R/S & E/Z Configuration Calculator

In organic chemistry, molecules with the same atoms can have different spatial arrangements — and those differences matter. The R/S & E/Z Configuration Calculator determines the stereochemical configuration of a chiral center or double bond using the Cahn-Ingold-Prelog (CIP) priority rules. Select your Configuration Type (R/S or E/Z), then position your priority groups and specify their stereochemistry to get the configuration label (R, S, E, or Z), along with the rule applied and the priority path direction traced.

Configuration Type *

Priority 4 Stereochemistry *

Priority Groups Arrangement *

Results

Stereochemical Configuration

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Rule Applied

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Priority Path Direction

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Frequently Asked Questions

What is the difference between R/S and E/Z configuration?

R/S configuration refers to the absolute stereochemistry at chiral centers (tetrahedral carbons), while E/Z configuration describes the stereochemistry of alkenes (double bonds). R/S uses priority rules around a central atom, while E/Z compares priority groups on each carbon of a double bond.

How do I assign CIP priorities for stereochemistry?

CIP (Cahn-Ingold-Prelog) priorities are assigned based on atomic number. Higher atomic number gets higher priority (1 > 2 > 3 > 4). For identical atoms, look at the next atoms in the chain. Double and triple bonds are treated as multiple single bonds to the same atom.

What happens if priority 4 is toward me instead of away?

If the lowest priority group (4) is pointing toward you (solid wedge), you must invert the result. If the 1→2→3 path appears clockwise, the actual configuration is S. If it appears counterclockwise, the actual configuration is R.

How do I determine E/Z configuration for alkenes?

First, identify the higher priority group on each carbon of the double bond using CIP rules. If the two higher priority groups are on the same side of the double bond, it's Z (zusammen = together). If they're on opposite sides, it's E (entgegen = opposite).

Can a molecule have both R/S and E/Z configurations?

Yes, complex molecules can have multiple stereocenters and double bonds, each with their own configuration. Each chiral center gets an R or S designation, and each double bond gets an E or Z designation independently.

What is a Fischer projection and how does it affect configuration?

Fischer projections show molecules as crosses where vertical lines represent bonds going away from you and horizontal lines represent bonds coming toward you. This affects how you visualize the stereochemistry and may require adjusting your analysis accordingly.

Why is stereochemistry important in chemistry?

Stereochemistry is crucial because molecules with the same connectivity but different spatial arrangements (stereoisomers) can have vastly different biological activities, physical properties, and chemical reactivities. This is especially important in drug design and biochemistry.