaldehyde, CH3CHO, and ethanol, which is oxidized by the I2 to
acetaldehyde.
Part II Identification of Carbonyl Unknown:
You will now identify a carbonyl unknown using the tests above and by preparing
derivatives of your unknown and taking the melting points. Your unknown will be one of
those listed in Table 13.1 at the end of this experiment. By comparing these melting
points to known values reported in the literature (see Table 13.1) you can identify exactly
your unknown compound. You can confirm this identification by taking the IR and
matching it with one of the spectra given in Appendix 13.1.
Procedure:
Get a sample of an unknown aldehyde or ketone from the teaching assistant.
Your first task is to determine whether or not it is an aldehyde or a ketone. Perform the
Chromic Acid Test, the Tollen’s Test, and the Iodoform Test. Based on these results
you should be able to determine whether your compound is one of the aldehydes or
ketones and whether you have the methyl ketone [CH3C(O)] group.
You will now prepare at least two derivatives of the unknown in order to
determine its exact identity. Looking at the list in Table 13.1 you see that there are three
possible derivatives that you can use. Begin by preparing the 2,4-
dinitrophenylhydrazone and the semicarbazone. If, however, these two derivatives do
not give you a clear answer, then you will have to prepare the phenylhydrazone
derivative. It is recommended that you prepare all three derivatives so as to give you
extra information in helping you make your final identification. You will ultimately be
making your identification on the basis of melting points and the purity of the sample that
you use is critical in helping you obtain accurate melting points. Wet compounds do not
melt at the expected range. In making your final judgment regarding the identity of the
compound, remember that any and all mistakes made in taking melting points give
numbers that are too low. You can also determine the boiling point so as to help narrow
your choices but it can be difficult to get an accurate boiling point.
Preparation of Semicarbazones
Dissolve 2.0 g of semicarbazide hydrochloride in 20 mL water. Add 3.0 g of
crystalline sodium acetate. Mix and divide the reagent into two equal portions. Set
aside one portion to use with your unknown. You will prepare one semi-carbazone using
methyl ethyl ketone (2-butanone) for practice in order to learn the proper technique and
to see if you can obtain an accurate melting point and then you will prepare a semi-
carbazone using your unknown.
Add 0.5 mL (10 drops) of methyl ethyl ketone to one portion of the semicarbazide
solution. Stopper the test tube with a cork and shake vigorously. Filter the crystals using
the Hirsch funnel and recrystallize from ethyl alcohol using the procedure described
below. Be careful that you do not use too much ethanol since the semi-carbazones are
generally very soluble in ethanol. When the product is dry, determine its melting point.
If the melting point agrees with that given in Table 13.1 then proceed to your unknown.
The reactions involved are shown in figure 13.7.
Figure 13.7 Semi-carbazone Formation
O O O
C C H2 O C
Cl- + NH3 NH NH2 + Na + -O CH3 NH2 NH NH2
semicarbazide sodium acetate
O O
R R C
C
C O + NH2 NH NH2 C N N NH2 + H2 O
H
R' R' semicarbazone
Recrystallization:
Transfer the crystals to a small or medium test tube and add 1-2 mL of ethyl
alcohol. The semi-carbazones are very soluble in ethanol so do not too much in the
beginning. A good rule-of-thumb is to add just enough solvent to cover the crystals.
Heat the mixture to boiling in a water bath using a hot plate.
CAUTION! ETHYL ALCOHOL IS VERY FLAMMABLE. It is best to not use
open flames when doing a recrystallization from ethanol but if your test tube does catch
fire do not panic. It burns with a gentle flame just like a candle. Simply cover the end of
the test tube with your watch glass to extinguish the flame.
If the product dissolves very readily in the initial amount of alcohol, it is best to
boil off some of the alcohol. Then, add a few drops of water until a faint turbidity
persists.
If your compound does not dissolve in the initial amount of ethyl alcohol used
when heated to boiling, then add more ethyl alcohol a few drops at a time and reheat to
boiling after each addition until the solution just becomes clear. Note: you should work
fairly quickly because your ethyl alcohol can all boil away if you leave it in the boiling
water bath. Once the solution becomes clear, add a few drops of water until a faint
turbidity persists.
Cool the test tube, first to room temperature slowly, and then in an ice bath.
Collect the crystals that form by suction filtration using the Hirsch funnel. Wash the
crystals with 1 mL of ice-cold ethyl alcohol and dry them as much as possible on a piece
of filter paper. Transfer a small amount of the product to a piece of filter paper on a
watch glass and complete the drying process by crushing the product on the paper.
Determine the melting point and record this in your notebook.
Preparation of 2,4-Dinitrophenylhydrazones
Obtain 20 mL of the 2,4-dinitrophenylhydrazine reagent. Divide this into two
equal portions. Save one portion for your unknown. To one portion, add 0.5 mL (10
drops) of benzaldehyde. Shake vigorously to ensure thorough mixing. The 2,4-
dinitrophenylhydrazone usually forms immediately. If no precipitate forms, set the
mixture aside for 15 minutes. Scratch the wall of the test tube occasionally with a glass
stirring rod to help induce crystallization. If the crystalline slurry which forms is too thick
and heavy to filter easily, dilute it with 10-15 mL water. Filter using the Hirsch funnel and
recrystallize a portion of your material (a large spatula tip is enough) from ethyl alcohol
using the procedure above as described for the semi-carbazone. Note, however, that
the 2,4-dinitrophenylkhydrazone is much less soluble in ethanol than the semi-
carbazone and you must use a lot more ethanol. Note that the solubility will also depend
on which unknown derivative you have made. Use a medium or large test tube. When
the material is dry, determine the melting point. If the melting point agrees with the
literature value given in Table 13.1 then proceed with your unknown. If not, make sure
the hydrazone is dry and pure. You may need to recrystallize them a second time.
Preparation of Phenylhydrazones:
First prepare a known phenylhydrazone using benzaldehyde to ensure the
proper technique and then repeat the preparation using your unknown.
Obtain 20 mL of phenylhydrazine-HCl reagent. Divide this into two 10 mL portions in
two medium sized test tubes. Save one portion for your unknown. To the other portion,
add 0.5 mL (10 drops) of benzaldehyde. Stopper the test tube with a cork and shake
vigorously until the product crystallizes. Filter the crystals with suction using your small
Hirsch funnel. Wash the solid thoroughly with water. Recrystallize the product from
ethyl alcohol using the procedure described above. If the melting point agrees with that
given in Table 13.1 then proceed with your unknown.
Procedure for Boiling Point Determination
Use the set-up shown in Figure 13.8 to determine the boiling point. You will need
2-3 mL of your unknown. Place this in a 25 mL round bottom flask along with 2-3 boiling
stones. Use the open ended Claisen adapter. Lower the thermometer so that it is just
below the mouth of the round bottom flask. Heat the unknown liquid to boiling and
continue heating until droplets of the liquid coat the thermometer and you reach a
constant temperature. Record this temperature. No vapors should escape but if you do
smell vapors escaping, then place a condensing column in the open mouth of the
Claisen adapter (not shown in the figure). You do not need to connect the water hoses
for such a small volume. When you are done, you should allow the liquid to cool and
then recover as much of the liquid as you can so that you have enough for the future
tests. Determination of the boiling point will help you narrow down the possible choices
for your unknown.
Table 13.1
Compound b.p. (°C) Phenylhydrazone Semicarbazone 2,4-Dinitrophenyl-
(m.p. °C) (m.p. °C) hydrazone (m.p. °C)
Propionaldehyde 48 oil 154, 89 155
Acetone 56 42 187 126
2-Butanone 80 oil 146 114
n-Butyraldehyde 74 oil 106, 96 123
3-Pentanone 102 oil 139 156
Cyclopentanone 131 50 205 142
2-Heptanone 151 oil 123 90
n-Heptaldehyde 153 oil 109 108
Cyclohexanone 156 77 166 162
2-Furaldehyde 161 97 202 229, 212
2-Octanone 173 oil 123 58
Benzaldehyde 179 158 222 237
Acetophenone 202 105 199 239
p-Tolualdehyde 204 114 234, 215 234
Note: Two melting values are given for some derivatives since they may occur in syn or
anti geometrical isomers or in different polymorphic forms.
After you have identified your unknown and written the Unknown report, your
instructor may be able to take the infrared spectrum to confirm your answer.
Appendix 13.1 Infrared Spectra of Carbonyl Unknowns
Propionaldehyde
Acetone
2-Butanone (Methyl Ethyl Ketone)
n-Butyraldehyde
3-Pentanone (Diethyl Ketone)
Cyclopentanone
2-Heptanone
n-Heptaldehyde
Cyclohexanone
2-Furaldehyde
2-Octanone
Benzaldehyde
Acetophenone
p-Tolualdehyde
