3.2.6Additional lessons
Cis-Trans Isomers (HL) — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 63 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Origin of Stereoisomerism
Restricted rotation
- Restricted rotation The π bond prevents rotation about a C=C double bond, so groups stay fixed on one side or the other.
Condition
- Condition Each carbon of the double bond must carry two different groups for isomerism to be possible.
Cis
- Cis In a cis isomer the two similar groups are on the same side of the double bond.
Trans
- Trans In a trans isomer the similar groups are on opposite sides of the double bond.
Stereoisomers
- Stereoisomers These isomers have the same structural formula and connectivity but different arrangements in space.
Part 2 of 5Using E/Z Labels
1
Cis and trans fail when there are three or four different groups, so the E/Z system is used.
2
On each carbon of the double bond, rank the two attached groups by the atomic number of the first atom, the higher number being higher priority.
3
If the two higher-priority groups are on the same side, the isomer is Z, from the German for together.
4
If they are on opposite sides, the isomer is E, from the German for opposite.
5
For but-2-ene the labels E and Z match trans and cis because the methyl groups are the higher-priority groups.
| Property | cis-but-2-ene | trans-but-2-ene |
|---|---|---|
| Boiling point | About 3.7 °C | About 0.9 °C |
| Polarity | Slightly polar | Almost non-polar |
| Packing | Poor | Better, higher melting point |
Part 3 of 5Explaining Differences
Explaining Differences
Polarity
- Polarity In the cis isomer the bond dipoles add up to a small molecular dipole, giving extra attraction between molecules.
Melting point
- Melting point The trans isomer packs more easily into a crystal, so it usually has a higher melting point.
Cycloalkanes
- Cycloalkanes Ring structures such as 1,2-dimethylcyclopropane also show cis-trans isomerism because ring bonds cannot rotate freely.
Biology
- Biology Cis and trans fats differ in shape, which affects the way they pack and how the body uses them.
Part 4 of 5Worked Example
Question Assign E or Z to the isomer of CHBr=CHCl in which Br and Cl are on the same side.
- On carbon 1 the groups are Br and H; bromine has the higher atomic number, so Br is the higher priority.
- On carbon 2 the groups are Cl and H; chlorine is the higher priority.
- Both higher-priority groups are on the same side.
- The isomer is therefore Z.
Answer (Z)-1-bromo-2-chloroethene
Part 5 of 5Exam Checklist & Tips
3.2.6 – Key Points for the Exam
- Stereoisomers Isomers with the same connectivity but a different arrangement of atoms in space.
- Cis The isomer with similar groups on the same side of a double bond.
- Trans The isomer with similar groups on opposite sides of a double bond.
- Restricted rotation The inability of atoms to rotate freely about a double bond.
Common Mistakes
- Assigning cis or trans using only the position of the carbon chain.
- Thinking that cis and Z always mean the same thing, which is only true when the higher-priority groups are the similar groups.
- Saying cis-trans isomerism occurs when one carbon has two identical groups.
- Believing that single bonds also show cis-trans isomerism at room temperature.
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