IB Chemistry HLTopic 3 — Classification of MatterPaper 1 & 2HL only~13 min read
Cis-Trans Isomers
Up to now, isomers have differed in the order the atoms are joined. Here the order is identical — every atom is bonded to exactly the same neighbours — and the molecules are still different substances, because one part of the molecule cannot rotate to match the other. Everything on this page comes back to that one idea: something is stopping the spin.
📘 What you need to know
Stereoisomers have the same atoms joined in the same order, but a different arrangement in space.
Conformational isomers arise from free rotation about a single bond. They interconvert constantly and cannot be separated.
Configurational isomers need bonds to be broken to interconvert, so they can be separated and studied.
Cis-trans isomerism is configurational. It happens when rotation is blocked by a C=C double bond or by a ring.
Cis means the two groups are on the same side; trans means opposite sides.
For cis-trans isomerism to exist, each carbon of the C=C must carry two different groups.
Cis and trans isomers have different physical properties because their shapes and polarities differ.
The E/Z system is not assessed on the IB course. Use cis and trans.
Where cis-trans isomerism sits
Isomerism has a family tree, and it is worth seeing the whole thing once so you know which branch you are on.
The single question that splits the tree is whether the atoms are joined in the same order. Answer that first and you know immediately whether you are looking at structural isomers or stereoisomers.
Conformational isomers: rotation is free
A single carbon–carbon bond is a sigma bond, and a sigma bond is symmetrical about its axis. That means the two ends can spin without weakening the bond at all. Ethane is the simplest case: looking straight down the C–C bond, the hydrogens on the front carbon can be lined up with those at the back (eclipsed) or turned 60° away from them (staggered).
Staggered is more stable, because the C–H bonds are as far apart as possible and the electrons in them repel each other least.
Eclipsed is less stable, because those bonding electrons are pushed closer together.
The energy difference is tiny, so at room temperature molecules flip between the two constantly. You cannot isolate a single conformer.
Cyclohexane does the same trick in a ring, flipping between a chair shape and a boat shape. The chair is more stable; the boat has four eclipsed bonds and its two end hydrogens crowd each other, which strains the ring.
Conformational isomers are worth understanding but they are not what exam questions on isomerism usually want. If a question asks you to “draw the two isomers”, it almost always means configurational ones — the sort you could actually put in two separate bottles.
Why a double bond changes everything
A C=C is one sigma bond plus one pi bond. The pi bond is formed from p orbitals overlapping above and below the axis, so to rotate one end you would have to break that overlap. That costs far too much energy at ordinary temperatures, and so the two ends are locked.
The blue lobes are the pi bond, formed above and below the line joining the two carbons. Twisting one end by 90° would pull those lobes out of overlap and destroy the pi bond entirely.
Rings do the same job. A cyclic alkane has only single bonds, but the ring itself holds the carbons in place — a carbon cannot rotate without dragging the whole ring round with it. So cis-trans isomerism also happens in rings, with groups either both on the same face of the ring or on opposite faces.
The condition for cis-trans isomerism
A double bond is necessary but not sufficient. You also need each carbon of the C=C to carry two different groups. If either carbon has two identical groups on it, swapping them changes nothing and there is only one molecule.
Try the swap test on the right-hand molecule: exchange the top and bottom methyl groups and you get back exactly the same structure. That is the quickest way to check whether cis-trans isomerism is possible.
🧩 Testing for cis-trans isomerism
Find something that blocks rotation — a C=C double bond, or a ring. No block means no cis-trans isomers.
Look at the first carbon of the double bond. List the two groups on it. If they are identical, stop: there are no cis-trans isomers.
Look at the second carbon and do the same. Again, two identical groups means stop.
Both carbons pass? Then two isomers exist. Draw one with the two “bigger” groups on the same side and label it cis.
Draw the second by moving one group to the other side, and label it trans. Check the two drawings are genuinely different.
Cis and trans really are different substances
Because you cannot interconvert them without breaking the pi bond, cis and trans isomers can be separated, bottled and measured independently. Their properties differ in two ways worth knowing:
Boiling point. A cis isomer often has a small overall dipole, because its two polar or bulky groups point the same way and their effects add up. A trans isomer is more symmetrical, so its dipoles tend to cancel. More polarity means stronger attractions and a higher boiling point. Cis-but-2-ene boils at 3.7 °C, trans at 0.9 °C.
Melting point. Here the trans isomer usually wins, because its neat, symmetrical shape packs into a crystal lattice far more efficiently. Trans-but-2-ene melts at −106 °C, but the awkwardly bent cis isomer has to be cooled to −139 °C.
These two trends run in opposite directions, and that is not a mistake in the data. Boiling depends on how strongly individual molecules attract each other; melting depends on how well they stack. Cis wins the first, trans wins the second.
Where cis-trans naming runs out
Cis and trans work when you can point at a pair of matching groups on opposite carbons. Once all four groups round the double bond are different, “same side” is meaningless — same side as what?
Two of the four groups identical: cis-trans works fine, as in 1,2-dichloropropene.
All four groups different: cis-trans cannot be used. 1-bromo-2-chloropropene is the standard example.
You do not need E/Z for IB. The E/Z system was invented to fix exactly this gap, and you may see it in textbooks or older papers as “geometric isomerism”. It is not on the current IB syllabus and will not be assessed, so stick with cis and trans.
Worked examples
WORKED EXAMPLE
State whether each of these shows cis-trans isomerism: propene, but-2-ene, and 1,1-dichloroethene.
Step 1: propene, CH3CH=CH2Carbon 1 of the double bond carries a CH3 and an H, which are different. Carbon 2 carries two hydrogens.two identical groups on one carbon, so noStep 2: but-2-ene, CH3CH=CHCH3Each double-bond carbon carries one CH3 and one H, so both pass the test.yes — cis and trans both existStep 3: 1,1-dichloroethene, CCl2=CH2One carbon has two chlorines; the other has two hydrogens. Both fail.noonly but-2-ene shows cis-trans isomerismCompare 1,1-dichloroethene with 1,2-dichloroethene, which does show it. The numbers in the name are doing real work here.
WORKED EXAMPLE
Explain why cis-but-2-ene and trans-but-2-ene can be separated from each other, but the staggered and eclipsed forms of ethane cannot.
Step 1: identify what would have to happen to interconvert each pairCis to trans needs one end of the double bond to rotate by 180°. Staggered to eclipsed needs the single bond in ethane to rotate by 60°.Step 2: work out the energy cost of each rotationRotating a C=C means pulling the p orbitals of the pi bond out of overlap, which effectively breaks the pi bond. Rotating a sigma bond costs almost nothing, because a sigma bond is symmetrical about its axis.Step 3: link the energy cost to whether they can be isolatedThe pi bond is far too strong to break at room temperature, so each isomer stays as it is. Ethane’s conformers swap over billions of times a second.the pi bond blocks rotation, so cis and trans are separable; ethane’s sigma bond does notThis is the difference between configurational and conformational isomers, in one sentence.
WORKED EXAMPLE
Cis-1,2-dichloroethene boils at 60 °C and the trans isomer boils at 48 °C. Explain the difference.
Step 1: compare the shapesIn the cis isomer both C–Cl bonds point to the same side of the double bond. In the trans isomer they point to opposite sides.Step 2: work out the overall dipole in eachEach C–Cl bond is polar. In the cis isomer the two bond dipoles point roughly the same way, so they add together and the molecule has a net dipole. In the trans isomer they point in opposite directions and cancel, so the molecule is non-polar overall.Step 3: link dipole to intermolecular forcescis: dipole-dipole plus London forcestrans: London forces onlythe cis isomer is polar, so its stronger intermolecular forces need more energy to overcomeAnswer with the dipoles and the type of force, not just “cis is bigger”. The two molecules have identical formulas and identical masses.
Conformational and configurational compared
Feature
Conformational isomers
Configurational isomers
What allows or blocks change
free rotation about a sigma bond
rotation blocked by a pi bond or a ring
To interconvert you must
simply rotate the bond
break and re-form bonds
Can they be separated?
no, they swap constantly
yes
Physical properties
effectively identical
measurably different
Examples
staggered and eclipsed ethane; chair and boat cyclohexane
cis and trans but-2-ene; pairs of optical isomers
💡 Exam tip
Check both carbons of the double bond before you commit. One carbon with two identical groups is enough to rule cis-trans isomerism out.
Always mention the pi bond when explaining why rotation is blocked. “The double bond is rigid” on its own rarely gets the mark.
Draw your C=C with proper 120° angles so the examiner can see which groups are up and which are down. A flat, cramped sketch is unmarkable.
For boiling point comparisons, argue through polarity. Identify the bond dipoles, say whether they cancel, then name the intermolecular force.
Remember that melting point goes the other way. Trans packs better, so it usually melts higher even though it boils lower.
Do not use E and Z. They are not assessed, and cis and trans are what the mark scheme expects.
⚠ Common mix-up
Assuming every alkene has cis-trans isomers. Propene and 2-methylpropene do not, because one of their double-bond carbons carries two identical groups.
Treating cis-trans isomers as structural isomers. The atoms are joined in exactly the same order, so they are stereoisomers.
Thinking conformational isomers can be separated. They interconvert far too fast, because nothing is stopping the rotation.
Forgetting that rings block rotation too. Cis-trans isomerism is not limited to alkenes.
Saying trans always boils higher because it is more stable. Stability is not the argument — polarity is, and for boiling point it usually favours cis.
Mixing up which isomer melts higher and which boils higher. Trans melts higher, cis usually boils higher.
Using cis-trans names when all four groups are different. The system cannot cope with that case, and IB will not ask you to.
Up next: Enantiomers — the other branch of configurational isomerism, where two molecules are mirror images that can never be laid on top of each other.
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