E, Z

The approved stereodescriptors of stereoisomeric alkenesR¹R²C=CR³R⁴ ( R¹ ≠ R² , R³ ≠ R⁴ ); neither R¹ nor R² need be different from R³ or R⁴ ), cumulenesR¹R²C(=C=C)_n=CR³R⁴ and related systems e.g. R¹R²C=NOH , HON=C{[CH₂]_n}₂C=NOH . The group of highest CIP priority attached to one of the terminal doubly bonded atoms of the alkene, oxime, etc. or cumulene (i.e. R¹ or R² ) is compared with the group of highest precedence attached to the other (i.e. R³ or R⁴ ). The stereoisomer is designated as Z (zusammen = together) if the groups lie on the same side of a reference plane passing through the double bond and perpendicular to the plane containing the bonds linking the groups to the double-bonded atoms; the other stereoisomer is designated as E (entgegen = opposite). The descriptors may be applied to structures with a fractional bond order between one and two; and to double bonds involving elements other than carbon. They are not used to describe ring substitution relationships.