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Cis-Trans Isomerism

An alkene or cyclohexane can have a cis or trans conformation if each side of the bond (double for alkene, single for cycloalkyl) is connected to two different groups.

Cis or Trans?

  • If the bulky groups are on the same side, it’s Cis
  • If the bulky groups are on opposite sides, it’s Trans

E or Z designation

Sometimes “bulky” is not sufficient for designating priority, in which case we can use the Cahn-Ingold-Prelog designations to identify which groups should have priority

If the higher priority groups are on the same side, it’s classified as Z

If the higher priority groups are on opposite sides, it’s classified as E

Rules for Priority

Rule 1

  • Consider each doubly bonded carbon independently
  • Look at the two atoms directly connected to it and rank them by atomic number (higher is more important)

Rule 2

  • If the two first atoms are the same, move up the chain to the next atom to compare

Rule 3

  • Multiple bonded atoms are considered to be equivalent to the same number of single bonded atoms

  • Double bonds should be considered as split into single bonds