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Geometrical isomerism; trans stability; propene

Short answer

Geometrical isomerism arises from restricted rotation about a double bond when each carbon has two different groups. The trans isomer is more stable because bulky groups are further apart, reducing steric repulsion. Propene (CH3–CH=CH2) shows no geometrical isomerism because one carbon carries two identical H atoms.

What is geometrical isomerism? Why is trans-isomer more stable than cis-isomer? Why is geometrical isomerism not possible in propene?

Geometrical (cis–trans) isomerism is a type of stereoisomerism in which isomers have the same structural formula but differ in the spatial arrangement of groups about a double bond (or ring) because rotation about it is restricted.

Condition: each doubly bonded carbon must carry two different groups (a≠b, c≠d in abC=Ccd).

Why trans is more stable than cis

In the cis isomer the two bulky groups are on the same side, so they come close and there is van der Waals (steric) repulsion, which raises the energy and gives a more strained molecule. In the trans isomer they are far apart, so there is little strain and lower energy. Example: trans-but-2-ene is more stable than cis-but-2-ene by about 4 kJ mol⁻¹.

Why propene shows no geometrical isomerism

Propene is CH₃–CH=CH₂. The terminal carbon holds two identical hydrogen atoms (not two different groups), so interchanging them gives the same compound. Hence no cis–trans isomers exist.

✓ Answer checked by Claude Opus 5.5 (high effort)

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