Cis (Cis Isomer)
Definition and meaning of Cis (Cis Isomer) in chemistry.
Cis describes a stereoisomer in which two similar or reference substituents lie on the same side of a double bond or ring, as opposed to the trans arrangement, where they lie on opposite sides.
In more detail
The cis/trans labels apply to geometric (configurational) isomers that arise when rotation is restricted, as around a carbon-carbon double bond or across a ring, so the two spatial arrangements cannot interconvert without breaking a bond. The distinction matters because cis and trans isomers are different compounds with different physical properties, such as melting point, boiling point, and dipole moment, even though they share the same molecular formula and connectivity. For more complex or asymmetric alkenes, the more rigorous E/Z system (using Cahn-Ingold-Prelog priority rules) is used instead of cis/trans, since "same side" becomes ambiguous when more than two different groups are attached.
Key facts
| Term type | Stereochemical descriptor (prefix), not a compound |
|---|---|
| Opposite term | Trans (substituents on opposite sides) |
| Common example | Cis-2-butene, C4H8 |
| Field | Organic Chemistry |
Cis-2-butene (CH3-CH=CH-CH2, with both methyl groups on the same side of the double bond) has a boiling point of about 4°C, while its isomer trans-2-butene boils at about 1°C, illustrating how geometry alone changes physical behavior.
Frequently asked questions
Is cis-trans the same as E/Z notation?
They overlap but are not identical. Cis/trans is based on which side substituents sit on relative to each other, while E/Z uses CIP priority rules; for a simple disubstituted alkene like 2-butene, cis corresponds to Z and trans corresponds to E, but this correspondence can break down with more substituents.
Can cis and trans isomers interconvert at room temperature?
No, not without breaking and reforming a bond, since rotation around a double bond (or around a ring) is restricted; interconversion typically requires added energy, such as heat or light, or a catalyst.