Difficulty: Introductory | Prerequisites: Lab Day 1 notes (isomers, structural representations, Newman projections)
Once you can draw and read structures, the next question is: when are two structures genuinely different molecules, and when are they just different views of the same one? This lab introduces the tools for answering that. Cis/trans isomerism sorts molecules by whether groups sit on the same or opposite side of a double bond. Symmetry elements (plane and centre) tell you whether a molecule is superimposable on its mirror image, and if it is not, you have an enantiomer. Cyclohexane conformations then show how a six-membered ring flexes in three dimensions and why that matters for stability.
Cis/trans isomers differ by which side of a double bond their substituents sit on. Molecules with a plane or centre of symmetry are achiral; those without either are enantiomers (non-superimposable mirror images). Cyclohexane rings flip between chair conformations, and whether a substituent lands axial or equatorial determines the molecule's stability.
Cis isomer
A stereoisomer in which two designated substituents are on the same side of a double bond (or ring). Requires a double bond (or ring) that prevents free rotation.
Think of it as both groups pointing the same way, up-up or down-down.
Trans isomer
A stereoisomer in which two designated substituents are on opposite sides of a double bond (or ring).
Think of it as the groups pointing in opposite directions: one up, one down.
Plane of symmetry (mirror plane, sigma plane)
An imaginary plane that bisects a molecule so that one half is the exact mirror image of the other. If a molecule has a plane of symmetry, it is achiral.
In simple terms, if you can slice the molecule in half and the two halves reflect each other, it has a plane of symmetry.
Centre of symmetry (inversion centre)
A point in the molecule through which every atom can be projected an equal distance on the opposite side and find an identical atom. Rarer than a plane of symmetry.
Enantiomer
One of a pair of molecules that are non-superimposable mirror images of each other. A molecule is an enantiomer (chiral) only if it has neither a plane of symmetry nor a centre of symmetry.
Think of it as left hand vs. right hand: mirror images that cannot be overlaid.
Chirality
The property of a molecule that is not superimposable on its mirror image. A chiral molecule has no internal plane or centre of symmetry.
Cyclohexane
A six-carbon saturated ring (C₆H₁₂). Because it has no double bonds, the ring is not flat; it adopts three-dimensional conformations.
Chair conformation
The most stable conformation of cyclohexane. Alternating carbons point up and down, forming a shape resembling a chair. All bond angles are close to the ideal 109.5° and all adjacent C–H bonds are staggered.
Boat conformation
A less stable cyclohexane conformation where the ring folds so that two opposite carbons point in the same direction (both up). Introduces eclipsing strain and flagpole interactions.
Twisted boat (skew boat) conformation
A slight twist of the boat that partially relieves flagpole strain. More stable than the boat, less stable than the chair.
Inverted chair conformation
The other chair form, obtained by a ring-flip of the original chair. Every axial substituent becomes equatorial and vice versa.
Axial position
Bonds that point straight up or straight down from the ring, roughly parallel to the ring's vertical axis. Substituents in axial positions experience more steric crowding (1,3-diaxial interactions), making them less stable when bulky.
Equatorial position
Bonds that point roughly outward from the ring, along the "equator." Substituents here have more room, making equatorial placement more stable for bulky groups.
Cis/trans isomerism requires restricted rotation, which in practice means a carbon-carbon double bond (or a ring).
Cis isomer
Two specified substituents sit on the same side of the double bond.
Example: cis-2-butene has both methyl groups on the same side.
Trans isomer
Two specified substituents sit on opposite sides of the double bond.
Example: trans-2-butene has one methyl group above and one below the double bond.
Why the double bond matters
Single bonds rotate freely, so "same side" and "opposite side" have no fixed meaning.
A double bond locks the geometry: the pi bond prevents rotation, making cis and trans genuinely different molecules with different physical properties (boiling point, dipole moment).
Symmetry and chirality
Plane of symmetry
The most common symmetry element tested.
If you can find any plane that cuts the molecule into two mirror-image halves, the molecule is achiral (not chiral).
Centre of symmetry (inversion centre)
Less common. Every atom has a matching atom equidistant from a central point, on the opposite side.
A molecule can lack a plane of symmetry but still be achiral if it has a centre of symmetry (rare, but examinable).
Enantiomers
If a molecule has neither a plane nor a centre of symmetry, it is chiral and exists as a pair of enantiomers.
Enantiomers have identical physical properties in an achiral environment but rotate plane-polarised light in opposite directions.
Cyclohexane is not flat. The ring puckers to relieve angle strain and keep bond angles close to the tetrahedral 109.5°.
Chair conformation (most stable)
Alternating carbons point up and down.
All adjacent C–H bonds are perfectly staggered, minimising torsional strain.
Each carbon has one axial bond (pointing up or down) and one equatorial bond (pointing roughly outward).
Boat conformation
The ring folds so two opposite carbons (the "bow" and "stern") point the same way.
Introduces eclipsing strain along the sides and flagpole interactions between the two bow/stern hydrogens.
Significantly less stable than the chair.
Twisted boat (skew boat)
A slight rotation of the boat that moves the flagpole hydrogens apart.
Partially relieves flagpole strain; still less stable than the chair.
Inverted chair
A ring-flip converts one chair into the other.
Every substituent that was axial becomes equatorial, and vice versa.
Both chairs are equal in energy for unsubstituted cyclohexane; for substituted cyclohexanes, the chair that places the bulkiest group equatorial is favoured.
Stability ranking: chair > twisted boat > boat. The inverted chair has the same intrinsic stability as the original chair; the preference depends on substituent placement.
Axial vs. equatorial: why it matters
Axial substituents are closer to other axial groups on the same side of the ring (1,3-diaxial interactions). Bulky groups in axial positions raise the energy.
Equatorial substituents point away from the ring, with more room. Bulky groups strongly prefer equatorial positions.
A classic exam scenario: given a substituted cyclohexane, draw both chair conformations and identify which is more stable based on where the largest group ends up.
Students often think cis/trans isomerism applies to any molecule. It does not. You need a double bond (or a ring) that prevents free rotation; single-bond molecules just rotate into each other.
Many students confuse "achiral" with "no symmetry." Achiral means the molecule is superimposable on its mirror image, which happens when it has at least one plane of symmetry or one centre of symmetry.
A common mistake is thinking the boat conformation is the least stable form of cyclohexane. The boat is less stable than the chair, but it is actually slightly more stable than a perfectly half-chair (the transition state). The twisted boat is more stable than the regular boat.
Students sometimes assume a ring-flip changes the molecule into a different isomer. It does not. Both chairs of the same molecule are conformations, not isomers; they interconvert without breaking any bonds.
⚠️ You will be asked to identify cis vs. trans isomers from a structural drawing. Look for a double bond first, then check which side the groups are on.
⚠️ Symmetry questions often ask: "Does this molecule have a plane of symmetry? Is it chiral?" Practise finding (or ruling out) mirror planes quickly.
⚠️ Drawing both chair conformations of a substituted cyclohexane and identifying the more stable one is a staple exam question. The rule: bulky groups prefer equatorial.
⚠️ Know the stability order of cyclohexane conformations: chair > twisted boat > boat. Expect energy diagram questions where you label each conformation on a potential energy curve.
True or False: Cis/trans isomerism can occur around a C–C single bond. (False: free rotation around single bonds means there is no fixed "same side" or "opposite side.")
A molecule with a plane of symmetry is ______ (chiral / achiral). (Achiral.)
True or False: In cyclohexane, the boat conformation is more stable than the chair. (False: chair is the most stable.)
After a ring-flip, an axial substituent becomes ______. (Equatorial.)
True or False: Enantiomers have identical melting points. (True, in an achiral environment.)
Q: What structural feature is required for cis/trans isomerism, and why?
A: A double bond (or ring) is required because it prevents free rotation. Without restricted rotation, the molecule simply rotates into the other arrangement, so there are no distinct "cis" and "trans" forms.
Q: A molecule has no plane of symmetry and no centre of symmetry. What can you conclude?
A: The molecule is chiral. It exists as a pair of enantiomers, non-superimposable mirror images that rotate plane-polarised light in opposite directions.
Q: Draw both chair conformations of methylcyclohexane. Which is more stable, and why?
A: In one chair the methyl group is equatorial; in the other it is axial. The equatorial chair is more stable because the methyl group avoids 1,3-diaxial interactions, which cause steric strain.
Q: Why is the boat conformation of cyclohexane less stable than the chair?
A: The boat has eclipsing interactions along the sides of the ring and flagpole interactions between the two hydrogens at the "bow" and "stern." The chair has all bonds staggered and no flagpole strain.
Q: Explain the difference between a plane of symmetry and a centre of symmetry.
A: A plane of symmetry is a flat surface that divides the molecule into two mirror-image halves. A centre of symmetry is a single point through which every atom can be projected an equal distance to find its identical partner on the other side. Both make a molecule achiral, but they are geometrically distinct tests.
Cis/trans isomerism connects back to Lab Day 1's structural representations: you need to read wedge-dash and line structures fluently before you can assign cis or trans.
Enantiomers become central in later organic chemistry when studying reactions at chiral centres (S_N1 vs. S_N2 mechanisms produce different stereochemical outcomes). Understanding symmetry now saves significant time later.
Cyclohexane conformational analysis extends into substituted rings, decalins, and eventually into sugar chemistry (glucose adopts a chair conformation) and steroid structures.
Cis isomer, trans isomer, geometric isomer, E/Z nomenclature, stereoisomer, plane of symmetry, mirror plane, sigma plane, centre of symmetry, inversion centre, enantiomer, chiral, achiral, chirality, non-superimposable mirror image, optical activity, cyclohexane, chair conformation, boat conformation, twisted boat, skew boat, inverted chair, ring flip, axial position, equatorial position, 1,3-diaxial interaction, steric strain, conformational analysis, organic chemistry lab, NYU chemistry, molecular models lab