Chemistry Reference
In-Depth Information
Table 9.4
Some properties of the unknown substances 1-4 being investigated by the students
1
2
3
4
Dichromate test
+
+
+
+
Cerium nitrate test
+
+
+
+
Esterifiable
+
+
+
+
Soluble in hexane
+
+
+
Soluble in water
+
+
+
Soluble in salt water
+
+
Molecular mass (
2 units)
32 u
46 u
61 u
73 u
Due to their solubility, they can be characterized as follows: alcohol 1 (methanol) is
the most polar, alcohol 4 (butanol) is the least polar.
The molecular masses of the four alcohols can be determined experimentally
according to the method of Heimann and Harsch [
16
]. Typical results are also listed
in Table
9.4
.
On this basis, the students can hypothesize on the structural formula of the
alcohol molecules. The hypotheses can be tested and verified with the help of the
13
C-NMR spectra (Fig.
9.11
). The term “homologous series” can now be defined on
an experimental and structural basis [
14
].
- Macroscopic definition of the homologous series:
Substances being members of a homologous series belong to the same substance
class. They have similar chemical properties (e.g., test behavior and syntheses).
Their physical properties (e.g., boiling temperatures, solubilities, molar masses)
follow a monotonous trend thus forming a rank.
- Submicroscopic definition of the homologous series:
Molecules being members of the homologous series have the same functional
groups, but they differ with respect to the length of their hydrocarbon chains by a
methylene group from one member to the next in the rank.
A demonstration experiment (Fig.
9.12
) on an overhead projector works well for
the visualization of the graded water solubility of homologous alcohols [
13
].
A graded movement during the solution process can be observed for the short-
chained alcohols due to the better water solubility. The long-chained alcohols react
rather “sluggishly.”
It has to be pointed out that alcohol molecules like methanol and ethanol, which
only differ by one CH
2
-group have similar chemical properties, but regarding their
physiological effects, the difference of one CH
2
-group can kill:
- An adult, who drinks 60 ml of ethanol, gets a blood alcohol concentration of
about 1 promille. This makes him or her dizzy without serious harms.
- Somebody who drinks 60 ml of methanol will probably be dead after only 20 h
of latency.
