Chemistry Reference
In-Depth Information
9.8
Integration of Other Substances
With their experience of the exemplary substances A-F, the students now have a
good basis for the integration of other substances. On this occasion, they can
strengthen, extend, and deepen their skills for the structure-properties-thinking.
Three examples will explain this matter.
9.8.1 Analysis of a Natural Substance
The students analyze an unknown natural substance X that can be found in the
leaves of wild vines. They find out: X has a positive result in the BTB test (group
test for carboxylic acids) and in the cerium nitrate test (group test for alcohols). The
dichromate test (for oxidizable substances) is also positive. All other test results are
negative. Possible formulae for the molecule of substance X are ([
1
], p. 70):
The
13
C-NMR spectrum of X gives two signals (at 176 ppm and at 59 ppm).
Therefore, only formulae (1) and (3) are possible. These formulae are also in
accordance with the rules for the ppm values which students have discovered before
(see Sect.
9.6
) by pattern recognition. The unknown substance could therefore be
glycolic acid (1) or tartaric acid (3), but not lactic acid (2).
The mass spectrum of X gives three signals (31 u, 45 u, and 76 u). This result goes
with the fragments -CH
2
OH (31 u) and -COOH (45 u). Formula (3) is therefore
excluded, because it cannot explain the fragment with amass of 31 u, and its molecular
mass (150 u) is too large. Formula (1), however, is in line with all results. The natural
substance X is glycolic acid, which is contained in the leaves of wild vines.
9.8.2 Development of the Term “Homologous Series”
Students analyze unknown substances, which are only labeled by numbers. The test
results are displayed in Table
9.4
(see [
1
], p. 65 and [
14
]):
Apparently, the substances are oxidizable alcohols. All four alcohols can be
esterified in a test tube experiment with acetic acid (in the presence of sulfuric acid).
