IB Chemistry Topic S3.2.7 Stereoisomers Revision Notes (HL)

IB Chemistry Topic S3.2.7 Stereoisomers Revision Notes (HL) | RevisionDojo

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Cis-Trans Isomerism: A Matter of Rigidity

Definition

Cis-trans isomerism

Cis-trans isomerism occurs in molecules where rotation about a bond is restricted.

This is most commonly observed in:
1. Alkenes (double bonds): The $π$-bond in a double bond prevents free rotation.
2. Cycloalkanes: The rigid ring structure locks substituents in place; this is examined for three-carbon (cyclopropane) and four-carbon (cyclobutane) rings.
For cis-trans isomerism to occur, the two substituents attached to each side of the double bond (or ring) must be different.
If any carbon in the double bond has two identical substituents, cis-trans isomerism is not possible.

Recognizing Cis-Trans Isomers

Cis-isomer: Identical groups are on the same side of the double bond or ring.
Trans-isomer: Identical groups are on opposite sides.

Example

Consider but-2-ene ($C_4H_8$):

In the cis-isomer, the two methyl groups ($CH_3$) are on the same side of the double bond.
In the trans-isomer, the two methyl groups are on opposite sides.

Example

In cycloalkanes, the reference plane is the flat face of the ring.
For 1,3-dichlorocyclobutane:
- Cis-isomer: Both chlorine atoms are on the same side of the ring.
- Trans-isomer: The chlorine atoms are on opposite sides of the ring.

Tip

Cis-trans isomerism only occurs if the molecule has restricted rotation and substituents that differ on each side of the double bond or ring.

Common Mistake

Students often forget that for cis-trans isomerism to occur, the groups attached to each carbon of the double bond must be different.
For example, propene ($CH_3-CH=CH_2$) cannot exhibit cis-trans isomerism because one carbon has two identical hydrogen atoms.

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Cis-Trans Isomerism: A Matter of Rigidity

Definition

Cis-trans isomerism

Cis-trans isomerism occurs in molecules where rotation about a bond is restricted.

This is most commonly observed in:
1. Alkenes (double bonds): The $π$-bond in a double bond prevents free rotation.
2. Cycloalkanes: The rigid ring structure locks substituents in place; this is examined for three-carbon (cyclopropane) and four-carbon (cyclobutane) rings.
For cis-trans isomerism to occur, the two substituents attached to each side of the double bond (or ring) must be different.
If any carbon in the double bond has two identical substituents, cis-trans isomerism is not possible.

Recognizing Cis-Trans Isomers

Cis-isomer: Identical groups are on the same side of the double bond or ring.
Trans-isomer: Identical groups are on opposite sides.

Example

Consider but-2-ene ($C_4H_8$):

In the cis-isomer, the two methyl groups ($CH_3$) are on the same side of the double bond.
In the trans-isomer, the two methyl groups are on opposite sides.

Example

In cycloalkanes, the reference plane is the flat face of the ring.
For 1,3-dichlorocyclobutane:
- Cis-isomer: Both chlorine atoms are on the same side of the ring.
- Trans-isomer: The chlorine atoms are on opposite sides of the ring.

Tip

Cis-trans isomerism only occurs if the molecule has restricted rotation and substituents that differ on each side of the double bond or ring.

Common Mistake

Students often forget that for cis-trans isomerism to occur, the groups attached to each carbon of the double bond must be different.
For example, propene ($CH_3-CH=CH_2$) cannot exhibit cis-trans isomerism because one carbon has two identical hydrogen atoms.

Definition

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Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

IB Chemistry S3.2.7 Stereoisomers Notes

Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

Cis-Trans Isomerism: A Matter of Rigidity

Recognizing Cis-Trans Isomers

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Cis-Trans Isomerism: A Matter of Rigidity

Recognizing Cis-Trans Isomers

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