Linus Pauling: Well, let me go on to discuss some things that we do know about. This model. This model represents the structure of ethylene.
It has something new in it; a new structural feature. Here we have two bonds connecting one carbon atom with another carbon
atom. A double bond represented in a conventional way by two lines between the carbon atoms.
Ethylene is an interesting substance. It causes oranges to ripen. If you have oranges that aren’t very ripe, look sort of
yellow in the freight car and put some ethylene into the freight car, the oranges develop a beautiful orange color. Nobody
knows why that goes on either.
Well here is a double bond, we can say two bent bonds holding the carbon atoms together. Now, if this double bond is described
as involving two tetrahedra, two tetrahedra that are attached together with one edge in common, then we can see that there
is restriction in the rotation. It is not possible to twist the molecule around, through, one end through a hundred and eighty
degrees relative to the other end. To do that, one would have to break a bond, and this takes a lot of energy.
The result of this is that a new sort of isomer is found in substituted ethylenes. If we replace one of the hydrogen atoms
on this end of the molecule with say a chlorine atom, and one on the other end with a chlorine atom, we may do this in either
one of two ways. This hydrogen and this hydrogen may be replaced with chlorine. That gives one substance. Or this hydrogen
and the opposite hydrogen may be replaced. That gives another substance. These substances have different properties, different
chemical and physical properties. They are represented by the model shown here. This is called cis-dichloroethylene, in
which the two chlorine atoms are on the same side of the double bond. This molecule, represented by this model, is called
trans-dichloroethylene in which the two chlorine atoms are on opposite sides of the double bond.
Well, here again, we have only two isomers with the formula C2H2C12 and with the chlorine atoms on separate carbon atoms. There is also a third isomer in which there are two hydrogen atoms
attached to one carbon atom, two chlorine atoms attached to the other carbon atom.
In addition to the double bond, the triple bond is known. There are substances, such as acetylene, that contain a carbon-carbon
triple bond. Here we have three bent bondsholding the two carbon atoms together. The other two bonds project out in opposite
directions. The molecule acetylene, C2H2, is a linear molecule. The conventional representation of this molecule is C, H, C, H.
No quadruple bond is known. Nobody has ever recognized the quadruple bond, at least so far as I am aware. In reading the
chemical literature, I have never seen mention of evidence that a quadruple bond exists. Perhaps we can understand that,
too, in terms of the tetrahedral carbon atom. For carbon, any rate, the four bonds come out in these directions. We can
have a double bond by bending the bond, triple bond by sharing two faces of the two tetrahedra and bending the bounds about
enough, but the fourth bond would have to make a terrific bend in order to get around from the backside of this carbon atom
to the backside of the other carbon atom.