With its double bond locked, but-2-ene comes in two forms: the same atoms, joined in the same order, set out in two different ways.
But-2-ene, C₄H₈, is a chain of four carbons with a double bond between carbons 2 and 3. Each of those two carbons carries one hydrogen atom and one CH₃ group. The two CH₃ groups can sit on the same side of the double bond, or on opposite sides:
- cis, from the Latin for "on this side": both CH₃ groups on the same side.
- trans, from the Latin for "across": the CH₃ groups on opposite sides.
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Turning one form into the other means breaking the second bond, so each form keeps its own arrangement at room temperature. They are two different substances, with their own boiling and melting points.
| Form | Boils at | Melts at |
|---|---|---|
| cis | 4 °C | −139 °C |
| trans | 1 °C | −106 °C |
In the cis form both CH₃ groups sit on one side, so the molecule has a slightly uneven spread of charge, like the polar molecules you met with water's partial charges. That makes cis molecules cling to each other a little more, and cis-but-2-ene boils a few degrees higher. In the trans form the two sides balance out. The trans molecules are more regular, so they stack neatly into a solid, and trans-but-2-ene melts at a higher temperature.
Who gets two forms
Each carbon of the double bond needs two different groups. In propene, CH₂=CH–CH₃, one carbon carries two hydrogen atoms. Swap them and you have the same molecule again, so propene has just one form. But-2-ene is the smallest alkene with a cis and a trans form.
Cis and trans forms are a new kind of isomer. Butane and isobutane have their atoms joined in a different order. A pair of mirror-image molecules, like the two carvones, are each other's reflection. Cis- and trans-but-2-ene have their atoms joined in the same order, and each one matches its own mirror image: the difference is which side of the double bond each group sits on.