Qualitative Tests for Carbonyls: Aldehydes vs. Ketones

Laboratory #6

Qualitative Tests for Carbonyls: Aldehydes vs. Ketones

Introduction

Various chemical tests identifying ketones and aldehydes are used in this experiment in order to identify an unknown carbonyl compound. The tests used are: 2,4-dinitrophenylhydrazone test, Tollen's Test, Benedict's Test, Chromic Acid Test, aka Bordwell-Wellman Test, Schiff's Test, and Iodoform Test. These classification tests provide results based on color change or formation of precipitation, which is then used to determine the identity of the functional group.

The first test, the 2,4-dinitrophenylhydrazone test, determines the presence of a carbon-oxygen double bond. Brady's reagent, another name for the test, is a solution of 2,4-dinitrophenylhydrazine, methanol, and sulfuric acid. When performing the test, a bright orange or yellow precipitate shows the presence of an aldehyde or ketone. This test proceeds via a nucleophilic addition-elimination reaction, as shown below.1

The Tollen's test is used to distinguish between aldehydes and ketones. The reaction works due to the fact that unlike ketones, aldehydes can be readily oxidized. The Tollen's test is also known as the silver-mirror test, because when the test is positive for aldehydes, a silver mirror forms on the side of the test tube. A sample reaction is shown below.

The Benedict's Test and Chromic Acid Test both work in similar fashion as the Tollen's Test. In the Benedict's Test, cupric salts are used as the oxidizing reagent rather than the silver nitrate. An aldehyde is identified by a brick-red precipitate, while ketones have no reaction.

The Chromic Acid Tests, sometimes known as the Bordwell-Wellman Test, uses chromic acid to oxidize the aldehydes to carboxylic acids. Ketones do not react. When oxidized, the color changes from orange to blue-green.

The Schiff's Test is a chemical test for the detection of aldehydes. When the sample is added to the decolorized Schiff reagent in the presence of an aldehyde, a bright purple color develops. Ketones do not react.

The last test, called the Iodoform Test, identifies the presence of a methyl ketone functional group via a halogenations reaction. A yellow precipitate tells us we have a methyl ketone.2

Materials and Hazards3

Name

MW

(g/mol)

MP

(°C)

BP

(°C)

Structure

Density

MSDS

Cyclohexanol

C6H11OH

100.16

20-22

160-161

0.948 g/mL

Harmful upon inflation, ingestion, and absorption

Water/Ice

18.02

0.0

100.0

H2O

----------------

Acetone

CH3CH3COCH3

58.08

-94

56

0.791 g/mL

Hazardous in case of skin contact, eye contact, ingestion, or inhalation.

Cyclohexa-none

C6H10O

98.14

-47

155

0.947 g/mL

Flammable; harmful if swallowed, inhaled, or absorbed through skin.

Benzaldehyde

C6H5CHO

106.12

-26

178-179

1.045 g/mL

May cause allergic skin reaction; harmful if swallowed or inhaled.

Tetrahydro-furan

C4H8O

72.11

-108

66

.8892 g/mL

Hazardous in case of skin or eye contact.

Ethyl Acetate

CH3COOC2H5

88.11

-84

77

0.902 g/mL

Flammable; harmful if swallowed or inhaled.

95% Ethanol

CH3CH2OH

46.07

-114

78

0.789 g/mL

Flammable

2,4-dinitro-phenylhydra-zine

C6H6N4O4

198.14

-----

197-200

---------------

Very hazar-dous in case of ingestion and inhalation

5% Silver Nitrate

AgNO3

169.87

444

212

Ag+NO3-

4.35 g/mL

Causes irritation to the eyes and skin.

Ammonium Hydroxide

NH3

35.04

37.7

-57

NH3

0.91 g/mL

Toxic; causes irritation to the skin, eyes and mucous membranes

Nitric Acid

HNO3

63.01

120

HNO3

1.48 g/mL

Flammable; toxic upon inhalation or if swallowed.

Chromic Acid

H2CrO4

118.01

-----

197

1.20 g/mL

Hazardous in case of skin or eye contact.

Methanol

CH4O

32.04

64.7

-97

0.7918 g/mL

Flammable; toxic if swal-lowed or inhaled

Acetaldehyde

C2H4O

44.05

20.2

-123

0.788 g/mL

Flammable; carcinogenic

Iodoform

CHI3

393.73

118-121

-------

4.008 g/mL

Harmful if swallowed or inhaled

Heptaldehyde

CH3(CH2)5CHO

114.19

153

-43

0.817 g/mL

Flammable; causes irritation to the skin and eyes

Cyclopen-tanone

C5H8O

74.12

130-131

-51

0.951 g/mL

Flammable; causes irri-tation to the eyes and skin

Procedures

The procedures are attached step-by-step, with no changes.

Data

Table 1

2,4-Dinitrophenylhydrazone Test

Reagent

Reaction

Cyclohexanol

Positive; turned orange

Cyclohexanone

Positive; formed yellow-orange precipitate

Acetone

Positive; turned orange

Benzaldehyde

Positive; yellow-orange precipitate

Tetrahydrofuran

Positive; turned orange

Ethyl Acetate

Positive; turned orange

Table 2

Tollen's Test

Reagent

Reaction

Cyclohexanone (Ketone)

No reaction

Benzaldehyde (Aldehyde)

Formed silver mirror on side of test tube

Table 3

Benedict's Test

Reagent

Reaction

Cyclohexanol

Negative; translucent blue

Cyclohexanone

Negative; translucent blue

Acetone

Negative; translucent blue

Benzaldehyde

Positive; Turned a cloudy blue-green

Tetrahydrofuran

Negative; translucent blue

Ethyl Acetate

Negative; translucent blue

Table 4

Chromic Acid Test

Reagent

Reaction

Cyclohexanol

Positive; turned a cloudy blue-green

Cyclohexanone

Negative; dark cloudy brown-orange

Acetone

Negative; translucent bright orange

Benzaldehyde

Positive; turned a cloudy blue green with precipitate

Tetrahydrofuran

Positive; turned a dark cloudy brown-orange

Ethyl Acetate

Positive; turned a dark cloudy brown-orange

Table 5

Schiff''s Test

Reagent

Reaction

Acetone (Aldehyde)

No Reaction

Benzaldehyde (Ketone)

Positive; turned bright translucent purple

Table 6

Iodoform Test

Reagent

Reaction

Acetone (Ketone)

Positive; formed a yellow precipitate

Table 7

Unknown Number 2 Data

Test

Results

Chromic Acid Test

Positive; formed silver mirror on sides of test tube

Tollen's Test

Positive; turned cloudy blue-green

Iodoform Test

Did not perform because it is an aldehyde

Boiling Point Determination Trial 1

129-131 °C

Boiling Point Determination Trial 2

131 °C

Boiling Point Average

130.5 °C

Table 8

Unknown Number 24 Data

Test

Results

Chromic Acid Test

Negative; No reaction

Tollen's Test

Negative; remained orange

Iodoform Test

Negative; transparent yellow

Boiling Point Determination Trial 1

125 °C

Boiling Point Determination Trial 2

125-126 °C

Boiling Point Average

125.25 °C

Discussion

Over the course of this two-week lab; students learned how to utilize six different chemical tests to obtain information regarding the tested compounds. This was a wonderful preparation for the second week, when students performed the tests on unknown compounds.

When working with the unknown compounds, students were only supposed to run the Tollen's Test, the Chromic Acid Test, and the Iodoform Test. The Tollen's and Chromic Acid Test both differentiate between aldehydes and ketones, and if the compound is determined to be a ketone, then the Iodoform Test is run. The Iodoform Test identifies whether the compound contains a methyl ketone.

The results for the two different unknowns were very clear; number 2 is an aldehyde, and number 4 is a ketone. According to the posted list of unknown compounds, unknown number 2 is heptaldehyde, and unknown number 24 is cyclopentanone. The chemical structures coincide with the chemical tests, as an aldehyde and a ketone were obtained.

Boiling points of the two unknown compounds also had to be determined. The obtained boiling point of heptaldehyde was 130.5 °C, in comparison to the literature value of 151 °C, and the boiling point of cyclopentanone was 125.5 °C, versus 130-131 °C lit.3

There are several things that may explain these changes in boiling points. There may have been contamination in the unknown vial, or the unknown vial could have been labeled incorrectly. The glassware used could have been cleaned improperly. Variance from standard temperature and pressure would have also affected the boiling point, since boiling point and the surrounding pressure are inversely related. There are almost a listless amount of possibilities that could have affected these deviances in boiling point.

Conclusion

This lab was very exciting, as there were many new and different concepts involved. Working with known substances helped to prepare the students to work with unknowns; they learned how to perform the tests and what to look for in the different situations of aldehydes and ketones. The difference in boiling point of the heptaldehyde is disappointing. However, the experiment as a whole was a success.

References

[1] Smith, J. Organic Chemistry, 2nd ed. New York: McGraw-Hill Companies; 2008.

[2] http://phoenix.liu.edu/~nmatsuna/che122/exp13.pdf

[3] http://www.sigmaaldrich.com

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