Relative and Absolute Configurations

Relative configuration compares one chiral molecule to another (used to classify enantiomers/diastereomers/identical). Absolute configuration describes one molecule's exact spatial arrangement on its own, via R/S.
E/Z nomenclature extends CIP priority logic to polysubstituted double bonds, where simple cis/trans breaks down: same side of highest-priority groups = Z (zusammen, 'together'); opposite sides = E (entgegen, 'opposite').
Fischer projections are 2D 'table' drawings of 3D molecules: vertical bonds point away from viewer, horizontal bonds point toward viewer. Common for carbohydrates and amino acids.
Converting Fischer → bond-line: set up backbone from top/bottom groups; left substituents become wedges, right become dashes; flipping to standard bond-line orientation swaps every wedge/dash; verify R/S is preserved.

1Identify the two substituents attached to each carbon of the double bond.
2Assign priority to each pair by atomic number (same rule as CIP for chiral centers).
3Compare the positions of the two highest-priority substituents, one per alkene carbon: same side → Z; opposite sides → E.

1Set up the backbone: top and bottom groups become the two ends of the bond-line structure, with a dot for each carbon in between.
2Add substituents: groups on the left become wedges (out of plane), groups on the right become dashes (behind plane).
3Reorient to bond-line: flipping the zig-zag backbone into standard orientation swaps every wedge to a dash and every dash to a wedge.
4Verify stereochemistry: re-check each chiral center's R/S using CIP to confirm configuration was preserved through the conversion.

Relative configuration: A chiral molecule's configuration described in relation to another chiral molecule — the tool for classifying enantiomers/diastereomers/identical molecules.
Absolute configuration: The exact spatial arrangement of one chiral molecule's atoms, independent of any other molecule, expressed via R/S.
E/Z nomenclature: Labels double-bond geometry for polysubstituted alkenes using CIP priority: same side of top-priority groups = Z, opposite sides = E.
Fischer projection: 2D 'table' representation of a 3D molecule: vertical bonds point away from viewer, horizontal bonds point toward viewer.

E/Z is not the same as trans/cis by default — E/Z is based on CIP priority of the two highest-ranked substituents, which may or may not correspond to the 'same group' comparison that cis/trans relies on.
In a Fischer projection, vertical = away from viewer, horizontal = toward viewer — reversing this convention flips every wedge/dash and inverts R/S incorrectly.
When converting Fischer to bond-line, the reorientation flip (standard zig-zag) swaps every wedge and dash — skipping this step gives the mirror-image (wrong) stereochemistry.

What's the difference between relative and absolute configuration?

In E/Z nomenclature, what does Z mean and where does the term come from?

In a Fischer projection, which bonds point toward the viewer and which point away?

When converting a Fischer projection to bond-line, what happens to left/right substituents during the reorientation flip?

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