Sequence Rules the E,Z Designation

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

Thomson MQW Click Organic Interactive to practice assigning priorities to groups according to the Cahn-lngold-Prelog rules. 

E double bond (Higher-priority groups are on opposite sides.) 

Called the Cnlm-Ingolrf-Prelog rules after the chemists who proposed them, the sequence rules are as follows  

Called the Cahn-lngohl-helog mlcK after the chemists vvlio proposed (hem, (he sequence rtiles are as follows  

Rule 1 Considering the double-bond carbons separately, look at the two atoms directly attached to each and rank them according to atomic number. An 

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A molecular structure such as the one shown in diagram XLII is achiral (presence of a plane of symmetry), but when a b and c = d it can exist in two diastereoisomeric forms. When the two largest or remarkable substituents are on the same side of the double bond, the isomer is cis, and it is designated trans in the other case. Ambiguities have been encountered, and it is recommended to designate as (Z) the isomer with the two sequence rule-preferred substituents on the same side of the double bond (usually the cis-form), and as (E) the other isomer (usually the Wwts-form) (XLIII a>H,c>H) ([53], and refs, therein). 

Ei/Ti (from entgegen/zusammen = on opposite sides/on the same side) Affixes to the name of a compound designating steric relations around a double bond and assigned by use of the sequence-rule procedure. For example, for a C=C double bond, the priorities of the two subtituents on the carbon atoms of the double bond are assigned (see R/S system). If the higher priority on each carbon is on the same side of the double bond the specification is Z, if it is on the opposite side it is E. 

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