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.
