Linus Pauling: A very important contribution to structure theory was made in 1874 by the Dutch chemist
van’t Hoff and the French chemist, Le Bel independently of one another. This is the
idea that the four bonds formed by the carbon atom are not directed out toward the
corners of the square in one plane as indicated here on the blackboard, or are not
so loose-jointed that they have no well defined direction, but instead are directed
toward the four corners of a tetrahedron.
All of our models are built in this way. Here we have the methane molecule with the
four bonds shown proceeding toward the four corners of a tetrahedron, a regular tetrahedron.
This has been found in recent years by the determination of the structures of crystals
and of gas molecules, by the x-ray diffraction method, the electron diffraction method,
and various spectroscopic methods, that the angles between single bonds formed by
a carbon atom remain in all substances quite close to the tetrahedral angle, the angle
for a regular tetrahedron, a hundred nine degrees, twenty-eight minutes.
The tetrahedral carbon atom is a very important part of chemistry. I think that the
discovery of the tetrahedral carbon atom was a wonderful thing – it shows the power
of man’s mind. The facts were that in 1874, it was known as a result of the work
of Pasteur that some substances can form crystals that have either a left-handed appearance
or a right-handed appearance.
Van’t Hoff and Le Bel asked how is it possible for substances to be built up of molecules
that are right-handed or left-handed? Two different kinds of molecules that are related
to one another in the way that the right hand and the left hand are related. They
showed that the tetrahedral carbon atom provides the explanation of these facts, that
the four bonds of a carbon atom are connected to four different kinds of atoms or
groups.
For example, a hydrogen atom, a methyl group CH3, a chlorine atom, a bromine atom. Then, this tetrahedral molecule can be either
a right-handed molecule or a left-handed molecule and the right-handed molecule does
not become left-handed by any translational or rotational motion in space. Only by
breaking the bond and moving it around to the other side can you convert the right-handed
molecule into the left-handed molecule. Recent investigations, recent structure determinations,
have of course, completely verified this idea of the tetrahedral carbon atom.
