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_Laboratory Experiments for GOB Chemistry
I
ORGANIC FUNCTIONAL GROUP ANALYSIS
I. OBJECTIVES AND BACKGROUND
This experiment will introduce you to some of the more common functional groups of organic chemistry. The functional group is that portion of the molecule that undergoes a structural change during a chemical reaction. The functional groups that will be studied in this experiment are carboxylic acid, amines aldehyde, ketone, alcohols and alkenes. You will learn chemical tests that will allow you to distinguish one functional group from another. You will use the chemical tests to identify the functionality of an unknown organic compound. In addition, you will use a water solubility test to determine whether your organic compound is of high or low formula weight.
The chemical tests you will perform make up a sequence of experiments designed to determine the absence of or suggest the presence of particular functional groups. The complete sequence is shown in the flow diagram on page 8. This diagram can serve you in several ways:
It is a summary of the procedure that you are to follow in classifying your unknown as one of the functional group types.
It can order your thoughts as you read the discussion of each test, and help you to understand the significance of that test.
It can enhance your appreciation for and enjoyment of this experiment. Your role is that of chemist and detective: you will employ this cleverly devised scheme to sleuth out the identity of your unknown's functionality.
_Laboratory Experiments for GOB Chemistry
Discussion of Chemical Tests
1. Water Solubility: Diagnosis of a Low Formula Weight Compound
Most organic compounds are slightly soluble or insoluble in water. Water solubility is uncommon and if observed is an indication of a molecule of low molecular weight. A litmus paper test of the solution’s pH will provide additional information about the water-soluble molecule.
A. Acidic Response to Litmus: Diagnosis of a Low Molecular Weight Organic Acid Carboxylic acids with fewer than five carbon atoms are generally water soluble and form solutions that give an acidic response to litmus (acidic response: blue litmus paper turns red).
R C
O
OH + H 2 O R C
O
O- + H 3 O+
B. Basic Response to Litmus: Diagnosis of a Low Molecular Weight Organic Base Amines are the organic derivatives of ammonia, and like ammonia, they are weak bases. Amines that have fewer than five carbon atoms are generally water soluble and form solutions that give a basic response to litmus (basic response: red litmus paper turns blue).
primary amine: RNH 2 + H 2 O RNH 3 +^ + OH−
secondary amine: R 2 NH + H 2 O R 2 NH 2 +^ + OH−
tertiary amine: R 3 N + H 2 O R 3 NH+^ + OH−
C. Neutral Response to Litmus: Diagnosis of Low Molecular Weight Neutral Compound Neutral compounds (ketones, aldehydes, alcohols, alkenes) with fewer than five carbons are generally water soluble, and the resulting solutions are neither acidic nor basic. The solutions give a neutral response to litmus (neutral response: litmus paper does not change colors).
_Laboratory Experiments for GOB Chemistry
5. Tollen’s Test: Diagnosis of Aldehyde
Aldehydes can be distinguished from ketones by the Tollen's Test because aldehydes can be easily oxidized to acids, but ketones cannot. The Tollen's reagent contains a silver-ammonia complex ion in a basic solution. Silver(I) ion is reduced to silver metal as the aldehyde is oxidized to acid.
R C
O
H + 2 Ag(NH 3 ) 2 +^ + 2 OH− R C
O
(^) O-+ NH 4 + (^) + 2 Ag(s) + H 2 O + 3NH 3
An aldehyde is indicated by the appearance of a silver mirror on the wall of the test tube or a precipitate of silver metal.
6. Reaction with Chromic Arid: Diagnosis of Alcohol
Primary and secondary alcohols are oxidized by chromic acid to carboxylic acid and ketone, respectively.
primary alcohol: R−CH 2 −OH H CrO^2 4 R^ C
O
H H CrO^2 4 R C
O
OH
secondary alcohol: R' CH
R
OH H CrO^2 4 R' C
O
R
tertiary alcohol: R'^ C
R
OH
R''
H CrO^2 4 no visible reaction
An alcohol is indicated by disappearance of the orange color of the chromic acid reagent and the formation of a green or blue-green solution.
_Laboratory Experiments for GOB Chemistry
7. Decolorization of Bromine: Diagnosis of Alkene
The halogen bromine will rapidly react with alkenes at room temperature to produce the dibromoalkane.
C
H
H
C
H
H
Br C
Br H H H H colorless red solution colorless or light yellow solution
Bromine solutions are reddish-brown; alkenes and bromoalkanes are colorless. Rapid decolorization of bromine is a positive indication of an alkene.
III. PROCEDURE
The following procedures are to be carried out first on samples of known compounds so that positive responses for each test can be observed. When you feel confident that you can recognize a positive test for each procedure, obtain an unknown from your instructor. Substitute your unknown for the known compound in each test. Use the flow diagram on page 8 to follow your progress in identifying the functionality and molecular weight (high or low) of your unknown.
Known compounds to be tested for solubility properties: ethanoic acid, propanone, cyclohexene, diethylamine, benzaldehyde, 1-pentanol.
1. Water Solubility: Diagnosis of a Low Molecular Weight Compound
Add ~1 mL of water to ~0.5 mL of the known compound. Agitate the mixture thoroughly by holding the test at the top and flicking the base with your finger. A liquid that is insoluble in water will form a second phase. Visualization of two liquid phases in a few drops of mixture can be difficult; holding the test tube nearly horizontal should make the liquid-liquid interface readily observable. If the compound is soluble in water, it is a low molecular weight compound, and the solution should be tested with litmus paper to determine whether the compound is a carboxylic acid, an amine, or a neutral compound (ketone, aldehyde, alcohol or alkene). If the compound is insoluble in water, it is a high molecular weight compound. The compound's solubility properties in 5% NaOH and 5% HCl will yield information about its functionality. Record your observations and draw conclusions about each compound tested.
_Laboratory Experiments for GOB Chemistry
provided and rinse the test tube thoroughly. It may deposit explosive products upon standing or drying out.
6. Reaction with Chromic Acid: Diagnosis of Alcohol
Known compound to be tested: 1-pentanol Place ~1 mL of the known compound in a test tube. Add 1 drop of the chromic acid reagent and swirl the mixture. An alcohol is indicated by the disappearance of the orange color of the chromic acid reagent and the formation of a green or blue-green solution. Record your results for each compound tested.
7. Decolorization of Bromine: Diagnosis of Alkene.
Known compound to be tested: cyclohexene. Place two drops of the known compound in a dry test tube. Add dropwise a 1 % solution of Br 2 in dichloromethane (CH 2 Cl 2 ). An alkene will rapidly decolorize at least ten drops of the bromine solution. Record your results for each compound tested.
_Laboratory Experiments for GOB Chemistry
_Laboratory Experiments for GOB Chemistry
Test 4, continued. Reaction with 2-4-Dinitrophenylhydrazine
Known compound tested: benzaldehyde Observations:
Chemical equation:
Test 5. Tollen's Test
Known compound tested: propanone Observations:
Chemical equation:
Known compound tested: benzaldehyde Observations:
Chemical equation:
Test 6. Reaction with Chromic Acid
Known compound tested: l-pentanol Observations:
Chemical equation:
Test 7. Decolorization of Bromine
Known compound tested: cyclohexene Observations:
Chemical equation:
_Laboratory Experiments for GOB Chemistry
UNKNOWN Compound Functional Group Tests
Unknown Number___________________
Chemical Test Observations Conclusions
Water Solubility/Litmus
5% NaOH Solubility
5% HCl Solubility
Reaction with 2,4- Dinitrophenylhydrazine
Tollen’s Test
Reaction with Chromic Acid
Decolorization of Bromine
Functional Group Present in Unknown____________________________
Molecular Weight of Unknown (high or low)________________________
