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pranay -askiitians expert Grade: 12

can any one explain me the nomenclature of compounds by entizezen and zussamen.

thanx in advance

6 years ago

Answers : (1)

879 Points

Dear student,

This is simple E and Z nomenclature.

E and Z system of nomenclature

 ‘Cis’and ‘Trans’ designations cannot be used if four different atoms or groups are attached to the carbon atoms of a double bond.

                     ba>C = C<de


 In such cases, E and Z system of nomenclature is used. This system is based on a priority system developed by Cahn, Ingold and Prelog.

In this system, the two atoms or groups attached to each of the doubly bonded carbon are put in order of preference on the basis of sequence rules.

The symbol ‘E’ is assigned to an isomer in which the atoms or groups of higher preference are on the opposite side (E from German word Entgegen = across or opposite).

The symbol ‘Z’ is assigned to an isomer in which the atoms or groups of higher preference are on the same side (Z from German word, Zusammen = together).


                          C= C        C = C

                                   E-isomer                             Z-isomer



 1 signifies higher preference and 2 signifies lower preference. Preference in most of the cases ‘Z’ corresponds to cis-form and ‘E’ to trans-form. However, there are many exceptions.

The following rules are followed for deciding the precedence order of the atoms or groups;

(i) Higher priority is assigned to the atoms of higher atomic number. For example, the order of preference in the following atoms, H, Cl, I Br  is : I (at. no. 53)> Br (at. no. 35)>Cl (at. no. 17)>H (at. no. 1).

(ii) If isotopes of the same element are attached, the isotope with higher mass number is given higher order of preference. For example, deuterium ( 21D )  is assigned higher priority in comparison to hydrogen ( 11H ) .

(iii) In the groups, the order of preference is also decided on the basis of atomic number of first atom of the group. For example, in the following set,

            - Cl, - OH, - COOH, - NH - CH3, - SO3H


Please feel free to ask your queries here. We are all IITians and here to help you in your IIT JEE preparation.

All the best.

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Askiitians Expert

Sagar Singh

B.Tech, IIT Delhi

6 years ago
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