when all the groups attached to doubly bonded carbon are different, then we can't use cis-trans nomenclature.... we use E-Z system.

E-means Entgegen-means opposite

Z-means Zusammen-means together

now we check the greater priority group on each side of double bond based on atomic number...

eg.-                   H   Br

                          |      |

                          C=C

                         |       |

                        Cl     I

On the right side of double bond, we give priority to I than to Br as atomic number of I is greater than Br.

Similarly, on the left side we give priority to Cl atom.

High priority groups are on same side => together =>Z compound

eg. 2-

      H    I

       |     |

      C= C

       |     |

      Cl   Br

On left side - priority group- Cl

On right side- priority group- I

Priority groups on opposite sides => opposite => E compound

eg 3-

When we have isotopes (same atomic number), we compare the atomic mass and give priority to atom with higher atomic mass.

H     Cl

|         |

C =    C

|          |

D       Br

D (Deuterium) is isotope of hydrogen but at. mass of D=2 and at. mass of H=1.

Left side- proirity-D

Right side- priority-Br

Priority groups on same side- together- Z compound

eg 4-

CH₃     CH₂-CH₃

|            |

C  =     C

|            |

H         CH₂-CH₂-CH₃

In such cases, we compare the atomic numbers of the first atom in the two groups that are different, in the chain.

Left side- obviously CH₃ is the priority group

Right side- the first two carbon atoms are same in the two groups. The difference comes where you get the third carbon in the chain in -CH₂-CH₂-CH₃ bonded to the second carbon in the chain instead of H in -CH₂-CH₃.

Therefore we give priority to -CH₂-CH₂-CH₃.

priority groups on opposite side- opposite- E compound

Similarly

CH₃-CH₂-CH₂            CH₂-CH₃

                  |                   |

                 C       =        C

                 |                    |

       CH₃-CH₂                   CH₂-CH₂-CH₂-CH₂-CH₃

 this is a E compound (check it for yourself)

eg 5-

                                      O

                                      ||

            CH₃                  C- H

                |                     |

                C        =       C

               |                     |

             H                     CH₂-OH

   Note-   >C=O is considered as-

                            > C - O

                                |       |

                               O      C

        It simply means C attached two times to O and O attached attached twice to C.

Left side - priority- CH₃

Right side- priority- -CHO

prority groups on same side- together- Z compound

eg 6-

CH₃            CN

|                   |

C  =          C

|                  |

H               CH₂-NH₂

As discussed in previous example,

here also -C=N can be drawn as (hypothetically)

             N          C

             |             |

           - C  -      N

              |            |

             N           C

left side- PG( priority group)- CH₃

right side- PG- -C=N

Question) Try this out and reply me the answer to this ( assign them E or Z configuration)?

1)

O

||

C         C=C

|            |

C =      C

|            |

O H      C=C

 

2)

H          CN

|             |

C =        C

|              |

D           C = C