Stereochemistry — Visual Field Guide

A pattern-recognition field guide. Each section is one visual. Read the picture, not the words.

Sources: FAD1018 W6 — Stereochemistry | FAD1018 Tutorial 6 — Stereochemistry | Stereochemistry

1. Wedge-Dash → Fischer Pipeline

The physical act of "projecting" a 3D molecule onto a 2D Fischer. You're looking from above, straight down the carbon chain.

One-liner: The vertical axis goes away from you. The horizontal axis comes toward you. You are the camera looking down from above.

2. Fischer Projection — The 4 Critical Rules

Four patterns. They'll save you every time.

One-liner: Every time you touch the drawing (rotate 90° or swap two groups), R becomes S. Touch twice (180° or two swaps), you're back where you started.

3. R/S Configuration — How to See It

Assign. Point away. Trace. Read.

Quick reference — The decision tree:

graph TB
    A[Assign CIP<br>priorities 1-2-3-4] --> B[Point group 4<br>away from you]
    B --> C[Trace 1 → 2 → 3]
    C --> D{Clockwise?}
    D -->|Yes| E[R configuration]
    D -->|No| F[S configuration]
    G[In Fischer: group 4<br>on horizontal?] -.-> H[Trace normally<br>then INVERT]
    H --> D

One-liner: Point the smallest (H usually) away. Trace 1→2→3. Clockwise = R. In Fischer, if the H is sideways, flip the answer.

4. The Meso Trap

The most common exam trap. Two chiral centres. But the molecule has a mirror inside it.

One-liner: Two chiral centres ≠ chiral molecule. If there's an internal mirror plane, the molecule is meso — achiral despite having stereocentres. 2ⁿ predicts 4, but you only get 3.

5. Enantiomers vs Diastereomers — At a Glance

One question to rule them all: Are they mirror images?

Flow chart cheat:

graph TB
    A[Two molecules] --> B{Are they mirror images?}
    B -->|No| C[Diastereomers<br>different physical<br>& chemical properties]
    B -->|Yes| D{Superimposable?}
    D -->|Yes| E[Identical<br>same compound]
    D -->|No| F[Enantiomers<br>same physical properties<br>different optical rotation]

One-liner: Mirror images ? enantiomers or identical : diastereomers. That's the whole classification.

Cheat Sheet (All 5 Rules)

Concept	Rule
Fischer→3D	Vertical = away, Horizontal = toward
90° rotation	R ↔ S (inverts)
180° rotation	R stays R (same)
Single swap	R ↔ S (inverts)
R/S	Point 4 away, trace 1→2→3. Clockwise = R
Fischer R/S	If 4 is horizontal, read then invert
Meso	Internal mirror plane → chiral centres but achiral
Enantiomers	Mirror images, non-superimposable
Diastereomers	Not mirror images
Max stereoisomers	2ⁿ, but subtract 1 for each meso
Optical activity	R can be + or -, S can be + or -. They are unrelated.