Alkene Stereochemistry and the E,Z Designation

The cis-trans naming system used in the previous section works only with disuhstituted alkenes—compounds that have two substituents other than hydrogen on the double bond. With trisubstituted and tetrasubstituted double bonds, however, a more general method is needed for describing doublebond geometry. Trisubstituted means three substituents other than hydrogen on the double bond tetrasubstituted means four substituents other than hydrogen.) 

Considering each of the double-bond carbons separately, look at the two substituents attached and rank them according to the atomic number of the first atom in each. An atom with higher atomic number ranks higher than an atom with lower atomic number. 

If a decision can t be reached by ranking the first atoms in the two substituents, look at the second, third, or fourth atoms away from the double-bond until the first difference is found. 

Multiple-bonded atoms are equivalent to the same number of single-bonded atoms. 

Zdouble bond (Higher-ranked groups are on the same side.) 

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