H3 Chemistry 9813 · Study focus: H3 Chemistry: Assign cis/trans and E/Z configurations
H3 Chemistry: Assign cis/trans and E/Z configurations
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Assign cis/trans and E/Z configurations
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Test whether configuration is locked
A carbon–carbon double bond prevents free rotation, while a saturated ring can lock substituents on the same or opposite faces.
Use cis/trans when an unambiguous same-face comparison exists; use E/Z for an alkene after ranking substituents on each double-bond carbon.
Cis, trans, E and Z
Cis means the specified comparable groups are on the same side or ring face; trans means they are on opposite sides or faces.
For E/Z, each alkene carbon must carry two different substituents: Z places the two higher-CIP-priority groups together, and E places them opposite.
Compare the atoms directly attached to one alkene carbon by atomic number; if tied, compare the next ordered set of attached atoms until the first difference, treating multiple bonds by duplicated-atom convention.
Do not choose groups by mass, chain length or visual size; after finding one winner on each alkene carbon, compare only those two winners across the double bond.
Text alternative: Every spatial relation is duplicated in text as same/opposite side or up/down, with named substituents and explicit priority comparisons.
Apply CIP one alkene carbon at a time
Compare the atoms directly attached to one alkene carbon by atomic number; if tied, compare the next ordered set of attached atoms until the first difference, treating multiple bonds by duplicated-atom convention.
Do not choose groups by mass, chain length or visual size; after finding one winner on each alkene carbon, compare only those two winners across the double bond.
Assign CHCl=CBrF
On the left alkene carbon, Cl outranks H; on the right, Br outranks F because bromine has the higher atomic number.
If Cl and Br are drawn on opposite sides, the alkene is E-CHCl=CBrF.
- Assign CHCl=CBrF when Cl is above the double bond and Br is below it.
Open the feedback checkpoint after attempting
- Award Cl > H, Br > F and E because the higher-priority groups are opposite.
Resolve a carbon-versus-carbon tie
In CH3CH=C(Cl)CHO, compare CH3 and H directly on the left, so CH3 wins; compare Cl and the carbonyl carbon directly on the right, so Cl wins.
If CH3 and Cl are on the same side, the configuration is Z even if CHO appears visually larger than Cl.
- State the CIP winners and descriptor when CH3 and Cl are drawn together in CH3CH=C(Cl)CHO.
Open the feedback checkpoint after attempting
- Credit CH3 on the left, Cl on the right and Z for same-side winners.
Classify a substituted ring
For 1,2-dimethylcyclohexane, two up substituents are cis and one up with one down is trans.
A chair flip exchanges axial and equatorial positions but retains each up/down label, so it cannot convert cis into trans.
- Classify a 1,2-dimethylcyclohexane chair with C1 methyl axial-up and C2 methyl axial-down.
Open the feedback checkpoint after attempting
- Name trans-1,2-dimethylcyclohexane because the methyl groups occupy opposite faces.
Do not equate drawn size with priority
CIP priority begins with atomic number at the first point of difference, not the apparent area of a skeletal group.
CH2=CCl2 has no E/Z descriptor because each carbon of the double bond does not have two different substituents.
- Repair: ‘The longest carbon chain always has highest priority in an E/Z assignment.’
Open the feedback checkpoint after attempting
- Reject it and compare atomic numbers outward only until the first difference.
Report an auditable descriptor
First prove that E/Z is defined, then write the priority comparison on each end and finally state same-side Z or opposite-side E.
For rings, label each substituent up or down before deciding cis/trans; next use four-group priority at tetrahedral stereocentres.
- Give the four checks needed before assigning an alkene E/Z descriptor.
Open the feedback checkpoint after attempting
- Require two different groups on each alkene carbon, a CIP winner on each end, their relative sides and the resulting E or Z name.