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2. E-Z system
• When atoms or groups of higher sequence-rule
precedence are on the same side of the double bond, the
double bond has the Z configuration, where Z stands for
the German word zusammen, meaning “together.”
• When they are on opposite sides, the configuration is E,
standing for the German word entgegen, meaning
“opposite.”
• The E or Z descriptor is placed in parenthesis and precedes
the rest of the name.
• Priority is based on atomic number: The higher the atomic
number, the higher is the priority.
• If priority cannot be assigned on the basis of the atoms that are
bonded directly to the double bond, look at the next set of
atoms, and continue until a priority can be assigned.
• Multiple (double or triple) bonds are assumed to have the same
number of single-bonded atoms.
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