In the E/Z system of geometrical isomerism, the priority rules are based on the atomic number and mass number of the substituents attached to the double bond. This system is also known as the Cahn-Ingold-Prelog (CIP) system. The use of atomic number and mass number for assigning priorities in the E/Z system is based on the following reasons:
Consistency: The atomic number and mass number are objective and well-defined physical properties of atoms. They provide a consistent and universally applicable criterion for assigning priorities. The use of these properties ensures that the priority rules can be applied consistently across different molecules and compounds.
Predictability: The atomic number and mass number reflect the relative sizes and masses of atoms. Atoms with higher atomic numbers and masses tend to have larger volumes and higher molecular weights. Assigning priorities based on these properties allows for predictability in the stereochemical assignments, as larger atoms or groups are typically assigned higher priorities.
Molecular Weight and Bulkiness: In the E/Z system, the assignment of priorities is based on the concept of the "priority sequence" of the substituents. The priority sequence is determined by comparing the atomic number or mass number of the atoms directly attached to the double bond. This comparison helps to determine the relative molecular weights and bulkiness of the substituents, which play a crucial role in determining the E/Z configuration.
Convention and Consensus: The use of atomic number and mass number in assigning priorities in the E/Z system is widely accepted and agreed upon by the scientific community. It has become a standard convention for designating stereoisomers. The CIP system provides a consistent and unambiguous method for describing the spatial arrangement of substituents around a double bond.
By prioritizing the atomic number and mass number, the E/Z system ensures a systematic and reliable approach for assigning geometrical isomerism, which allows for clear communication and understanding of the stereochemistry of molecules.