Here's a breakdown:
Cis:
* Meaning: "On the same side"
* Structure: In a cis isomer, the substituents are located on the same side of the double bond or ring. Imagine them pointing towards each other.
Trans:
* Meaning: "Across from" or "On the opposite side"
* Structure: In a trans isomer, the substituents are located on opposite sides of the double bond or ring. Imagine them pointing away from each other.
Example:
Consider a molecule with a double bond and two different substituents (e.g., CH3 and Cl) on each carbon of the double bond.
* Cis isomer: The CH3 and Cl groups are both on the same side of the double bond.
* Trans isomer: The CH3 and Cl groups are on opposite sides of the double bond.
Important Note:
* Double Bonds: The cis/trans designation is primarily used for molecules with double bonds because rotation around a double bond is restricted.
* Ring Structures: The cis/trans designation can also be used for cyclic molecules (rings) where the substituents are on the same or opposite sides of the ring.
* E/Z Notation: For more complex molecules with multiple substituents, a more rigorous system called the E/Z notation is used. This notation considers the priority of the substituents based on their atomic numbers.
The cis/trans nomenclature is a helpful way to describe the spatial arrangement of substituents in molecules and understand their different properties.