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.
Step 3: repeat for the other carbon of the C=C.If the two higher-priority groups are on the same side of the double bond, the isomer is Z. If they are on opposite sides, it is E.Worked example: 2-chlorobut-2-ene, CH3C(Cl)=CHCH3. On carbon 2, Cl (atomic number 17) outranks CH3 (carbon, 6). On carbon 3, CH3 outranks H. In the isomer where Cl and the CH3 on carbon 3 are on the same side, the higher-priority groups are together, so it is Z-2-chlorobut-2-ene.This is why E and Z do not always match trans and cis. In that Z isomer the two CH3 groups are on opposite sides, so it would look 'trans' by the methyl groups, but it is Z by priority.
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.
Step 3: repeat for the other carbon of the C=C.If the two higher-priority groups are on the same side of the double bond, the isomer is Z. If they are on opposite sides, it is E.Worked example: 2-chlorobut-2-ene, CH3C(Cl)=CHCH3. On carbon 2, Cl (atomic number 17) outranks CH3 (carbon, 6). On carbon 3, CH3 outranks H. In the isomer where Cl and the CH3 on carbon 3 are on the same side, the higher-priority groups are together, so it is Z-2-chlorobut-2-ene.This is why E and Z do not always match trans and cis. In that Z isomer the two CH3 groups are on opposite sides, so it would look 'trans' by the methyl groups, but it is Z by priority.