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Cis-trans isomerism is a type of stereoisomerism found in molecules that have restricted rotation around a bond, often a double bond. It is also referred to as

• geometric isomerism
• configurational isomerism

In a molecule exhibiting cis-trans isomerism, the arrangement of substituent groups differs across the double bond.

• Cis isomer: The same substituent groups are on the same side.
• Trans isomer: The same substituent groups are on opposite sides.

In the case of 2-butene, the cis form has both methyl groups on the same side, while the trans form has them across from each other.
This distinction is significant because the spatial arrangement affects the physical and chemical properties of these isomers. For example, trans-2-butene typically has a higher melting point than cis-2-butene due to more symmetrical packing, leading to stronger intermolecular forces.

Molecular symmetry refers to the distribution of identical components or features within a molecule and its ability to be mapped onto itself through rotations or reflections. Symmetry is crucial when it comes to determining the chirality of a molecule.
A molecule with a plane of symmetry means it can be divided into two mirror-image halves. This is essential in distinguishing chiral molecules from achiral ones.

• Cis-2-butene: Has a plane of symmetry where the two identical halves mirror each other.
• Trans-2-butene: Also possesses a plane of symmetry since the two halves can reflect across the center.

Both of these isomers are symmetric, meaning they're not chiral. To remember, a molecule with any plane of symmetry will be achiral by definition.

Achiral compounds are molecules that are superimposable on their mirror images. They do not have a chiral center, such as a carbon atom bonded to four different groups. Achiral molecules often possess a plane of symmetry, which makes one half a mirror image of the other.
Because they can be superimposed, achiral compounds do not exist as enantiomers (pairs of non-superimposable mirror-image isomers).
In the context of 2-butene:

• Cis-2-butene and trans-2-butene exhibit symmetry. This makes them achiral since both can be divided into two identical halves.
• The presence of a plane of symmetry in both ensures they are not distinguishable from their mirror images.

An easy way to identify achiral compounds is by checking for symmetry. If a molecule shares symmetry in any form (plane, center, or axis), it is achiral.