Chemistry Reference
In-Depth Information
-
Note
: Aldehydes are oxidized by copper ions Cu
2+
in alkaline solution to acid
ions; Cu
2+
ions are reduced to Cu
1+
, and these ions form insoluble red copper
oxide Cu
2
O.
Not only glucose but also fructose gives a positive test result because of rapid
isomerization in the alkaline solution, in contrast to saccharose.
Diethyl acetal gives a negative test result because acetals are not hydrolized in
alkaline solutions. (Note that saccharose is also an acetal!)
This test reaction was discovered by Hermann Fehling (1812-1885) who was a
Professor for Chemistry at the University of Stuttgart (Germany).
1.6 Copper Sulfate Test
-
Reagent 1
: Copper sulfate in water (
c
0.1 mol/l).
-
Reagent 2
: Sodium hydroxide in water (
c
¼
3 mol/l).
-
Procedure
: 1 ml of the liquid test sample (or a small spatula amount of a solid
sample) is dissolved in 1 ml copper sulfate solution. The mixture must be clear.
Then 5 drops of sodium hydroxide solution are added drop by drop (continuous
shaking).
-
Result
: A color change from light blue to deep blue or purple without a permanent
precipitate (the solution must be clear after adding 5 drops of sodium hydroxide
solution) indicates molecules with alcohol groups on adjacent carbon atoms (diol
¼
groups).
-
Note
: In the copper sulfate test, the test substance (diol) has the same function as
the tartrate ions (they are diols) in the Fehling test: They form deep blue and
stable chelate compounds with Cu
2+
ions in alkaline solution.
1.7 Bromine Test
-
Reagent
: 10 drops of bromine are dissolved in 250 ml water.
-
Procedure
: 2 ml test substance (or 1 spatula) are dissolved or suspended in 2 ml
bromine water. The solution is shaken well. (Close the test tube with a stopper
and use gloves.)
-
Result
: Decolorization indicates molecules with double bonds (alkenes).
-
Note
: Decolorization occurs by addition of bromine molecules to the double
bonds of the test molecules. (The addition products are colorless.)
If bromine is only extracted from the water phase into the organic phase without
decolorization (e.g., with alkanes or esters as test substances),
the test is
negative.
