Linus Pauling: The simple chemical structure theory permits one to understand the existence of isomers. I have here a model. Let me start
with this one. I have here a model representing one of the two forms of butane, C4H10, 3, 6, 7, 8, 9, 10, C4H10. This is normal butane, a straight chain hydrocarbon. It is called a straight chain even though the bonds are at the tetrahedral
angle here so that the chain is a zigzag chain.
Normal butane is one substance. There is another substance with formula C4H10 that has somewhat different properties. This other substance, called isobutane, is represented by this model. Here we have
again four carbon atoms and 3, 6, 9, 10, ten hydrogen atoms, but the bonding between atoms is different for isobutane from
that of normal butane.
These are the only two ways in which four carbon atoms and ten hydrogen atoms can be attached together with each carbon atom
forming four bonds and each hydrogen atom one. And, corresponding to this, there are only two isomers known, two substances
known, that have the composition C4H10. This provides an interesting, simple example of the power of chemical structure theory.
Now, there are many other sorts of molecules that one can build that are compatible with the simple principles of structure
theory. I have here a model of a new sort in which there is a ring, a cycle, of carbon atoms. This molecule is the molecule
of cyclopentane, C5H10. The angle in a pentagon is a hundred and eight degrees, so very close to the tetrahedral angle so that there is practically
no strain in this molecule.
On the other hand, a smaller ring involves some strain. No longer are we able to use half-inch wood dowel rods in representing
the bond. We have to have strings in this model which represents cyclopropane, C3H6. Here we have bent bonds connecting the carbon atoms. If the bonds came directly from carbon atom to carbon atom, the angle
would be sixty degrees instead of the tetrahedral angle, a hundred and nine degrees, twenty-eight minutes. We can represent
the structure by the use of these bent bonds. There is some strain associated with the bent bonds so that cyclopropane is
somewhat less stable than one would expect from a molecule with the same composition and without the bent bonds.
It is interesting that this substance, cyclopropane, is used as a general anesthetic. It produces anesthesia. I think this
is an illustration of the present situation in physiology, biochemistry, our lack of understanding of the nature of the human
body. Why is it that this particular molecule produces anesthesia? Why is it that chloroform, CHC13, produces anesthesia? Nobody really knows the answer. No one is able to predict what substances will be anesthetics and
what not. Here we have chloroform with a hydrogen atom down here - I’ll hide this chlorine atom to make it into chloroform.
No one knows why chloroform serves as an anesthetic too.
Some light on this question is provided by the fact that xenon is also a general anesthetic. Now, xenon is one of the noble
gases, atomic number fifty-four. It forms no chemical compounds in which it forms chemical bonds. It will, in fact, form
a hydrate, xenon hydrate, in which the water molecules are arranged together, attached to one another in such a way as to
make cages, little rooms, in which the xenon atoms fit. It is interesting that cyclopropane forms a similar hydrate, and
chloroform forms a similar hydrate. In the case of chloroform, the hydrate is something like CHC13 17H2O.
I think that it may well be that the effect of these substances in producing general anesthesia is related to their power
to form moderately stable hydrates, stable at temperatures ten degrees or more above the freezing point of water. Perhaps
somewhere in the tissues in the nervous system, there are little regions where the water is tied down into a sort of pseudo-crystalline
aggregate by the molecules of the general anesthetic and the normal metabolic activities of the nervous system are not able
then to go on. But we really don’t know enough about the chemistry of the human body to be able to give an explanation of
this.
