How can #"E/Z"# isomers arise?

Question

Jacob Anderson · Accepted Answer

Consider the simple molecule #"2-butylene..........."#

Now for #H_3C-CH=CH-CH_3# we can have 2 geometric isomers:

#"cis-2-butylene"# or #"Z-butene"#; or #"trans-2-butylene"# or #"E-butene"#

For both isomers, the connectivity is MANIFESTLY the same: #C1# connects to #C2# connects......to #C4#. However, the geometry is manifestly different, and this geometric isomerism gives rise to different physical and chemical properties.

For example, #"cis-2-butene"# has a boiling point of #3.7# #""^@C#; on the other hand, #"trans-2-butene"# has a boiling point of #0.9# #""^@C#;

Ellie Adams · Answer

undefined "E/Z" isomers arise due to the restricted rotation around a double bond in organic molecules. When two different groups are attached to each carbon of the double bond, the arrangement of these groups can either be cis (Z) or trans (E) with respect to each other. The priority of substituent groups is determined using the Cahn-Ingold-Prelog priority rules based on atomic number. If the higher priority groups are on the same side of the double bond, it is termed as the "Z" isomer (from German zusammen, meaning "together"). If they are on opposite sides, it is termed as the "E" isomer (from German entgegen, meaning "opposite").

Nicholas Antonuccio · Answer

undefined "E/Z" isomers, also known as geometric isomers, arise due to restricted rotation around a double bond or a ring in a molecule. This restricted rotation leads to the formation of different spatial arrangements of atoms or groups attached to the double bond or within the ring. The "E" and "Z" designations refer to the configuration of substituent groups relative to each other across the double bond.

The "E" (from the German word "entgegen," meaning "opposite") configuration refers to the arrangement where the highest priority substituents are on opposite sides of the double bond. On the other hand, the "Z" (from the German word "zusammen," meaning "together") configuration refers to the arrangement where the highest priority substituents are on the same side of the double bond.

The determination of priority of substituents is based on the Cahn-Ingold-Prelog (CIP) priority rules, which assign priority based on atomic number. The substituent with the higher atomic number is given higher priority.

The presence of different functional groups or atoms attached to the carbon atoms of the double bond can result in the formation of "E/Z" isomers. For example, in a molecule with two different substituents attached to each carbon of the double bond, the spatial arrangement of these substituents can lead to the formation of "E" and "Z" isomers.

Overall, "E/Z" isomers arise due to the restricted rotation around a double bond or within a ring, leading to different spatial arrangements of substituent groups and the designation of "E" or "Z" based on the relative positions of the highest priority substituents across the double bond.