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
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
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.