Chemistry Reference
In-Depth Information
Propanoic propyl ester has been transformed by sodium hydroxide solution to
propanoic acid and propanol.
- For reasons of analogy and simplicity, it can be assumed that the only products
of the alkaline hydrolysis of ethanoic ethyl ester are acetic acid and ethanol, not
propanol.
To support this hypothesis it will be tried to reproduce the corresponding ester from
the cleavage products of the ester hydrolysis in the next step. Sulfuric acid can be used
as synthesis reagent (as an antagonist for the sodium hydroxide solution, used in the
ester hydrolyses). Note that the antagonistic reason is not compulsory, but it makes the
choice of the synthesis reagent reasonable for the student. Mixtures of ethanol/acetic
acid and propanol/propionic acid, respectively, are thus to be laced with concentrated
sulfuric acid ([
1
], p. 247). The reaction mixtures can already be poured onto water
after a couple of minutes. Ester phases indeed separate out in both cases. They show
the behavior pattern of ethanoic ethyl ester and propionic propyl ester, respectively
(after washing with sodium carbonate solution). The discovered synthesis relations
are displayed and summarized in a net of synthetic pathways (Fig.
9.5
).
9.6 Leaping to the Particle Level
The leap from the phenomena to the particle level is abstract by nature and requires
formal operational thinking skills for a deep understanding: “Science is by its very
nature formal” (Herron [
41
]). However, it cannot be ignored that the majority of
students at high school level (and certainly at middle school level) has substantial
problems with reflective, formal operational thinking (Graber and Stork [
37
],
Shayer and Adey [
42
], Lawson [
43
], Haußler, Bunder, Duit, Graber, Mayer [
44
]).
For educational psychological reasons, but also due to time limits, it is not
possible to simultaneously work on the structural formulae of alcohols, carboxylic
acids, and esters with conventional methods of structure analysis. There is no best
way that directly leads from the phenomena to the formula. Constructive
assumptions are necessary, where eligibility cannot be derived from single phe-
nomena. The explanation for the formula rather results in retrospective from the
immediate success of its application in the interpretation of a system of interrelated
phenomena, which have been discovered empirically without formulae.
At this point we make use of spectroscopic measurements (simplified
13
C-NMR
and mass spectra [
17
] of the substances A-F). Students cannot measure these
spectra by themselves, of course, but they are able to work on their concrete
operational analysis. The fundamentals of these analytical techniques do not have
to be explained in detail before starting with the analysis.
The following assumptions are necessary:
- Molecules of the substances A-F only contain carbon, hydrogen, and oxygen
atoms [this can also be determined experimentally, if necessary ([
1
], p. 317)].
- Covalence and relative masses of the three kinds of atoms C/H/O, as well as the
possibility of double bonds, have to be familiar or have to be assumed.
