BRAINCAKENomenclature (Cahn-Ingold-Prelog)
Cis/trans only works when there are identical groups to compare. For any alkene, the E/Z label is decided with the Cahn-Ingold-Prelog (CIP) priority rules.
Step 1: look at one carbon of the C=C. Compare the atoms directly attached to it. The atom with the higher atomic number has the higher priority. For example Br (35) > Cl (17) > F (9) > O (8) > C (6) > H (1).
Step 2: if the attached atoms are the same, move outwards and compare the sets of atoms attached to them, at the first point of difference. CH3 carries (H, H, H) but CH2CH3 carries (C, H, H), so the ethyl group has the higher priority.
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More in Alkenes
- Naming Alkenes
- Bonding
- Position Isomerism
- Geometric Isomerism
- Physical Properties of Alkenes
- Combustion
- Electrophiles
- Electrophilic Addition
All 23 lessons in Alkenes · All AQA AS-level Chemistry topics