This axis corresponds to the straight line joining the two
carbon atoms. In the case in which double union exists no such
revolution is possible. Again, if, by addition to an unsaturated
compound like ethylene, a saturated compound is formed, the kind of
union between the carbon atoms is changed, and the possibility of
revolution of the two parts of the compound is given. Whether such
revolution take place or not will be determined largely by the structure
of the compound. The tendency will be for those parts of the molecule
which have the greatest specific affinity for one another to take those
positions in which they are nearest to one another. Thus, suppose that
chlorine is added to ethylene. By following the change on the model, it
is seen that in the resulting figure the two chlorine atoms in ethylene
chloride are situated at angles of the two tetrahedrons which are
nearest each other. But chlorine has a stronger affinity for hydrogen
than it has for chlorine, and therefore each chlorine atom would tend to
get as near a hydrogen atom as possible. This involves a partial
revolution of the two tetrahedrons in opposite directions around their
common axis. So also hydrogen would tend to take a position as near as
possible to hydroxyl and to carboxyl, while hydroxyl would avoid
hydroxyl, and carboxyl would avoid carboxyl.
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