Microchem
Microscale Chemistry Experimentation for Teachers

24. C₂H₂Br₂ orbital model (sp² hybridization): cis-trans-isomerismDuring the reaction between bromine water and acetylene (ethine) two isomers of 1,2-dibromoethene were obtained.

Left: cis-1,2-dibromoethene isomer showing the sigma skeleton from side and one p orbital (pi bond) above and under the planar molecule.  Both Br atoms can be seen in front of the sigma skeleton.
Photo 2: The same molecule model showing the complete sigma skeleton from the top.
The model of the cis isomer  shows that the two Br atoms are on the same side of the sigma skeleton.
Photo 3: trans-1,2-dibromoethene isomer showing the sigma skeleton from side and one p orbital (pi bond) above and under the planar molecule. Only one of the two Br atoms is in front of the sigma skeleton.
Right: The same molecule model showing the complete sigma skeleton from the top.
The model of the trans isomer shows that the two Br atoms are on different sides of the sigma skeleton.

The molecules of these two stereoismers are visualized by assembling white, black and green beads (models of H, C, Br atoms) long   microbeads (sigma bonds), round microbeads and plastic tubings ( pi bonds) by wire.