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Cis and trans isomers; relative stability

Short answer

Cis and trans isomers are geometrical isomers where similar groups are on the same side (cis) or opposite sides (trans) of a double bond. The trans isomer is more stable because bulky groups are farther apart, minimizing steric repulsion.

What are Cis and trans-isomers? Which of these two isomers is more stable and why?

Cis and trans isomers are geometrical isomers of a compound with a C=C double bond (or ring). In the cis isomer, similar groups are on the same side of the double bond; in the trans isomer, they are on opposite sides.

 CH₃     CH₃      CH₃     H
    \   /            \   /
     C=C              C=C
    /   \            /   \
   H     H          H     CH₃
cis-but-2-ene   trans-but-2-ene

The trans isomer is more stable. In the cis form the two bulky CH₃ groups are close together and repel each other (steric strain), which raises the energy. In the trans form they are far apart, so the repulsion is minimum. Trans-but-2-ene has a heat of hydrogenation about 4 kJ mol⁻¹ lower than the cis form. Similarly, fumaric acid (trans) is more stable than maleic acid (cis), and maleic acid changes into fumaric acid on heating.

✓ Answer checked by Claude Opus 5.5 (high effort)

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