Stereochemistry

Learning Goals

• Read wedge-dash drawings and translate them into 3D arrangements.
• Determine whether a molecule is chiral or achiral.
• Find stereocenters in a structure.
• Assign R/S configuration using CIP priority rules.
• Assign E/Z configuration to alkenes.
• Classify two structures as enantiomers, diastereomers, or the same compound.
• Recognize meso compounds and explain why they are achiral.
• Read Fischer projections and assign R/S from them.

Key Terms

Tap any term to see its definition.

Section 1

Reading 3D Drawings

Before any of the naming systems make sense, you have to read a molecule in three dimensions. Organic chemists do this with three kinds of bonds: a plain line stays in the plane of the page, a wedge points out toward you, and a dash points back away from you.

Line in plane, wedge toward you, dash away from you

Here are four molecules I want you practicing on. Read each one and picture which groups are coming toward you and which are going back.

Methane

2-Butanol

Bromochlorofluoromethane

Lactic acid

The core idea underneath all of this: stereochemistry is about molecules with the same connectivity but a different arrangement in space. Same atoms, same bonds, different 3D shape.

Section 2

Chirality

A molecule is chiral if it is not superimposable on its mirror image. It is achiral if it is superimposable on its mirror image. That word "superimposable" is doing all the work here, so the way I always introduce it is with your own hands.

Your hands are mirror images that cannot overlap

Hold your hands up palm to palm and they look like perfect mirror images. Now try to stack one directly on top of the other, both palms down. They never line up. That failure to overlap is exactly what chirality means.

Chiral vs achiral

A plane of symmetry usually means achiral

Section 3

Finding Stereocenters

The rule I want you reaching for first: a stereocenter is usually an sp³ carbon attached to four different groups. 2-butanol is the clean example. Its central carbon carries an OH, an H, a CH₃, and a CH₂CH₃, which are four genuinely different groups, so that carbon is a stereocenter.

2-butanol: one stereocenter, four different groups

Now the warnings I do not want you skipping. Not every sp³ carbon is a stereocenter. If a carbon has even two identical groups, it is not a stereocenter. And a molecule can contain stereocenters and still be achiral overall if it turns out to be meso, which is a case we will get to.

Two identical groups: not a stereocenter

Section 4

Assigning R/S Configuration

This is the core skill for chiral centers, and it comes in two halves: first rank the groups using the CIP rules, then trace them. Let me give you the priority rules.

Ranking four groups by atomic number

Multiple bonds become duplicate attachments

Once the four groups are ranked, here is the exact sequence I trace every single time.

Priority 4 away, trace 1 to 2 to 3

Group 4 toward you: reverse the result

Section 5

E/Z Alkene Stereochemistry

A double bond cannot freely rotate the way a single bond can, and that locked geometry means alkenes can have stereochemistry too. We name it with E and Z.

Rank the two groups on each alkene carbon

Z: highest priorities on the same side

E: highest priorities on opposite sides

One caution: do not lean only on cis and trans. Those labels work for simple alkenes, but they break down once each carbon carries more than one kind of substituent. E/Z is the more general system and always gives a clear answer.

cis-2-butene = Z, trans-2-butene = E

Section 6

Comparing Two Structures

When you are handed two structures and asked how they relate, I want you running the same decision process every time instead of guessing. Walk these four questions in order.

Isomer decision flowchart

Enantiomers: mirror images

Diastereomers: not mirror images

Section 7

Meso Compounds

This is the section that breaks the assumption most students walk in with: that stereocenters automatically mean a chiral molecule. Not always. A meso compound has stereocenters but is achiral.

Meso compound: two stereocenters, internal symmetry

A molecule is meso when all of the following are true:

Section 8

Optical Activity and Racemic Mixtures

A substance is optically active if it rotates plane-polarized light. A racemic mixture is a 50:50 mixture of two enantiomers. Enantiomeric excess describes how much one enantiomer outweighs the other in a sample.

Pure enantiomers rotate polarized light

A racemic mixture shows no net rotation

A pure single enantiomer rotates plane-polarized light. A racemic mixture shows no net rotation, because the two enantiomers rotate light by equal and opposite amounts that cancel out.

Section 9

Fischer Projections

A Fischer projection is a flattened way to draw a stereocenter, and it has its own built-in convention: every horizontal bond points toward you, and every vertical bond points away from you. Once you trust that convention, these are fast to read.

Horizontal toward you, vertical away

Assigning R/S from a Fischer projection works the same as always, with one shortcut for the lowest-priority group. If priority 4 sits on a vertical bond, it is already pointing away, so read the 1 to 2 to 3 path normally. If priority 4 sits on a horizontal bond, it is pointing toward you, so reverse your result.

Priority 4 horizontal: reverse the answer

180 is safe, 90 inverts

Section 10

A First Look at Stereochemistry in Reactions

Here is why all of this pays off. Reactions can create, destroy, or invert stereochemistry, and that turns everything in this lesson into a predictive tool rather than just a naming exercise.

Reactions can set or flip 3D arrangement

Two ideas to carry forward. Some reactions favor one stereochemical product over another, and some reactions depend heavily on the stereochemistry of the starting material. I am keeping this short on purpose. The full mechanisms come in the substitution, elimination, and addition lessons, where you will use exactly the R/S and E/Z skills you just built.

Common Mistakes

Watch Out for These

Lesson Summary

Stereochemistry is about molecules with the same connectivity but different 3D arrangements. Read wedges and dashes first, then find stereocenters (sp³ carbons with four different groups), assign R/S by ranking with CIP rules and tracing 1 to 2 to 3 with priority 4 facing away, and assign E/Z by comparing the top-priority groups across a double bond. Compare two structures with the same four-step process every time to land on enantiomers, diastereomers, or identical. Watch for meso compounds, which carry stereocenters but stay achiral through internal symmetry. All of this becomes a prediction tool the moment reactions start changing 3D arrangement.