Chemistry

Spatial structure of the molecules - isomerism

Spatial structure of the molecules - isomerism


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The spatial shape of the molecules

In 1874 the Dutchman Jacobus Hendricus van't Hoff and the Frenchman Achille Le Bel came to the conclusion that in molecules with carbon atoms bearing four different substituents, these substituents can be arranged in different ways. Van't Hoff suggested a tetrahedral arrangement for this, while Le Bel did not comment on it.

The geometry of the molecules has important consequences. As an explanatory example, consider the compound dichloromethane with the empirical formula CH2Cl2 called. One of the reasons for van't Hoff's assumption was the fact, already known at the time, that all four bonds of a four-coordinate carbon atom are identical.

Only two arrangements are possible in which all four substituents are uniformly arranged around the carbon atom:

  • the square planar, in which there are two different CH2Cl2 Connections must be and
  • the tetrahedral arrangement in which only one connection is possible.

(By rotating the 3D images it can be seen that the pseudo-cis- and the pseudo-trans- shape are identical.)

Since only a connection with the empirical formula CH2Cl2 was found, the tetrahedral arrangement was the only possible one.

The bonds of the carbon in a tetrahedron have an angle of 109.5 ° to each other. For example, longer hydrocarbon chains (alkanes), such as those found in gasoline, are not to be regarded as straight chains, but rather as a series of tetrahedra connected from tip to tip (Fig. 6).

It follows that two consecutive carbon-carbon bonds to each other never form a straight line. The graphics ((Fig. 7) and (Fig. 8)) show two different forms of representation of such an angled hydrocarbon chain.

There are different methods to represent this spatial structure of the molecules in a two-dimensional drawing, e.g. the Fischer projection and the Newman projection.


Video: Δρ David MARΤIN: Η M0DERNA γεννήθηκε από μια επιχορήγηση (July 2022).


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