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Cis, trans, Z, E notation for but-2-enedioic acid

Short answer

Maleic acid is the (Z) isomer with COOH groups on the same side; fumaric acid is the (E) isomer with COOH groups opposite. Priority: COOH > H, so Z when higher groups are same side.

Write Cis, Trans and Z, E notation for the possible isomer of but-2-enedioic acid.

But-2-enedioic acid, HOOC–CH=CH–COOH, has two isomers because each doubly bonded carbon has two different groups (H and COOH).

      cis / Z                         trans / E
 HOOC       COOH                 HOOC        H
    \      /                        \      /
     C == C                          C == C
    /      \                        /      \
   H        H                      H        COOH
 maleic acid                    fumaric acid
  • cis (Z)-isomer, maleic acid: the two COOH groups are on the same side.
  • trans (E)-isomer, fumaric acid: the two COOH groups are on opposite sides.

Z/E assignment: on each carbon, COOH has higher priority than H (C > H). When the two COOH groups are on the same side, it is Z (zusammen); on opposite sides, it is E (entgegen).

Here cis = Z and trans = E. Maleic acid has m.p. 130 °C; fumaric acid has m.p. about 287 °C and is more stable.

✓ Answer checked by Claude Opus 5.5 (high effort)

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