This Demonstration describes the rules of Cahn, Ingold and Prelog (CIP) to assign priority to the substituents associated with double bonds and chiral carbon atoms. The first case leads to the classification (for geometric isomerism), the second to the classification (optical isomerism). The method assigns the following order of priority [1, 2]:
1. Priority shall be assigned on the basis of the atomic number. The higher the atomic number, the higher the priority of the substituent; with the same atomic number (isotopes), priority is given to the isotope with the highest atomic mass.
2. If the two atoms compared are identical, it is then necessary to take account of the atoms to which they are bonded. We proceed to the comparison of the successive atoms in the chains up to the point in which a difference in priority occurs. It is useful, to facilitate comparison, to associate each atom of the chain with its atomic number, followed in parentheses by the atomic numbers of the three atoms that are bound to it, in decreasing order.
3. Multiple bonds are transformed into an equivalent number of single bonds, to which is joined a replica of the previously connected atom (usually enclosed in square brackets).
Different organic compounds can be selected with the drop-down menu:
Along with "linear chains", "branched chains" or "chain with double or triple bonds (formally converted)," it is possible to use "carbon atoms of substituent" to change the atom of the substituent chain and thus evaluate the priority of the elements connected to it. The numbering begins with the atom directly involved in the double bond or chiral carbon; the bond relative to this first atom is shown in blue.
Selecting "formal transformation of double and triple bonds" instead allows you to observe the transformation of double and triple bonds to single bonds. In line with the CIP convention 3, the evolution of the structural formulas can be followed with the slider "evolution of the structural formula."
The atom responsible for the first change in the difference in priority and, therefore, responsible for assigning the priority, is represented in green, as well as the atomic number now associated. Shown in orange are the atom and its relative atomic number, which would have had priority if not previously assigned.
Snapshot 1: double and triple bonds are formally converted into single bonds
Snapshot 2: Moving along the chain of the substituent in the branch associated with oxygen (element with the highest atomic number), the methyl carbon introduces the first difference between the two structures and causes the prioritization. Forming oxygen, only two bonds are introduced, with virtual bonds with phantom atoms of zero atomic number.
Snapshot 3: after the formal transformation of multiple bonds into an equivalent number of single bonds (the duplicated atom is enclosed in square brackets), the priority is assigned according to the criterion of the first difference
 H. Hart, L. E. Craine and D. J. Hart, Organic Chemistry: A Short Course, 10th ed., Boston: Houghton Mifflin, Co., 1999.