Question

What is eandz system of nomenclature with an example?

Answer 1

Introductory Organic and Biochemistry

The E-Z system for naming alkenes; 
examples of using the CIP rules

A previous X402 textbook, Bettelheim 6/e, did not cover the E-Z system. Therefore, I wrote this page as a supplement to introduce the topic for those who wanted to go beyond the book. The new X402 book, Ouellette 2/e, covers the E-Z system fine, in Ch 4. Students taking my X402 with the Ouellette book are responsible for the E-Z system, as covered in the book. I have since added more material, especially more examples, here, to supplement Ouellette.

Textbook references on this page are to the previous X402 book, Bettelheim et al, 6/e -- except as noted. Students using that book will probably find it easier to learn the E-Z system for naming alkene isomers after covering the R-S system for chiral compounds in Ch 15.

On my page RasMol - An Introductory Guide, much use is made of Dave Woodcock's collection of molecular structure files at Okanagan University College. Most of the alkene files there are labeled in E-Z nomenclature.

Contents: 
   Introduction to the E,Z system 
   E,Z will work -- even when cis,trans fails 
   E,Z will work -- but may not seem to agree with cis,trans 
   Multiple double bonds. How to show E,Z for more than one double bond in a molecule. 
   The double-bond rule in determining priorities. How a double bond in the attached group helps determine E,Z. 
   The "first point of difference" rule. How do you really compare the three atoms on one C with the three atoms on another C? 
      Bottom of page; return links and contact information


Introduction to the E,Z system

The traditional system for naming the geometric isomers of an alkene, in which the same groups are arranged differently, is to name them as cis or trans. However, it is easy to find examples where the cis-trans system is not easily applied. IUPAC has a more complete system for naming alkene isomers. The Bettelheim textbook does not introduce it, but it is actually rather straightforward.
In Ch 15 Sect 3 Bettelheim presents the R-S system for naming stereoisomers that differ only in the arrangement of groups around a chiral center. The R-S system is based on a set of "priority rules", which allow you to rank any groups. These rules are summarized in Bettelheim 6/e Table 15.1, p 359. (They are in Ouellette 2/e, pp 108-9.)

The priority rules are often called the Cahn-Ingold-Prelog (CIP) rules, after the chemists who developed the system.

The rigorous IUPAC system for naming alkene isomers, called the E-Z system, is based on the same priority rules.
The general strategy of the E-Z system is to analyze the two groups at each end of the double bond. At each end, rank the two groups, using the CIP priority rules, discussed in Ch 15. Then, see whether the higher priority group at one end of the double bond and the higher priority group at the other end of the double bond are on the same side (Z, from German zusammen = together) or on opposite sides (E, from German entgegen = opposite) of the double bond.