91Ó°ÊÓ

The cis-trans naming system works only for disubstituted alkenes having two substituents other than hydrogen on the double bond. But when trisubstituted and tetra substitution of the double bond is present then the E and Zsystem is used for describing alkene stereochemistry.

• Multiple-bonded atoms are equivalent to the same number of single-bonded atoms.
• Once the groups attached have been ranked look at the entire molecule. If the higher ranked groups on each carbon are on the same side of the double bond then alkene is said to have Z geometry and if present on opposite sides then the alkene has E geometry.

As ranking is atomic number dependent, more the atomic number higher will be ranking.

So,

In${\text{-CH}}_{3}O$and $−\text{Cl}$:

• In ${\text{-CH}}_{3}O$carbon is attached to oxygen.
• In $−\text{Cl}$has a higher atomic number than carbon.

So, $−\text{Cl}$rank 1.

In${\text{-CH}}_2{\text{CH}}_2{\text{CH}}_3$and${\text{-CH}}_2{\text{CH}}_3$ :

• In ${\text{-CH}}_2{\text{CH}}_3$, second carbon is attached to three hydrogens.
• In${\text{-CH}}_2{\text{CH}}_2{\text{CH}}_3$, second carbon is attached to two hydrogens and one carbon.

So, ${\text{-CH}}_2{\text{CH}}_2{\text{CH}}_3$rank 1.

From structure of given compound, it can be seen both high ranking group on opposite side so E configuration.

$\begin{array}{l}{\text{-CH}}_{3}O\\ {\text{-CH}}_2{\text{CH}}_2{\text{CH}}_3\end{array}$