WEEK 7:
QUALITATIVE ANALYSIS OF ALDEHYDES AND KETONES
PURPOSE:
This experiment will introduce the student to qualitative
organic analysis. A number of test tube reactions will be done
to identify an aldehyde and/or a ketone unknown.
IMPORTANT REACTIONS AND DATA:
Part 1: Tollen's Test
O
O
+ 2 Ag(NH3)2OH
C
R
+ 2 Ag + 3 NH3 + H2O
C
H
O-NH4+
R
Silver
Mirror
Aldehyde
Part 2: Iodoform Test
O
+
C
R
O
3 I2 + 4 OH-
+ CHI3 + 3 I- + 3 H2O
C
CH3
R
OIodoform
methyl ketone
Part 3: 2,4-Dinitrophenylhydrazones
O2N
O
+
C
R
R'
H+
H2N
N
NO2
H
2,4-dinitrophenylhydrazine
R
O2N
C
N
N
R'
NO2
H
2,4-dinitrophenylhydrazone
Part 4: Semicarbazones
ClO
+
C
R
R'
H3
+N
N
O
C
N C
H
R
- H2O
NH2
O
N
N
R'
C
H
semicarbazide
hydrochloride
NH2
+
N+ClH
semicarbazone
Week 6: Table of Unknown Aldehydes and Ketones
Compound
2-heptanone
3-heptanone
n-heptanal
n-butanal
Acetone
3-pentanone
Benzaldehyde
Acetophenone
Cinnamaldehyde
Hexane-2,5-dione
Mpt of 2,4-DNP
89
81
108
123
126
156
237
238
255
257
Mpt of
semicarbazone
123
101
109
95-106
187
138
222
198
215
224
BACKGROUND INFORMATION:
This experiment will be the first of a few experiments that
will explore qualitative organic analysis. The analysis of
unknown compounds is routinely done by chemists and occupies
much of the time expended by scientists. Analytical chemistry
is an entire separate study which, by its very nature, crosses
into other branches such as organic chemistry. Analytical
chemistry can be qualitative (interested only in the
identification of an unknown) or quantitative (interested in the
nature and percentage composition of unknowns). Of the two
qualitative is less demanding. Qualitative analysis may use
instruments or classical wet chemical test tube reactions.
Instrumental analysis was introduced last semester in the
discussions on infrared and proton magnetic resonance
spectroscopy. Wet chemical analysis was also presented in the
experiment that studied Sn1 and Sn2 reactions. This experiment
will expand on that brief introduction last semester.
Wet chemical analysis usually involves using simple and
quick test tube reactions that have clear observations.
However, since many factors can affect the result, the observer
must look beyond the observation and carefully interpret the
data in making a decision about the identity of an unknown.
Reactions that give a precipitate, color change or gas evolution
are ideally suited for qualitative tests.
Usually, in organic chemistry a qualitative test only
identifies the functional group class to which the unknown
belongs. Further work must be done to fully determine the exact
identity of the unknown compound. This aspect of qualitative
organic analysis is called ‘making a derivative’.
In today’s experiment there are four tests. The first two
(Tollen’s test and iodoform reaction) are used to get a general
class of possibilities. The last two (2,4dinitrophenylhydrazone
formation and semicarbazone formation) are used to make
derivatives. Actually, in the broader scheme of qualitative
organic analysis, the 2,4-dinitrophenylhydrazone reaction
identifies the class and makes a derivative. However, since we
know that today’s unknowns are aldehydes or ketones, this test
does not supply any useful differentiation as it is positive for
both these classes.
The Tollens test gives a precipitate of silver metal
(either as a beautiful silver mirror on the inside of the test
tube or a black precipitate of colloidal silver) if the unknown
is an aldehyde. This mild oxidizing agent can also give a
positive test for any very easily oxidized organic compound such
as some carbohydrates. Further note that an insoluble unknown
aldehyde may give a false negative test if the reagents are not
mixed thoroughly.
The iodoform test gives a light yellow precipitate of
iodoform if the unknown has a methyl carbonyl group as part of
the molecule. Since the reagent is also mildly oxidizing, a
methyl group next to a secondary alcohol group will give a
positive test. The test is positive for 2-pentanone, acetone,
2-butanone, ethanol, 2-propanol and like compounds. The test is
negative for 3-pentanone, for example. The iodoform that forms
has a melting point of 117 degrees but cannot be used as a
derivative as it is the same product no matter what the unknown
is.
The formation of a 2,4-dinitrophenylhydrazone is a messy
reaction. Sulfuric acid is one of the ingredients in the test
reagent. The precipitate is difficult to filter and dry. Some
success in cleaning the product can be achieved by washing the
solid with cold ethanol. Since percent yields of derivatives
are never an issue, loss of material is acceptable if a purer
solid results. Sometimes, the color of the
2,4dinitrophenylhydrazone (2,4-DNP for short) can give some
information about the unknown. Highly conjugated carbonyl
groups give deep red 2,4-DNP derivatives while non conjugated
carbonyl groups give yellow 2,4-DNP compounds.
The formation of a semicarbazone derivative is a nice
second choice in identifying an unknown. Since precipitation
can be difficult it is usually not a first choice.
Considering the table of data above, if the unknown were 3pentanone and a successful 2,4-DNP was prepared with a melting
point of, say, 153 degrees, there would be no need to make a
second derivative.
However, if the unknown were acetone, the
2,4-DNP would not clearly rule out n-butanal as their 2,4-DNP
derivatives have similar melting points. A semicarbazone would
clearly identify what the unknown would be in this case.
However, other data would help distinguish acetone from nbutanal. A Tollen’s test would be positive for n-butanal and
the iodoform test would be positive for acetone. So, the final
identification of an unknown is achieved by an analysis of the
data available rather than following a rigid set of rules or
procedures.
EXPERIMENTAL PROCEDURE:
Your goal is to identify the unknown aldehyde or ketone
sample assigned to you from the table. The tests in parts 1 & 2
will enable you to narrow your search to a particular type of
compound, and measuring the melting points of the derivatives
you synthesize in parts 3 & 4 will indicate identity of a
particular aldehyde or ketone.
PART 1: Tollens Test
This test must be performed on three samples: a known
aldehyde (postive test), a known ketone (negative test) and your
unknown sample.
Clean three micro-scale reaction tubes by adding sodium
hydroxide solution (2 mL) to them and heating them in a water
bath.
To prepare the Tollens reagent, place a 0.03 M solution of
silver nitrate solution (2 mL) into a 10 ml volumetric flask and
add a 3 M of sodium hydroxide solution (1 mL) to produce a
grayish suspension of silver oxide. To this, add a 2.8 %
solution of ammonia (0.5 mL) dropwise with shaking. If the
precipitate does not dissolve, add more ammonia dropwise until
it does to a maximum of 3 mL. Then dilute the entire mixture to
10 mL total by addition of water. The Tollens reagent can form
explosive compounds on standing and should nnot be stored.
Remove the sodium hydroxide solution from your three
reaction tubes, rinse them with water, and add your freshly
prepared Tollens reagent (1 mL) to each of the tubes. To the
first tube add one drop of your known aldehyde, to the second
tube add one drop of your known ketone and to the third tube add
one drop of your unknown. Allow the tubes to stand for several
minutes and look for the appearance of a silver mirror coating
on the inside of the tube.
PART 2: Iodoform Test
This test must be performed on three samples: a known
methyl ketone (postive test), a non-methyl ketone (negative
test) and your unknown sample.
Place the sample to be tested (approximately 50 mg) in a
reaction tube and dissolve it in water (2 mL). (If it does not
dissolve in water, repeat the procedure and dissolve it in 2 mL
of 1,2-dimethoxyethane instead). Add 3 M sodium hydroxide
solution (2 mL) followed by the careful addition of the iodoform
solution (3 mL). For a positive test with a known methyl ketone,
the iodoform reagent’s colour should disappear and the yellow
iodoform product separates. Repeat the procedure for a known
non-methyl ketone and your unknown.
PART 3: 2,4-Dinitrophenylhydrazones
Place the 2,4-Dinitrophenylhydrazine solution (5 mL)
in a reaction tube and add approximately 50 mg of your unknown
sample. Heat the sample tube in a water bath for several minutes
until the presence of a precipitate is observed. Then cool the
reaction tube in an ice bath and filter off the product using a
Hirsch funnel. Wash the product and the reaction tube with icecold water, and then wash the product with ice-cold ethanol and
allow to dry thoroughly. Record the melting point range of your
product.
PART 4: Semicarbazones
Place the semicarbazide hydrochloride solution (0.5
mL) in a reaction tube and add approximately 100 mg of your
unknown sample. To this add methanol (1 ml) and pyridine (10
drops, use in a fume hood), then heat the sample tube in a water
bath for 10 minutes. Cool the reaction tube in an ice bath and
allow the product to crystallize (you may need to scratch the
tube with a glass rod to induce crystallization). Filter off the
product using a Hirsch funnel and wash the product and the
reaction tube with ice-cold water. Finally, wash the product
with ice-cold methanol and allow to dry thoroughly. Record the
melting point range of your product.
IMPORTANT INFORMATION ABOUT THE REPORT:
The report for this experiment will be different from the
usual format of the past few experiments. No percent yield
calculation will be done. Melting point ranges of any
derivatives must be recorded and compared to the melting points
given in the table to determine the identity of any unknown.
The identity of any unknown and its unknown number should be
given along with conclusive evidence that supports the claim
that is made.
END OF EXPERIMENT.
© 2007 STEPHEN ANDERSON AND ROBERT SHINE