What defines an E-isomer?

An E-isomer is defined by having the highest priority substituents on opposite sides of a double bond. The term "E" comes from the German word "entgegen," meaning "opposite." In terms of molecular geometry, this means that if you have a double bond between two carbon atoms, the groups or substituents with the highest atomic numbers (or priorities, determined by the Cahn-Ingold-Prelog priority rules) will be located on opposite sides of that double bond.

This configuration leads to specific physical and chemical properties distinct from those of Z-isomers, where the highest priority groups are on the same side. The ability for the groups to be positioned this way allows for different interaction possibilities with other molecules, which is significant in biological systems and material properties.

In contrast, the other options do not accurately capture the definition of E-isomers, as they either describe arrangements that would correspond to Z-isomers or refer to characteristics unrelated to the specific arrangement of substituents in the context of double bonds.