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Geometrical isomerism with Z and E notation

Short answer

Geometrical isomerism arises from restricted rotation about a double bond when each carbon bears two different groups. In but-2-ene, the isomer with CH3 groups on the same side is Z, opposite side is E, based on CIP priority (higher atomic number first).

Define the geometrical isomerism and write an example with Z and E notation.

Geometrical (cis–trans) isomerism is a type of stereoisomerism in which isomers have the same structural formula but differ in the spatial arrangement of groups about a double bond (or ring), because rotation about it is restricted.

Condition: each doubly bonded carbon must carry two different groups (a≠b, c≠d in abC=Ccd).

Example: but-2-ene, CH₃–CH=CH–CH₃

  CH₃     CH₃              CH₃     H
     \   /                    \   /
      C=C                      C=C
     /   \                    /   \
    H     H                  H     CH₃
  (Z)-but-2-ene           (E)-but-2-ene
   (cis)                    (trans)

CIP priority rules for E/Z: on each doubly bonded carbon, rank the two attached groups by the atomic number of the atom directly attached (higher atomic number = higher priority; if they are the same, compare the next atoms). If the two higher-priority groups lie on the same side, the configuration is Z (zusammen); if they are on opposite sides, it is E (entgegen). Here CH₃ has higher priority than H on each carbon, so the isomer with both CH₃ groups on the same side is (Z) and the one with them on opposite sides is (E).

✓ Answer checked by Claude Opus 5.5 (high effort)

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