Geometrical isomerism, also known as cis-trans isomerism, occurs in compounds with restricted rotation around a bond, leading to distinct spatial arrangements of substituents. It is commonly shown by alkenes with carbon- carbon double bonds and cyclic compounds where substituents can be on the same or opposite sides.

Key Compounds Exhibiting It

Geometrical isomerism requires each carbon of a double bond (or adjacent carbons in rings) to have two different substituents, preventing free rotation.

  • Alkenes like 2-butene (CH₃-CH=CH-CH₃) : The cis isomer has methyl groups on the same side; trans has them opposite. This classic example illustrates how the double bond's rigidity creates stable isomers with different properties.
  • Substituted cycloalkanes, e.g., 1,2-dibromocyclopentane : Cis places bromines on the same ring face; trans on opposite faces, due to the ring's cyclic constraint mimicking double-bond restriction.
  • Complexes like [Pt(NH₃)₂Cl₂] : Square planar coordination allows cis (ligands adjacent) and trans (opposite) forms, a staple in coordination chemistry.

Conditions for Geometrical Isomerism

For a molecule to display this isomerism:

  1. Double bonds : Each C=C carbon must bear two unique groups (e.g., not symmetric like in propene).
  2. Rings : Substituents on adjacent carbons in small-to-medium rings (C3-C8 typically), with differing groups.
  3. No free rotation : Single bonds rotate freely, so they don't qualify—only rigid structures do.

Compounds like 1-butene or acyclic alkanes lack it due to symmetry or flexibility.

Real-World Examples and Properties

2-butene isomers (from web visuals): Cis boils higher (boiling point 4°C vs. trans 1°C) due to polarity differences, impacting reactivity in reactions like hydrogenation.

In pharmaceuticals, cis-trans forms affect bioactivity—think retinal in vision, where isomer switching triggers signals.

Compound Type| Example| Cis/Trans Distinction| Physical Difference 1
---|---|---|---
Alkene| 2-Butene| Methyl groups same/opposite side| Cis: higher bp; Trans: more stable
Cycloalkane| 1,2-Dichlorocyclohexane| Substituents same/opposite face| Cis: polar; Trans: often achiral
Coordination| [Co(NH₃)₄Cl₂]⁺| Ligands adjacent/opposite| Trans: symmetric, violet; Cis: green

TL;DR : Geometrical isomerism is shown by alkenes (e.g., 2-butene), cycloalkanes (e.g., 1,2-disubstituted), and certain metal complexes with rigid structures.

Information gathered from public forums or data available on the internet and portrayed here.