Isomerism in Organic Chemistry

Isomerism is a phenomenon by which compounds with the same molecular formula have different physical or chemical properties due to the different arrangement of their atoms in space or different connectivity between them.

Structural Isomerism

Structural isomers have the same molecular formula but differ in the connectivity of their atoms. There are several types:

Chain isomerism: Different arrangement of the carbon skeleton (e.g. butane vs. isobutane)
Position isomerism: Same functional group in different positions (e.g. 1-propanol vs. 2-propanol)
Functional group isomerism: Different functional groups (e.g. ethanol vs. dimethyl ether)

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Stereoisomerism

Stereoisomers have the same connectivity but differ in the spatial arrangement of their atoms. There are two main types:

Configurational Isomerism R/S (Chirality)

A carbon is chiral (or asymmetric) when it is attached to four different substituents. These carbons act as chiral centers and can have two different spatial configurations, called R and S according to the Cahn-Ingold-Prelog (CIP) rules:

1. Assign priorities to the four groups attached to the chiral carbon (according to atomic number). 2. Orient the molecule with the lowest priority group (4) pointing away from the observer. 3. Observe the direction (clockwise or counterclockwise) when going through the remaining groups in decreasing order of priority (1→2→3). 4. If the direction is clockwise: R configuration (Rectus); if counterclockwise: S configuration (Sinister).

Example: S configuration (counterclockwise) with priorities Br > Cl > CH₃ > H

Geometric Isomerism Z/E and cis/trans

This occurs when there is restricted rotation around a bond, typically in:

Double bonds C=C (Z/E isomerism)

In alkenes, the restricted rotation of the double bond allows two possible configurations:

Z isomer (Zusammen): The higher priority groups are on the same side of the double bond.
E isomer (Entgegen): The higher priority groups are on opposite sides of the double bond.

Z isomer: priority groups on the same side

E isomer: priority groups on opposite sides

Cyclic compounds (cis/trans isomerism)

In cyclic compounds, the rigidity of the ring prevents free rotation, generating cis/trans isomerism:

cis isomer: The substituents are on the same side of the ring plane.
trans isomer: The substituents are on opposite sides of the ring plane.