Geometric (cis / trans) isomers

What geometric isomers are

• Geometric isomers have their atoms joined in the same order, but arranged differently in space across a C=C double bond.
• They exist because a C=C double bond cannot rotate. The groups on either end are locked in position.
• A single C–C bond can rotate freely, which is why saturated compounds never show this kind of isomerism.

• cis — the two matching groups are on the same side of the double bond.
• trans — the two matching groups are on opposite sides.

The test for whether they exist

Geometric isomers only exist if both conditions hold:

• There is a C=C double bond, and

• each of the two double-bond carbons carries two different groups.

• If either double-bond carbon carries two identical groups, swapping sides changes nothing — there is no cis/trans isomerism.

Worked through:

• But-2-ene, $CH_3CH{=}CHCH_3$ — each C=C carbon holds one H and one $CH_3$. Two different groups on both. ✓ cis and trans exist.
• But-1-ene, $CH_2{=}CHCH_2CH_3$ — carbon 1 holds two H atoms. Identical. ✗ No geometric isomers.
• 1,2-dichloroethene, $CHCl{=}CHCl$ — each carbon holds one H and one Cl. ✓ cis and trans exist.
• 2-methylbut-2-ene, $CH_3CH{=}C(CH_3)CH_3$ — carbon 3 holds two $CH_3$ groups. Identical. ✗ No geometric isomers.

How to check a structure

• Find the C=C.
• Look at the left-hand carbon. List the two groups on it. Are they different?
• Look at the right-hand carbon. List its two groups. Are they different?
• Both different → geometric isomers exist. Otherwise they do not.

Naming them

• Put cis- or trans- at the front of the ordinary IUPAC name:
  • cis-but-2-ene, trans-but-2-ene.
• They are genuinely different compounds — trans-but-2-ene melts at −106 °C and cis-but-2-ene at −139 °C.