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