Qualitative Analysis of Alcohols, Aldehydes,
and Ketones
 2012, Sharmaine S. Cady
East Stroudsburg University
Skills to build:
 Using qualitative tests to identify an alcohol
 Using qualitative tests to identify an alcohol as primary,
secondary, or tertiary
 Using qualitative tests to identify an aldehyde
 Using qualitative tests to identify a ketone
 Using a flow chart to identify an unknown alcohol,
aldehyde or ketone
Introduction
Alcohols, aldehydes, and ketones are organic molecules that have recognizable
oxygen-containing functional groups. These functional groups react differently with
various reagents. An unknown structure may be identified as an alcohol, aldehyde, or
ketone by performing a set of reactions and noting the results. Further testing can
distinguish an alcohol as primary, secondary, or tertiary
Alcohols are a class of organic compounds which contain the -OH functional
group covalently bonded to carbon. An alcohol may be represented as a water molecule
in which one of the hydrogens is replaced with a hydrocarbon R group. Like water,
H
O
H
R
O
H
alcohols are polar; however, their polar character decreases as the size of the nonpolar R
group increases.
Alcohols can be further classified as primary, secondary, or tertiary:
H2
C
HO
CH3
CH3
CH3
primary
C
HO H
C
HO
CH3
secondary
CH3
CH3
tertiary
Qualitative Analysis of Alcohols, Aldehydes, and Ketones
Alcohols which have one R group bonded to the carbon attached to the -OH
functional group are primary (1); two R groups are secondary (2); three R
groups are tertiary (3). In the above examples, R is a methyl group, but any
aliphatic or aromatic group may be bonded to the hydroxyl carbon. All three
classes of alcohols are capable of hydrogen-bonding in the pure state or to other
alcohol molecules because the -OH functional group is always present. Alcohols
also have the ability to hydrogen bond with water; however, their solubility
decreases as the size of the R group and their hydrocarbon-like character
increases.
Aldehydes and ketones are classes of organic compounds in which oxygen is
covalently joined to a carbon atom by a double bond. The carbon and oxygen are
collectively referred to as a carbonyl group. Ketones differ from aldehydes by
replacement of a hydrogen with an R group. Consequently, aldehyde functional groups
are always found at the end of hydrocarbon chains.
O
O
C
carbonyl group
R
C
O
H
R
aldehyde
C
R
ketone
The geometry around the carbon atom of a carbonyl group is different from the
tetrahedral geometry of sp3- hybridized carbon atoms attached to -OH functional groups.
The presence of the double bond results in sp2 hybridization on the carbon and a trigonal
planar arrangement of atoms.
Oxidation of Alcohols
Alcohols undergo several types of chemical reactions. The most important
reaction alcohols undergo is oxidation to carbonyl compounds. Primary alcohols are
oxidized to aldehydes, which themselves are sensitive to oxidation to carboxylic acids.
Oxidizing agents convert secondary alcohols to ketones, and tertiary alcohols lack the
ability to undergo oxidation. The following reactions show the expected products for
different alcohols with the Jones reagent, a solution of chromic acid (H2Cr2O7) prepared
by dissolving CrO3 in concentrated H2SO4. The chromic acid serves as the oxidizing
agent as it changes from an oxidation number of +6 to +3. In the reactions, R represents
the remaining hydrocarbon groups (methyl, isopropyl, etc.) on the carbon.
O
3 R
CH2
OH + 4 CrO3 + 6 H2SO4
3 R
C
OH
1 alcohol
blue-green ppt.
R
3 R
CH
+ 2 Cr2(SO4)3 + 9 H2O
R
OH + 2 CrO3 + 3 H2SO4
2 alcohol
3 R
C
O + Cr2(SO4)3 + 6 H2O
blue-green ppt.
2
Qualitative Analysis of Alcohols, Aldehydes, and Ketones
R
R
No Reaction
OH + CrO3 + H2SO4
C
R
Lucas Test for Alcohol Classification
Alcohols also react to form alkyl halides, in which the intermediate is a
carbocation. Carbocations can be primary, secondary, or tertiary:
H
+
C
R
R
H
+
C
H
R
1
2
R
R
+
C
R
3
Stable intermediates form faster than unstable ones. The order of stability of
carbocations is 3>2>1; hence, tertiary alcohols react faster than secondary alcohols,
which react faster than primary alcohols. The reactions may be written
ZnCl2
R
CH2
OH + HCl
1 alcohol
CH
OH + HCl
+ H2O
R
CH Cl + H2O
cloudy reaction within 3-5 min
R
C
Cl
R
ZnCl2
2 alcohol
R
CH2
clear after 5 min
R
R
R
R
ZnCl2
OH + HCl
R
3 alcohol
R
C
Cl + H2O
R
cloudy reaction within 1 min
where ZnCl2 in concentrated HCl is referred to as the Lucas reagent. Tertiary alcohols
produce a cloudy suspension immediately; secondary alcohols turn the reaction mixture
cloudy within 3-5 minutes; and primary alcohols do not react under these conditions. The
cloudiness results from the insolubility of the alkyl halide in the reaction mixture.
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Qualitative Analysis of Alcohols, Aldehydes, and Ketones
Oxidation of Aldehydes
Similar to alcohols, aldehydes and ketones differ in their ability to be oxidized. As
noted previously, aldehydes can be oxidized to carboxylic acids. Ketones, which lack a
hydrogen bonded to the carbonyl group, cannot be oxidized. The following reactions
show the expected products aldehydes, and ketones with the Jones reagent.
H
3 R
O
C
+ 2 CrO3 + 3 H2SO4
O
2 R
C
OH + Cr2(SO4)3 + 3 H2O
aldehyde
blue-green ppt.
R
R
C
O + CrO3 + H2SO4
No Reaction
ketone
2,4-Dinitrophenylhydrazine (2,4-DNP) Test for Aldehydes and Ketones
Aldehydes and ketones undergo a condensation reaction with 2,4-dinitrophenylhydrazine to produce yellow to orange precipitates as products. Alcohols do not
undergo this reaction. Note the condensation reaction involves the removal of water with
loss of the carbonyl oxygen.
R
O2N
R
H
C N
H2N
C O
N
H
+
NO2
aldehyde
O2N
N
H
NO2
+ H2O
+
O2N
R
C N
H2N
C O
ketone
H
yellow-orange precipitate
R
R
O2N
N
H
NO2
R
N
H
NO2
+ H2O
yellow-orange precipitate
In this experiment the above chemical reactions will be used to qualitatively identify
alcohols, aldehydes, and ketones. The flow chart on the following page shows how an
unknown may be identified as an aldehyde, ketone, or primary, secondary, or tertiary
alcohol. Not all tests may need to be performed before an identification can be made.
Table 1 summarizes expected results with each test.
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Qualitative Analysis of Alcohols, Aldehydes, and Ketones
Alcohols, Aldehydes, Ketones
2,4-dinitrophenylhydrazine
none
alcohols
yellow ppt.
ketones, aldehydes
Jones reagent
blue-green ppt.
none
ketone
aldehyde
Jones reagent
Lucas test
cloudy
none
3o
blue-green ppt.
< 1 min
1o, 2o
3-5 min
none
1o
3o
2o
Table 1. Summary of Results
+  positive
  negative
*  not applicable
Lucas Test
Jones
Compound 2,4-DNP
Room
Test
Test
temperature
50 C
Aldehyde
+
+
*
*
Ketone
+

*
*
1 alcohol

+


2 alcohol

+

+
3 alcohol


+
+
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Qualitative Analysis of Alcohols, Aldehydes, and Ketones
Experimental Methods and Materials
Safety considerations
Wear suitable protective clothing, gloves, and eye/face protection!
You should read the online MSDS for:
tert-Amyl Alcohol
Jones Reagent
1-Butanol
2-Pentanol
2-Butanol
1-Propanol
tert-Butyl Alcohol
Propiophenone
2,4-Dinitrophenylhydrazine
Salicylaldehyde
2-Heptanone
p-Tolualdehyde
Hydrochloric Acid
Zinc Chloride
6
Qualitative Analysis of Alcohols, Aldehydes, and Ketones
2, 4-Dinitrophenylhydrazine test
2, 4-Dinitrophenylhydrazine is used to distinguish aldehydes and ketones from
alcohols. Clean and thoroughly rinse with distilled water five test tubes. Label a
test tube for formaldehyde, acetone, absolute ethanol, isopropyl alcohol, and tertbutyl alcohol. Add 10 drops of each compound to its labeled test tube. Add 5
drops reagent grade ethanol to each test tube. Add 10 drops 2, 4dinitrophenylhydrazine to each test tube. Record your results. The presence of
a yellow to orange-red precipitate is a positive test.
Chromic acid test (Jones reagent)
The chromic acid test may be used to distinguish an aldehyde and primary and
secondary alcohols from ketones and tertiary alcohols. Clean and thoroughly
rinse with distilled water five test tubes. Label a test tube for formaldehyde,
acetone, absolute ethanol, isopropyl alcohol and tert-butyl alcohol. Add 5 drops
of each compound to its test tube. Add 10 drops reagent grade acetone to
each test tube. Add 2 drops of the Jones reagent to each test tube. Stir to mix
the contents of each test tube. Allow the solutions to stand for 1 minute. Record
your results. The appearance of a blue-green color or precipitate is a positive
result.
Lucas test
The Lucas test is used to distinguish between primary, secondary, and tertiary
alcohols. Clean and thoroughly rinse with distilled water three test tubes. Label a
test tube for absolute ethanol, isopropyl alcohol, and tert-butyl alcohol. Add 5
drops of the compound to its labeled test tube. Add 15 drops Lucas reagent to
each test tube. If no reaction has occurred after 5 minutes, heat the test tube in a
50C water bath for 5 minutes. Record your results. The immediate formation of
a white cloudy suspension is a positive test for a tertiary alcohol. The appearance
of the cloudy layer after 5 minutes or heating is a positive test for a secondary
alcohol. If no cloudiness is observed after heating, the alcohol is primary.
Identification of unknown
Obtain an unknown sample from your instructor. Mark the letter or number on
your report sheet. Using the flow chart and Table 1, identify your unknown as a
1, 2, or 3 alcohol, aldehyde, or ketone. Determine the refractive index to
identify which alcohol, aldehyde, or ketone is your unknown. If necessary, correct
the refractive index.
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Qualitative Analysis of Alcohols, Aldehydes, and Ketones
UNKNOWN POSSIBILITIES
1-butanol
salicylaldehyde
1-propanol
p-tolualdehyde
2-butanol
2-heptanone
2-pentanol
propiophenone
tert-butyl alcohol
tert-amyl alcohol
References
Pasto, D. J.; Johnson, C.R.; Miller, M. J. Experiments and Techniques in Organic
Chemistry; Prentice Hall: Upper Saddle River, NJ, 1992, pp 306, 324, 328-331.
Laboratory Report
Using the flow chart, describe how you classified your unknown. Explain how you
identified your unknown compound. Give the structure of the unknown. Give the
chemical reaction for each known compound that undergoes a positive reaction.
8