Isomers and Stereochemistry

The question this page answers: what is the relationship between two molecules that share a single chemical formula — and how do we name the ways they can differ?

Deeper reading: Clayden, Organic Chemistry 2e, ch. 14 (pp. 302–327) — see our chapter-by-chapter practice map for Clayden.

Same formula, three ways to differ

How can two molecules share a formula yet differ?

Molecules that share a formula can differ at three levels: connectivity (structural isomers), configuration (stereoisomers — interconverted only by breaking bonds), and conformation (the same molecule, twisted about its single bonds).

Molecules with the same formula but different connectivity between atoms are structural isomers (also called constitutional isomers):

Three pairs of structural isomers: 2-methylbutane and pentane (C5H12), pyridine and an isomeric imine (C5H5N), acetone and allyl alcohol (C3H6O) — Each pair shares a formula; nothing else about them needs to be similar.

Live examples of structural isomers load as you scroll here…

Conformers, by contrast, interconvert by rotation about single bonds alone — no bonds break — so we treat them as one compound striking different poses rather than as distinct isomers. The rest of this page is about the middle case: configurational stereoisomers, where the difference is locked in place:

Live examples of stereoisomers load as you scroll here…

Stereoisomers and stereocenters

What is a stereocenter — and what counts as four different groups?

Molecules with the same formula and the same connectivity that are still not identical are stereoisomers. An atom carrying four different substituents is a stereocenter.

Examples of stereoisomers: E/Z alkene isomers, R/S isomers of 2-fluorobutane drawn with wedge and hash bonds, and a sulfoxide stereocenter where the lone pair counts as a group — Wedged and hashed bonds carry the 3D information. Note the sulfoxide: a lone pair counts as one of the four groups.

Even though every atom is bonded the same way in two stereoisomers, the only way to interconvert them is to break a bond and re-form it with different geometry. That's what makes stereoisomerism permanent rather than conformational.

Enantiomers, diastereomers, and meso compounds

Two stereoisomers in front of me — how do I name their relationship?

Two stereoisomers that are mirror images are enantiomers; two that are not mirror images are diastereomers; and a molecule with stereocenters that is nevertheless achiral is a meso compound.

Two pairs of enantiomers: CHBrClF mirror images and trans-1,2-dimethylcyclohexane mirror images — Pairs of enantiomers: exact mirror images.

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Two pairs of diastereomers: 2,3-dibromopentane stereoisomers and cis vs trans 1,3-dimethylcyclohexane — Pairs of diastereomers: stereoisomers that are *not* mirror images.

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Meso compounds: cis-1,3-dimethylcyclohexane and meso-2,3-dibromobutane — Meso compounds contain stereocenters, yet an internal mirror plane makes the whole molecule achiral.

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Chirality: the mirror-image test

How do I decide whether a molecule is chiral?

A molecule is chiral if it cannot be superimposed on its own mirror image. Chirality is a property of whole objects — hands, screws, and molecules alike.

The relationships above beg a question: why are enantiomers different compounds, while a meso compound and its mirror image are the same one? The answer is chirality — whether an object differs from its own mirror image.

CHFClBrI-type molecule and its mirror image cannot be superimposed: a chiral molecule. The two are not the same molecule although they are the same constitutional isomer — Not superimposable on its mirror image → chiral. These are two different molecules with identical connectivity.

Methane and its mirror image are superimposable: an achiral molecule, the same molecule on both sides of the mirror — Superimposable on its mirror image → achiral: both drawings are the same molecule.

A practical shortcut: achiral molecules have an internal mirror plane of symmetry — exactly what makes meso compounds achiral despite their stereocenters.

CH2Br2 is achiral, with an internal mirror plane in the plane of the screen; a molecule bearing F, I, Cl on adjacent carbons is chiral with no possible internal mirror plane — Find a mirror plane inside the molecule and it's achiral; if no conformation has one, it's chiral.

CIP priority rules

How do I rank substituents by CIP priority?

Stereochemical labels (R/S, E/Z) are assigned with the Cahn–Ingold–Prelog rules: rank the atoms attached to the stereocenter by atomic weight, break ties by walking down the chain through the highest-ranked atoms, and split multiple bonds into phantom atoms.

CIP rules 1 and 2: rank by heaviness (I outranks Br, and deuterium outranks hydrogen) and continue down the chain, where C(FHH) outranks C(HHH) because it carries a fluorine — Rule 1: heavier atoms rank higher (deuterium beats hydrogen). Rule 2: on a tie, compare the atoms one bond further out.

CIP rule 3: multiple bonds are split into phantom atoms, shown for formaldehyde and for ranking an aldehyde against a CH(OH)2 group, where following the highest-ranked atoms leads to the winning group — Rule 3: split each π bond into phantom atoms, then keep ranking as usual. Phantom atoms usually make multiple bonds outrank single bonds.

Assigning R and S

How do I assign R or S from a drawing?

Prioritize the four substituents, look at the stereocenter with priority 4 pointing away from you, and read 1→2→3: clockwise is R, counterclockwise is S.

A silicon stereocenter bearing H, D, Br, I with CIP priorities 1 through 4 assigned, and an arrow showing the viewing direction opposite the lowest priority — Step 1: assign priorities. Step 2: view the stereocenter down the axis that puts priority 4 behind it.

The same stereocenter viewed with the lowest priority behind: priorities 1 to 2 to 3 run counterclockwise, so the assignment is S — Reading 1→2→3 counterclockwise: this stereocenter is S.

Live examples load as you scroll here… or jump straight to R/S practice.

Assigning E and Z

How do I assign E or Z on a double bond?

If each alkene carbon has two different substituents, rank them by CIP priority. Higher priorities on the same side of the double bond: Z (cis). Opposite sides: E (trans).

E and Z assignments: 1,2-difluoroethylene drawn as E (trans) and Z (cis), plus two tetrasubstituted alkenes assigned E and Z by comparing CIP priorities on each carbon — E/Z generalizes cis/trans to any substitution pattern — the comparison is always between CIP priorities.

Live examples load as you scroll here… or jump straight to E/Z practice.

Beyond R and S: the lowercase descriptors r and s

Why do some stereocenters get lowercase r/s?

A pseudoasymmetric center carries two substituents with identical connectivity but opposite configuration — an R arm and an S arm. It is assigned the lowercase descriptors r and s, which, unlike R and S, do not flip when the molecule is reflected.

Consider pentane-2,3,4-triol with C2 assigned R and C4 assigned S. The two arms attached to C3 contain the same atoms connected in the same order, yet they are not interchangeable: one is the (R)-arm and the other the (S)-arm. C3 therefore holds four distinguishable groups (H, OH, and the two arms) and is a real stereocenter — but exchanging its H and OH converts one achiral diastereomer into another, not a molecule into its enantiomer. The ordinary CIP rules can't rank the two arms, so an auxiliary rule breaks the tie (an R-configured branch ranks above S) and the resulting descriptor is written lowercase: r or s.

The lowercase letter is a flag with a precise meaning: reflection converts every R center to S, but r stays r and s stays s. That mirror-invariance is why pseudoasymmetric centers turn up inside meso and other achiral molecules — among the achiral isomers of cyclohexane-1,2,3,4,5,6-hexol (the inositol family), there is one whose six centers are all labeled r.

You will not need lowercase descriptors in a typical sophomore course — textbooks rarely mention them. But real compound names carry them, and our Isomer Relationships game draws from real PubChem structures, so you may occasionally meet one. Now you know what it means — and that it is telling you the center survives a mirror image unchanged.

Put it together: what's the relationship?

Two molecules, one formula — can I classify the relationship?

Given two molecules with one formula, work the decision tree: different connectivity → structural isomers. Same connectivity → compare configurations: all inverted → enantiomers; some inverted → diastereomers (E/Z pairs included); none → identical — and watch for meso mirror images.

Every category from this page in one deck. Browse examples by type, or try classifying them yourself — and when you're warmed up, the Isomer Relationships game keeps score.

Live examples load as you scroll here… or jump straight to isomer relationship practice.

Optical activity

Does (+)/(−) rotation follow from R/S?

Enantiomers rotate plane-polarized light in opposite directions, labeled (+) and (−). This is measured, not derived: + and − have no correlation with R and S.

Optical rotation is how stereoisomerism is observed at the bench: one enantiomer of a chiral compound rotates linearly polarized light clockwise (+), the other counterclockwise (−). It's a useful experimental gauge, but the direction cannot be predicted from the R/S label — a classic exam trap.

Live examples load as you scroll here… or jump straight to meso practice.