- But-2-ene is a structural isomer of but-1-ene.
- However, if you look at how the but-2-ene can be formed, then you will realize that there are two variations of the molecule:
- To turn the first molecule into the second; on end of the molecule must be rotated; however this would involve breaking one of the bonds in the C=C double bond.
- This would require a chemical reaction, and so the two variations of but-2-ene are isomers.
- They are cis and trans isomers.
Cis and Trans Isomers
- Cis and trans isomers are molecules with the same atoms within them, but a different layout of the atoms. The order of the atoms in cis trans isomers is the same, but the geometries are different.
- This form of isomerism is sometimes called geometric isomerism, and is a type of stereo-isomerism.
- Stereo-isomers have the same molecular formula, and the atoms are bonded together in the same order; however the arrangement of the atoms in space is different in each isomer.
- Cis and trans isomers are given different names- if the substituent groups are on adjacent sides of the double bond to each other, the prefix cis is added. If they are on opposite sides, the suffix trans is added.
- They must have different groups on each C of the double bond.
- Cis and Trans isomers can also be represented by skeletal formula.
- As cis and trans isomers are different compounds, they have different physical properties:
Useful books for revision:
Revise AS Chemistry for Salters (Written by experienced examiners and teachers of Salter's chemistry)
Revise AS Chemistry for Salters (OCR) (Salters Advanced Chemistry)