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Organic chemistry - the basics

Stereoisomerism

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Explainer Video

Tutor: Alisha

Summary

Stereoisomerism

In a nutshell

Stereoisomers are molecules with the same structural formula but the atoms are arranged differently in space. E/Z isomerism is a type of stereoisomerism. CIP rules can be used to determine whether an isomer is E or Z isomer. Cis-trans isomerism is a sub-category of E/Z isomerism.

Double bonds

$C=C$ bonds and the atoms directly attached to these carbon atoms are arranged in a trigonal planar shape, with a bond angle of $120 \degree C$ .

Example

Chemistry; Organic chemistry I; KS5 Year 12; Stereoisomerism — A. Planar - rigid rotation; B. Tetrahedral - free rotation.

The atoms directly attached to the carbons in the $C=C$ bond cannot rotate around them - the rotation is restricted. Although some molecules may have the same structural formula, they are arranged differently in space. This gives rise to stereoisomers.

E/Z isomerism

Stereoisomers have the same structural formula but are arranged differently in space. The restricted rotation around the $C=C$ bond
leads to some alkenes to have stereoisomers, where each carbon atom in the double bond is attached to two different atoms or group of atoms. This gives rise to E/Z isomerism. Z isomers have the same atom or group of atoms on the same side. They are either, both above the double bond or both below the double bond. Whereas E isomers have the same atoms or groups of atoms on opposite ends.

Example

Cahn-Ingold-Prelog (CIP) rules

Many alkene molecules have different atoms attached to each carbon in the $C=C$ bond. You can follow the Cahn-Ingold-Prelog rules of priority to determine whether you have an E or Z isomer. Atoms or groups with a higher atomic number are higher priority.

Procedure

1.	Label the carbons in the double bond as $1$ and $2$ .
2.	Determine which atom directly attached to $C_1$ has a higher atomic number. This will be the higher priority atom/group.
3.	Determine which atom directly attached to $C_2$ has a higher atomic number. This will be the higher priority atom/group.
4.	If the atoms directly attached to the carbon are the same, move on to the next atom attached until there is a difference in atomic number and you can determine which side has a higher priority.
5.	Atoms/groups with the same priority on the same side will be classed as Z isomers. Atoms/groups with the same priority on opposite ends will be classed as E isomers.

Example

Determine whether the molecule is a E or Z isomer.

Following the procedure to determine the type of isomer, you find that it is an E isomer. This is because the higher priority groups are on opposite ends.

Cis-trans isomerism

Cis-trans isomerism is a type of E/Z isomerism. When there is at least one atom or group in common, the isomers can be called cis-trans isomers.

Example

The molecule below has two hydrogens, which is the common factor between the carbons in the double bond. The hydrogen atoms are on the same side so this isomer is cis- $1$ -bromopropene.

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Structural isomerism

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