ORGANIC CHEMISTRY EXPERIMENT II
FUNCTIONAL GROUPS
An organic functional group is a combination of
atoms, which imparts special chemical properties to the
parent compound. Organic compounds with the same
functional group are a family of compounds. In this
experiment, we will study four common functional
groups. They are the alcohol, aldehyde, ketone and
carboxylic acid.
The alcohol functional group is an OH group
attached to a carbon with no other oxygens. We will test
for this group by oxidizing it with chromic acid;
however, only primary and secondary alcohols can be
oxidized in this way.
OH
H3C
C
H
+
CH3 + 2H2CrO4 + 6H
O
H3C
C
+3
+ 2Cr
CH3
+ 8H2O
Both aldehydes and ketones have a carbonyl, C=O
group. Aldehydes have the carbonyl at the end of a chain
while ketones have a carbonyl between carbon atoms in
the middle of a chain. We will first test for the presence
of the carbonyl by forming a precipitate with 2,4-D
(2,4-dinitrophenylhydrazine). We will distinguish
aldehydes from ketones by using the Tollens test. The
test takes advantage of the fact that aldehydes can be
oxidized under mild conditions but ketones cannot.
O
H3C
C
+ 2Ag(NH3)2OH
H
O
H3C
C
-
+
O NH4
2Ag + NH3 + H2O
The last functional group we will investigate is the
O
carboxylic acid, C OH . We will react the acid with
sodium bicarbonate solution as our test.
O
H3C
C
OH
+ NaHCO3
O
H3C
C
O-Na+
+ H2O
Experimental Procedures:
A) Chromic Acid Test: Dissolve 1 drop of test
compound in 1-mL of acetone. Add 1 drop of chromic
acid test reagent. A positive test is the appearance of a
blue green color within a few seconds. Test compounds:
acetic acid, ethyl alcohol, 2-butanone and unknown.
(Note: aldehydes give a positive test but also give the
2,4-D test.)
B)
2, 4-D test: Place 1 drop of the test compound in
a small test tube and add I ml of 2, 4-D reagent. Shake
the mixture vigorously. Most aldehydes and ketones will
give a yellow to red precipitate immediately. However,
some may require up to 15 minutes. Test Compounds:
acetone, benzaldehyde, ethyl alcohol, and unknown.
C)
Tollens Test: The reagent must be prepared
immediately before use in a thoroughly cleaned test
tube. In four clean test tubes, a drop of 3 M NaOH is
added to 20 drops of 5% silver nitrate, and 2%
ammonium hydroxide is added dropwise until the silver
oxide is almost completely dissolved. Then one drop of
test substance is added to each tube. Shake the solution
well and let stand for 10 minutes. If no reaction has
occurred, place the tube in a beaker of hot tap water for
an additional 5 min.. A silver mirror on the test tube or
dark precipitate is a positive test for an aldehyde.
(Caution: Some alcohols may also give a positive test.)
Test Compounds: Use the same compounds as in Part B.
D)
Sodium Bicarbonate Test: Dissolve 3 drops of
the test compound in 1 ml of 5% aqueous sodium
bicarbonate solution. If the compound is a carboxylic
acid, bubbles of carbon dioxide will be seen. Test
compounds: acetic acid, ethyl alcohol and unknown.
Aldehyde Boiling Points
propanal
2-methylpropanal
butanal
3-methylbutanal
2-butenal
2-ethylbutanal
hexanal
heptanal
benzaldehyde
Ketone Boiling Points
48°
64°
75°
92°
104°
117°
130°
153°
179°
Alcohol Boiling Points
methanol
2-propanol
2-butanol
1-butanol
3-methyl -1-butanol
1-hexanol
1-heptanol
acetone
2-butanone
2-pentanone
4-methyl-2-pentanone
cyclopentanone
2-heptanone
2-octanone
56°
80°
101°
117°
131°
151°
173°
Carboxylic Acid Boiling Points
65°
82°
99°
118°
130°
157°
176°
acetic acid
propanoic acid
2-methylpropanoic acid
2-methylpropenoic acid
3-methylbutanoic acid
118°
141°
154°
163°
176°
Name__________________________________
ORGANIC FUNCTIONAL GROUPS REPORT SHEET
1.
Chromic Acid Test
a. Test compounds giving a positive chromic acid test are:
Give the equation for the reactions observed. (Use structural formulas for organic compounds).
2.
2, 4-D Test:
Test compounds giving a positive 2, 4-D test:
3.
Tollens Test:
a. Test compounds giving a positive Tollens Test are:
b. Give the equation for the reactions observed.
4.
Sodium Bicarbonate Test:
a. Test compounds giving a positive NaHCO3 are:
c. Give the equations for the reactions observed.
5
Draw the structure of the following compounds:
1 –butanol
6.
2-pentanone
butanal
propanoic acid
Unknown Number: ______________
Name of unknown: ___________________________
Boiling point: ____________
Structure of Unknown:
Test Results: (List + or -)
Chromic Acid _____
2, 4-D Test
_____
Tollens Test
_____
NaHCO3 Test _____
Name_____________________________________
PRELABORATORY ASSIGNMENT - ORGANIC FUNCTIONAL GROUPS
1.
How does an aldehyde differ from a ketone?
2.
Both an alcohol and aldehyde give a positive chromic acid test. How can they be distinguished?
3.
An unknown is added to aqueous NaHCO3 solution. No gas is formed. What does this mean?
4.
How does the alcohol functional group differ from the carboxylic acid functional group?
5.
An unknown gives a precipitate when 2, 4-D is added but does not react with Tollens reagent. The boiling
point is 116±2°. The unknown is: