The physical basis for stereoisomerism in alkenes is the restricted rotation of the double bond. Unlike single bonds ( bonds), which allow free rotation, the bond in a double bond prevents the atoms from twisting without breaking the bond.
In cyclic compounds, the ring structure itself acts as a constraint. Atoms or groups attached to the ring are locked into positions 'above' or 'below' the plane of the ring, preventing them from interconverting.
Because these isomers cannot easily interconvert at room temperature, they exist as stable, separate substances with unique properties such as different boiling points and densities.
The Cahn-Ingold-Prelog (CIP) priority rules are used to systematically name stereoisomers. This method involves assigning a priority to the two groups attached to each carbon of the double bond.
Priority Assignment: Priority is determined by the atomic number of the atom directly attached to the double-bonded carbon. The higher the atomic number, the higher the priority (e.g., ).
Determining E or Z: If the two highest-priority groups are on the same side of the double bond, the isomer is designated Z (from the German zusammen, meaning together). If they are on opposite sides, it is designated E (from the German entgegen, meaning opposite).
The 'Split' Technique: When naming, mentally draw a line through the double bond. Evaluate the left carbon and right carbon completely independently before comparing the sides.
Atomic Number, Not Mass: Always use the atomic number. While usually consistent with mass, isotopes (like Deuterium vs. Hydrogen) require strict adherence to atomic number rules.
Tie-Breaking: If the first atoms attached to the carbon are identical (e.g., two different alkyl groups), move along the chain atom-by-atom until a point of difference is found. The group with the first atom of higher atomic number at the point of difference wins priority.
Ignoring the 'Two Different Groups' Rule: Students often try to assign E/Z to molecules like propene. However, because one carbon is bonded to two hydrogens (identical), it cannot have stereoisomers.
Confusing Size with Priority: A large group like a tert-butyl group might seem higher priority than a single Chlorine atom, but because Chlorine has a higher atomic number than Carbon, the Chlorine takes priority.
Rotation Misconception: Remember that stereoisomers are not just 'different views' of the same molecule. You cannot turn an E-isomer into a Z-isomer without breaking chemical bonds.
