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The Three-Dimensional Structure of Molecules - Isomerism

The Three-Dimensional Shape of Molecules

Fig.1
Tetraeder

In 1874 the Dutchman Jacobus Hendricus van't Hoff and the Frenchman Achille Le Bel came to the conclusion that four substituents can be arranged in different ways around a carbon atom. Van´t Hoff proposed a tetrahedral arrangement.

The geometry of molecules results in important consequences. To illustrate these, the compound dichloromethane with the empirical formula CH2Cl2 is described. One reason for van´t Hoff´s hypothesis was the fact that all four bonds of a tetrasubstituted carbon atom are equivalent.

Therefore, there are only two possible geometrical arrangements of a given four substituents around a carbon atom:

  • the square planar orientation, which would result in two different compounds of the same empirical formula CH2Cl2 and
  • the tetrahedral arrangement, which only gives one compound with the empirical formula CH2Cl2.
Tab.1
planar tetrahedral
cis?
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Fig.2
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Fig.3
trans?
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Fig.4
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Fig.5

(By rotating the three-dimensional molecular models it can be shown that the pseudo-cis- and the pseudo-trans-form of the tetrahedron are identical.)

In all experiments every compound with the empirical formula CH2Cl2 had the same physical and chemical properties. The arrangement of the substituents around the tetrasubstituted carbon atom has thus proven to be tetrahedral.

The bond angles of a tetrasubstituted carbon atom are 109.5°. Therefore, long hydrocarbon chains (alkanes), which can be found among other things in gasoline, are not hooked up in a straight line but rather in a series of tetrahedrons, connected at their corners (see (Abb. 6) ).

Fig.6
Tetraederkette
Tab.2
Tetrahedron series Space-filling model
Fig.7
Fig.8

Therefore, two consecutive carbon-carbon bonds never form a straight line. The illustration ( (Abb. 7) and (Abb. 8) ) shows two different ways to describe such an angled hydrocarbon chain. The Fischer and Newman projections are examples of different two-dimensional illustrations for three-dimensional structures.

Exercise

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