IDENTIFICATION OF AN ALCOHOL OR CARBONYL COMPOUND
BY MEANS OF THREE REAGENTS (12/24/09)
The reactions of alcohols, aldehydes, and ketones with the reagents, chromic acid in acetone,
2,4-dinitrophenylhydrazine, and sodium hypoiodite, are useful for identification purposes. The
use of these three reagents, along with proper interpretation of the results, makes it possible to
identify an unknown alcohol or carbonyl compound provided there is a known limit to the
number of possibilities for the unknown.
As an illustration, it is possible to decide on the identity of any one of the compounds in the
following sets of five by using only the three reagents. In some instances only two tests may be
required. Your instructor will provide you with an unknown compound labeled as belonging to
one of the sets. Identify the compound by using a minimum number of tests.
Set A
1-Pentanol
2-Butanol
2-Butanone
Benzaldehyde
2-Methyl-2-butanol
Set B
2-Propanone
2-Methyl-1-propanol
2-Methylpropanal
2-propanol
2-Methyl-2-propanol
Set C
1-Hexanol
Cinnamaldehyde
Acetophenone
4-Methyl-2-pentanol
Cyclohexanone
PROCEDURE
Lab Notebook Preparation
In lieu of the standard preliminary notebook, prepare tables similar to the example (linked to the
syllabus following the experiment) for each of the three tests. These tables (complete with
introductions, general reactions, and procedures for each test) will serve as your preliminary
notebook, but will not be handed in until after the completion of the experiment. While in lab,
enter data and observations directly into the table. The experiment’s special summary/report
/postlab requirements are included at the end of the procedure section.
Before and during the experiment, fill in the table. Include balanced equations for all positive
reactions you observe when running tests with the three reagents on known compounds and your
unknown.
Reactions Of Alcohols And Carbonyl Compounds With Chromic Acid
The greater ease of oxidation of aldehydes than of ketones is the basis of several classical
qualitative tests. Fehling's and Benedict's reagents contain complex salts (tartrate and citrate,
respectively) of cupric ion as oxidizing agents, and Tollen's reagent consists of a silver diammine
salt. The first two have been used as tests for aliphatic aldehydes (although they are more often
used to detect reducing sugars), and the Tollen's test has been widely employed to distinguish
between aldehydes (both aliphatic and aromatic) and ketones.
Tertiary alcohols are much more resistant toward oxidation than primary and secondary alcohols.
Potassium permanganate and certain other oxidizing agents have been used to distinguish tertiary
alcohols from those of the other two types.
There are some disadvantages to all of these tests. A reagent that gives quick and definitive
results is chromic acid, prepared by dissolving chromic anhydride in sulfuric acid and used in
acetone solution. This reagent oxidizes primary and secondary alcohols and all aldehydes with a
distinctive color change, and gives no visible reaction with tertiary alcohols and ketones under
the conditions of the test.
Thus, the chromic acid reagent gives a clear-cut distinction between primary and secondary
alcohols and aldehydes on the one hand and tertiary alcohols and ketones on the other.
Aldehydes may be distinguished from primary and secondary alcohols by means of Tollen's,
Benedict's, or Fehling's test, and lower molecular weight primary and secondary alcohols may be
differentiated on the basis of their rates of reaction with concentrated hydrochloric acid
containing zinc chloride--the Lucas reagent.
Chromic Acid Test. Dissolve 1 drop of a liquid (about 10 mg of a solid) alcohol or carbonyl
compound in 1 mL of reagent grade acetone in a test tube. Add 1 drop of the acidic chromic
anhydride reagent to the acetone solution, and shake the tube to mix the contents. A positive
oxidation reaction is indicated by the disappearance of the orange color of the reagent and the
formation of a green or blue-green precipitate or emulsion.
Primary and secondary alcohols and aliphatic aldehydes give a positive test within 5 sec.
Aromatic aldehydes require 30 to 45 sec. Color changes occurring after about 1 min should not
be interpreted as positive tests; other functional groups such as ethers and esters may slowly
hydrolyze under the conditions of the test, releasing alcohols which in turn provide "false
positive" tests. Tertiary alcohols and ketones produce no visible change in several minutes.
Phenols and aromatic amines give dark precipitates, as do aromatic aldehydes having hydroxyl
or amino groups on the aromatic ring.
Conduct tests on the following classes of compounds: primary alcohols, secondary alcohols,
tertiary alcohols, aldehydes, ketones.
Reactions Of Carbonyl Compounds Involving Nucleophilic Additions
Nitrogen compounds that may be considered to be derivatives of ammonia are an important type
of nucleophiles that add to the carbonyl group of carbonyl compounds (equation 1). With
primary amines the addition product (2) is not stable and readily dehydrates to form an imine (3).
Imines are sometimes referred to as Schiff bases. Imines formed from aliphatic amines are
generally unstable and cannot be isolated, whereas those formed from aromatic amines are stable
and can be isolated. Imines undergo hydrolysis very readily in the presence of water to
regenerate the carbonyl compounds and aromatic amines; that is to say, the reaction in equation 1
is reversible.
O
-
C6 H5
O
+
••
C
NH 2 R
H
C6 H5
H
C
NH2 R
+
1
(1)
OH
R
C6 H5
C
N
H
3
+
H2 O
C6 H5
H
C
NHR
2
Other very useful derivatives of ammonia that undergo addition reactions with carbonyl
compounds are hydroxylamine (R = OH), semicarbazide (R = NHCONH2), and aryl hydrazines
(R = NHAr). The utility of these three types of ammonia derivatives resides in the fact that their
addition products (oximes, semicarbazones, and arylhydrazones) are almost always crystalline
solids, whereas the carbonyl compounds from which they are derived are often liquids. In
general, crystalline solids are easier to work with and simplify purification and identification
procedures. Phenylhydrazine, p-nitrophenylhydrazine, and 2,4-dinitrophenylhydrazine are the
arylhydrazines most often used to make crystalline derivatives of carbonyl compounds. The
formation of a 2,4-dinitrophenylhydrazone (4) is shown in equation 2.
OH
R2 C
NH2
R2 C
O+
NH
R2 C
NH
NH
NO2
N
NH
+
NO2
NO2
NO2
H2 O
(2)
NO2
NO2
4
Because oximes, semicarbazones, and arylhydrazones usually precipitate from solution, and are
thus removed from equilibrium, they can be isolated in high yield.
2,4-Dinitrophenylhydrazine Test. Add a solution of 1 or 2 drops of a liquid compound (about 100
mg of a solid compound) dissolved in 2 mL of 95% ethanol to 2 mL of the 2,4dinitrophenylhydrazine reagent. Shake the mixture vigorously; if a precipitate does not form
immediately, let the solution stand for 15 min.
Run a 2,4-dinitrophenylhydrazine test on an aldehyde and a ketone. Note: Do not wash test tubes
to be used in this test with acetone, a ketone, or a false positive may result.
-Halogenation of Carbonyl Compounds: The Haloform Reaction
The two preceding reactions occur by attack on the carbonyl group or by formation of a carbonyl
group through oxidation. The presence of a carbonyl group also has an effect on the chemical
reactivity of the adjacent carbons, the-carbons. The hydrogen atoms on the -carbons (the "hydrogens") of carbonyl compounds are fairly acidic, and are easily removed, because of
resonance stabilization of the resultant carbanion by the carbonyl group. This resonance
stabilized anion is called an enolate anion. (e.g. 5 in equation 3).
O
R2 C C R + OH
H
R
R
O
C C
-
O
R
R
C
R
C
+ H2 O
(3)
R
5
When a carbonyl compound that has -hydrogens is treated with a halogen in the presence of a
base, an enolate anion forms which reacts almost instantaneously with the halogen to produce an
-halogenated carbonyl compound (equation 4). When the carbonyl
O
5
+ X2
R2 C C
R
-
+ X
(4)
X
compound is acetaldehyde or a methyl ketone, all three -hydrogens of the methyl group are
replaced by halogen atoms (equation 5, R = H or alkyl).
O
CH3 C
O
R + 3 NaOH + 3 X2
R + 3 NaX + 3 H2 O
CX 3 C
(5)
6
After the first -hydrogen has been replaced with a halogen, the -hydrogens that remain on that
carbon atom are replaced by halogens even more rapidly because successive substitution of
electronegative halogens for -hydrogens successively increases the acidity of the remaining hydrogens. Once initiated, -halogenation proceeds at the same -carbon until all of its hydrogens have been replaced before any reaction occurs at the other -carbon. The inductive
effect of the three halogens not only makes the carbonyl group particularly susceptible to
nucleophilic attack by hydroxide ion, but also makes the trihalomethyl group a fairly good
leaving group. Thus the tetrahedral intermediate 7 readily undergoes cleavage, as shown in
equation 6, to give a trihalomethyl anion (8) and a car- boxylic acid (9), which are immediately
converted into the final products, a trihalomethane or haloform (10) and a carboxylate anion
(11), as shown in equation 7. The overall equa- tion for the reaction of a methyl carbonyl
compound is given by equation 8.
O-
O
CX3 C R + OH
CX 3
C
O
R
CX 3
OH
7
CX 3 + H2 O
O
CH3 CR + 4 NaOH + 3 X2
8
+
HO C R
(6)
9
HCX 3 + OH
10
(7)
O
HCX 3 + NaOCR + 3 NaX + 3 H2 O
(8)
Although all aldehydes and ketones with -hydrogens undergo -halogenation, only those with
methyl groups directly attached to the carbonyl group undergo carbon-carbon bond cleavage as
in the production of a haloform, presumably because a trihalomethyl group is needed to have a
sufficiently stable carbanion to serve as a leaving group. The facts that (1) the only carbonyl
compounds that give the haloform reaction are methyl ketones and acetaldehyde and (2) of the
haloforms, only iodoform (CHI3) is a crystalline solid with a characteristic color (yellow) and
odor form the basis for a qualitative test for the structural features shown below (R =H, alkyl, or
aryl). Alcohols of the structural type shown below exhibit a positive reaction because halogens in
a basic medium are oxidizing
O
OH
CH3 C R
CH3 CH R
agents and oxidize them to the corresponding methyl carbonyl compounds. The haloform
reaction is often used synthetically to convert of a methyl ketone into a carboxylic acid of one
less carbon atom. Chlorine is the halogen of choice for these synthetic uses because it is the least
expensive and the most readily available.
O
O
R C
CH3
(1) Cl2 , NaOH
+
(2) H2 O, H
R C OH
Iodoform Test. Dissolve or suspend about 0.2 g of the unknown in 1 mL of 10% potassium
iodide solution, add 0.5 mL of 1M sodium hydroxide, and then 1 mL of 5% sodium hypochlorite
solution. Shake the solution, and if necessary warm for a few minutes. The precipitation of
iodoform, CHI3, as yellow crystals, or a yellow turbidity constitutes a positive test. The
distinctive color and odor of iodoform is normally satisfactory for identification, but more
definite identification may be made by taking its melting point, 119 OC.
Run iodoform tests on a methyl ketone, a non-methyl ketone, a methyl secondary alcohol, and
non-methyl secondary alcohol. Note: Do not wash test tubes to be used in this test with acetone,
a methyl ketone, or a false positive may result. Also commercial compounds often contain trace
impurities of methyl ketones and methyl secondary alcohols which may result in false positives.
When in doubt , always compare your unknown result to that of an authentic methyl ketone or
methyl secondary alcohol.
SUMMARY/DISCUSSION (12/24/09)
In lieu of a post lab exercise, turn in a short (< 2 pages), legible (type-written preferred) report that
summarizes this experiment. The report is due in the lab, the week after completion of the
experiment. Please include the information listed below.
A. TITLE INFORMATION
1. Title of the experiment/report
2. Section number
B. ABSTRACT
1. A brief introductory statement that describes/explains the experiment performed and/or it's purpose.
2. A short/concise statement of results (the type of tests performed on your unknown and whether they
were positive or negative)
3. Your interpretation of results (include the CODE and identity of your unknown)
C. RESULTS/DISCUSSION
1. Prepare a table that summarizes your findings for each of the tests/reagents used in the
identification of your unknown.
2. Include a specific (not general) reaction of your "unknown" with each of the test reagents it reacted
positively with.
3. In explicit terms, explain how the observed reactions (positive and/or negative) of the three reagents
with your unknown allowed you to identify it among the five possibilities in your list.
WRITING/STYLE INFORMATION
1. Be concise: avoid wordy, flowery explanations!
2. Be complete/clear: make sure your reader will understand your intended meaning!
 Do not include procedure in the report!
D. POSTLAB QUESTION (6pts)
Please complete the following two-part question and attach or include with the report for this experiment.
Each of the four compounds in the two series that follow can be unequivocally identified using only two of
the three tests introduced in this experiment. Specify the two tests that should be used for the verifications
for each series. Explain your selection.
Series A
2-pentanone
butanal
1-heptanol
2-methyl-2-pentanol
Series B
2-pentanol
3-pentanone
hexanal
2-heptanone