Final answer:
Halogen addition to cis alkenes results in vicinal dihalides with halogen atoms on the same side, while addition to trans alkenes results in dihalides with halogen atoms on opposite sides, leading to different physical properties.
Step-by-step explanation:
The products formed by molecular halogen addition to cis alkenes vs trans alkenes will differ in their stereochemistry. When a molecular halogen, such as bromine, is added to a cis alkene, the halogen atoms end up on the same side of the former double bond, resulting in a vicinal (neighboring) dihalide. In contrast, when the addition occurs on a trans alkene, the halogen atoms are positioned on opposite sides, forming a trans dihalide. These products are distinctive due to their different physical properties like boiling points, dipole moments, and reactivity. For example, if we consider the IUPAC naming for the halogenation reaction of cis-2-pentene, we would obtain 2,3-dichloropentane with the chlorines on the same side. However, halogenation of trans-2-pentene would give us a product with chlorines on opposite sides, resulting in a molecule with different physical properties.