Geometrical Isomers
Definition and meaning of Geometrical Isomers in chemistry.
Geometrical isomers are stereoisomers that arise from the restricted rotation around a carbon-carbon double bond or within a ring, causing substituents to be locked into fixed spatial arrangements relative to one another rather than freely interconverting.
In more detail
Because a pi bond prevents free rotation, groups attached to the two doubly bonded carbons stay fixed on the same side (cis) or opposite sides (trans) of the double bond. This type of isomerism requires that each doubly bonded carbon bear two different substituents; if either carbon carries two identical groups, cis-trans isomerism is impossible. For more complex cases with multiple different priority groups, the E/Z (entgegen/zusammen) system based on Cahn-Ingold-Prelog priority rules gives an unambiguous name. Geometrical isomers often have distinctly different physical properties, such as melting point, boiling point, and dipole moment, because their shapes differ.
Key facts
| Field | Organic Chemistry |
|---|---|
| Type | Stereoisomerism (cis-trans / E-Z) |
| Requirement | Restricted rotation (double bond or ring) plus two different groups on each doubly bonded carbon |
| Example formula | C4H8 (but-2-ene) |
But-2-ene (CH3-CH=CH-CH3) exists as cis-but-2-ene, with both methyl groups on the same side of the double bond, and trans-but-2-ene, with the methyl groups on opposite sides; these are distinct compounds with different boiling points (3.7 degrees C for cis versus 0.9 degrees C for trans).
Frequently asked questions
Is geometrical isomerism the same as cis-trans isomerism?
Yes, cis-trans isomerism is the traditional name for the simplest cases of geometrical isomerism, used when only two distinct substituent positions exist. The more general E/Z naming system, based on Cahn-Ingold-Prelog priority rules, is used when more than two different groups are involved.
Why can't single bonds show geometrical isomerism?
A single (sigma) bond allows free rotation around its axis, so any spatial arrangement of substituents rapidly interconverts and cannot be isolated as a separate isomer, unlike a double bond or ring, which restricts rotation.