Chemistry Reference
In-Depth Information
To give an impression of the 3D structures, it is advisable to build the
corresponding molecular models with the help of a molecule kit. The only thing,
however, that is important for the explanation at this level is the constitution of the
molecules.
9.7 Use of Formulae (“Combining Structure and Properties”)
The advantage of the depicted approach is that students are able to construct
formulae rather quickly. What is even more important is that students can immedi-
ately assure themselves of the formulae's potential value. They can use these
symbols for the interpretation of all the analytical and synthetic relations between
the substances A-F, which they have already discovered on the phenomena level:
• The pair wise similar analytical properties (Fig.
9.2
, see color chart) of
substances A/D (alcohols), B/E (carboxylic acids), and C/F (esters) can appar-
ently be ascribed to the pair wise similar molecular structures. The functional
groups that are responsible for the similar properties in pairs can now be
identified (Fig.
9.9
).
• The pair wise similar synthesis behavior (Fig.
9.5
) of substances A/D, B/E, and
C/F can also be ascribed to the functional groups. First (very simplified) reaction
symbols can now be formulated (Fig.
9.10
). Students have to find out the
invariant structure characteristics of the educt and product molecules, by com-
paring their structural formulae. They recognize the partial structure of the educt
molecule that survives the reaction unaltered. The synthesis reagent and, if
applicable, the solvent (water) have to be responsible for the difference. This
leads to simplified chemical equations ([
1
], p. 60 ff.). Formal operational skills
are not necessary for their deduction. Complete redox reaction equations can be
discussed here or later.
• The pair wise similar spectroscopic properties (Fig.
9.6
) of substances A/D, B/E,
and C/F do not only support the functional group concept, but also make the
discovery of important rules possible which govern the positions of the signals in
the
13
C-NMR spectra. Pattern recognition reveals (Fig.
9.8
):
- Carbon atoms that are bonded to
two
oxygen atoms give a signal at a high
ppm value (165-185 ppm). Partial structures of this kind are typical for
carboxylic acid and ester molecules.
- Carbon atoms that are bonded to
one
oxygen atom give a signal at an
intermediate ppm value (55-65 ppm). Partial structures of this kind are
typical for alcohol and ester molecules.
- Carbon atoms that are bonded to
no
oxygen atom give a signal at a low ppm
value (5-30 ppm). This is typical for the hydrocarbon chains of all molecules
including the alkanes.
